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23
24 title: 'JAVAC(1) JDK @@VERSION_SHORT@@ | JDK Commands'
25 date: @@COPYRIGHT_YEAR@@
26 lang: en
27 ---
28
29 ## Name
30
31 javac - read Java declarations and compile them into class files
32
33 ## Synopsis
34
35 `javac` \[*options*\] \[*sourcefiles-or-classnames*\]
36
37 *options*
38 : Command-line options.
39
40 *sourcefiles-or-classnames*
41 : Source files to be compiled (for example, `Shape.java`) or
42 the names of previously compiled classes to be processed for annotations
43 (for example, `geometry.MyShape`).
44
45 ## Description
46
47 The `javac` command reads _source files_ that contain module, package and type
48 declarations written in the Java programming language, and compiles them into _class files_
49 that run on the Java Virtual Machine.
50
51 The `javac` command can also [process annotations](#annotation-processing)
52 in Java source files and classes.
53
54 Source files must have a file name extension of `.java`.
55 Class files have a file name extension of `.class`.
56 Both source and class files normally have file names that identify the contents.
57 For example, a class called `Shape` would be declared in a source file
58 called `Shape.java`, and compiled into a class file called `Shape.class`.
59
60 There are two ways to specify source files to `javac`:
61
62 - For a small number of source files, you can list their file names on
63 the command line.
64
65 - For a large number of source files, you can use the [`@`*filename*](#option-at)
66 option on the command line to specify an _argument file_ that lists
67 their file names. See [Standard Options] for a description of the
68 option and [Command-Line Argument Files] for a description of
69 `javac` argument files.
70
71 The order of source files specified on the command line or in an
72 argument file is not important. `javac` will compile the files together,
73 as a group, and will automatically resolve any dependencies between
74 the declarations in the various source files.
75
76 `javac` expects that source files are arranged in one or more directory
77 hierarchies on the file system, described in [Arrangement of Source
78 Code].
79
80 To compile a source file, `javac` needs to find the declaration of
81 every class or interface that is used, extended, or implemented by the
82 code in the source file. This lets `javac` check that the code has the
83 right to access those classes and interfaces. Rather than specifying
84 the source files of those classes and interfaces explicitly, you can
85 use command-line options to tell `javac` where to search for their
86 source files. If you have compiled those source files previously, you
87 can use options to tell `javac` where to search for the corresponding
88 class files. The options, which all have names ending in "path", are
89 described in [Standard Options], and further described in
90 [Configuring a Compilation] and [Searching for Module, Package and Type Declarations].
91
92 By default, `javac` compiles each source file to a class file in the
93 same directory as the source file. However, it is recommended to
94 specify a separate destination directory with the [`-d`](#option-d) option.
95
96 Command-line [options] and [environment variables] also control how
97 `javac` performs various tasks:
98
99 - Compiling code to run on earlier releases of the JDK.
100 - Compiling code to run under a debugger.
101 - Checking for stylistic issues in Java source code.
102 - Checking for problems in `javadoc` comments (`/** ... */`).
103 - Processing annotations in source files and class files.
104 - Upgrading and patching modules in the compile-time environment.
105
106 `javac` supports [Compiling for Earlier Releases Of The Platform]
107 and can also be invoked from Java code using one of a number of [APIs]
108
109 ## Options
110
111 `javac` provides [standard options], and [extra options] that are either
112 non-standard or are for advanced use.
113
114 Some options take one or more arguments.
115 If an argument contains spaces or other whitespace characters,
116 the value should be quoted according to the conventions of the
117 environment being used to invoke javac.
118 If the option begins with a single dash (`-`) the argument should
119 either directly follow the option name, or should be separated with a
120 colon (`:`) or whitespace, depending on the option. If the option begins with
121 a double dash (`--`), the argument may be separated either by whitespace
122 or by an equals (`=`) character with no additional whitespace.
123 For example,
124
125 -Aname="J. Duke"
126 -proc:only
127 -d myDirectory
128 --module-version 3
129 --module-version=3
130
131 In the following lists of options, an argument of *path* represents
132 a search path, composed of a list of file system locations separated
133 by the platform path separator character, (semicolon `;` on Windows,
134 or colon `:` on other systems.) Depending on the option, the
135 file system locations may be directories, JAR files or JMOD files.
136
137 ### Standard Options
138
139 <a id="option-at">`@`*filename*</a>
140 : Reads options and file names from a file. To shorten or simplify the
141 `javac` command, you can specify one or more files that contain arguments
142 to the `javac` command (except [`-J`](#option-J) options). This lets you to create
143 `javac` commands of any length on any operating system.
144 See [Command-Line Argument Files].
145
146 <a id="option-A">`-A`*key*\[`=`*value*\]</a>
147 : Specifies options to pass to annotation processors. These options are not
148 interpreted by `javac` directly, but are made available for use by
149 individual processors. The *key* value should be one or more identifiers
150 separated by a dot (`.`).
151
152 <a id="option-add-modules">`--add-modules` *module*`,`*module*</a>
153 : Specifies root modules to resolve in addition to the initial modules, or
154 all modules on the module path if *module* is `ALL-MODULE-PATH`.
155
156 <a id="option-boot-class-path">`--boot-class-path` *path* or `-bootclasspath` *path*</a>
157 : Overrides the location of the bootstrap class files.
158
159 **Note:** This can only be used when compiling for releases prior to JDK 9.
160 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
161 [`-target`](#option-target) for details. For JDK 9 or later, see [`--system`](#option-system).
162
163 <a id="option-class-path">`--class-path` *path*, `-classpath` *path*, or `-cp` *path*</a>
164 : Specifies where to find user class files and annotation processors. This
165 class path overrides the user class path in the `CLASSPATH` environment
166 variable.
167
168 - If `--class-path`, `-classpath`, or `-cp` are not specified, then the
169 user class path is the value of the `CLASSPATH` environment variable,
170 if that is set, or else the current directory.
171
172 - If not compiling code for modules, if the [`--source-path`](#option-source-path) or -sourcepath`
173 option is not specified, then the user class path is also searched for source files.
174
175 - If the [`-processorpath`](#option-processor-path) option is not specified, then the class path is
176 also searched for annotation processors.
177
178 <a id="option-d">`-d` *directory*</a>
179 : Sets the destination directory (or _class output directory_) for class files.
180 If a class is part of a package, then `javac` puts the class file in a
181 subdirectory that reflects the module name (if appropriate) and package name.
182 The directory, and any necessary subdirectories, will be created if they
183 do not already exist.
184
185 If the `-d` option is not specified, then `javac` puts each class file in
186 the same directory as the source file from which it was generated.
187
188 Except when compiling code for multiple modules, the contents of the
189 class output directory will be organized in a package hierarchy.
190 When compiling code for multiple modules, the contents of the output
191 directory will be organized in a module hierarchy, with the contents of each
192 module in a separate subdirectory, each organized as a package
193 hierarchy.
194
195 **Note:**
196 When compiling code for one or more modules, the class output directory will
197 automatically be checked when searching for previously compiled classes.
198 When not compiling for modules, for backwards compatibility,
199 the directory is _not_ automatically checked for previously compiled classes,
200 and so it is recommended to specify the class output directory as one
201 of the locations on the user class path, using the `--class-path` option or one of
202 its alternate forms.
203
204 <a id="option-deprecation">`-deprecation`</a>
205 : Shows a description of each use or override of a deprecated member or
206 class. Without the `-deprecation` option, `javac` shows a summary of the
207 source files that use or override deprecated members or classes. The
208 `-deprecation` option is shorthand for `-Xlint:deprecation`.
209
210 <a id="option-enable-preview">`--enable-preview`</a>
211 : Enables preview language features. Used in conjunction with either
212 [`-source`](#option-source) or [`--release`](#option-release).
213
214 <a id="option-encoding">`-encoding` *encoding*</a>
215 : Specifies character encoding used by source files, such as EUC-JP and
216 UTF-8. If the `-encoding` option is not specified, then the platform default
217 converter is used.
218
219 <a id="option-endorseddirs">`-endorseddirs` *directories*</a>
220 : Overrides the location of the endorsed standards path.
221
222 **Note:** This can only be used when compiling for releases prior to JDK 9.
223 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
224 [`-target`](#option-target) for details.
225
226 <a id="option-extdirs">`-extdirs` *directories*</a>
227 : Overrides the location of the installed extensions.
228 `directories` is a list of directories, separated by the platform path separator
229 (`;` on Windows, and `:` otherwise).
230 Each JAR file in the specified directories is searched for class files.
231 All JAR files found become part of the class path.
232
233 If you are compiling for a release of the platform that supports the
234 Extension Mechanism, then this option specifies the directories that
235 contain the extension classes.
236 See [Compiling for Other Releases of the Platform].
237
238 **Note:** This can only be used when compiling for releases prior to JDK 9.
239 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source), or
240 [`-target`](#option-target) for details.
241
242 <a id="option-g">`-g`</a>
243 : Generates all debugging information, including local variables. By default,
244 only line number and source file information is generated.
245
246 <a id="option-g-custom">`-g:`\[`lines`, `vars`, `source`\]</a>
247 : Generates only the kinds of debugging information specified by the
248 comma-separated list of keywords. Valid keywords are:
249
250 `lines`
251 : Line number debugging information.
252
253 `vars`
254 : Local variable debugging information.
255
256 `source`
257 : Source file debugging information.
258
259 <a id="option-g-none">`-g:none`</a>
260 : Does not generate debugging information.
261
262 <a id="option-h">`-h` *directory*</a>
263 : Specifies where to place generated native header files.
264
265 When you specify this option, a native header file is generated for each
266 class that contains native methods or that has one or more constants
267 annotated with the [`java.lang.annotation.Native`](
268 ../../api/java.base/java/lang/annotation/Native.html)
269 annotation. If the class is part of a package, then the compiler puts the
270 native header file in a subdirectory that reflects the module name
271 (if appropriate) and package name.
272 The directory, and any necessary subdirectories, will be created if they
273 do not already exist.
274
275 <a id="option-help">`--help`, `-help` or `-?`</a>
276 : Prints a synopsis of the standard options.
277
278 <a id="option-help-extra">`--help-extra` or `-X`</a>
279 : Prints a synopsis of the set of extra options.
280
281 <a id="option-help-lint">`--help-lint`</a>
282 : Prints the supported keys for the `-Xlint` option.
283
284 <a id="option-implicit">`-implicit:`\[`none`, `class`\]</a>
285 : Specifies whether or not to generate class files for implicitly referenced
286 files:
287
288 - `-implicit:class` --- Automatically generates class files.
289
290 - `-implicit:none` --- Suppresses class file generation.
291
292 If this option is not specified, then the default automatically generates
293 class files. In this case, the compiler issues a warning if any class files
294 are generated when also doing annotation processing. The warning is not
295 issued when the `-implicit` option is explicitly set.
296 See [Searching for Module, Package and Type Declarations].
297
298 <a id="option-J">`-J`*option*</a>
299 : Passes *option* to the runtime system, where *option* is one of the Java
300 options described on [java](java.html) command. For example, `-J-Xms48m`
301 sets the startup memory to 48 MB.
302
303 **Note:** The `CLASSPATH` environment variable, `-classpath` option, `-bootclasspath`
304 option, and `-extdirs` option do not specify the classes used to run
305 `javac`. Trying to customize the compiler implementation with these options
306 and variables is risky and often does not accomplish what you want. If you
307 must customize the compiler implementation, then use the `-J` option to
308 pass options through to the underlying Java launcher.
309
310 <a id="option-limit-modules">`--limit-modules` *module*`,`*module*\*</a>
311 : Limits the universe of observable modules.
312
313 <a id="option-module">`--module` *module-name* (`,`*module-name*)* or `-m` *module-name* (`,`*module-name*)*</a>
314 : Compiles those source files in the named modules that are newer
315 than the corresponding files in the output directory.
316
317 <a id="option-module-path">`--module-path` *path* or `-p` *path*</a>
318 : Specifies where to find application modules.
319
320 <a id="option-module-source-path">`--module-source-path` *module-source-path*</a>
321 : Specifies where to find source files when compiling code
322 in multiple modules. See [The Module Source Path Option].
323
324 <a id="option-module-version">`--module-version` *version*</a>
325 : Specifies the version of modules that are being compiled.
326
327 <a id="option-nowarn">`-nowarn`</a>
328 : Generate only mandatory warnings.
329
330 <a id="option-parameters">`-parameters`</a>
331 : Generates metadata for reflection on method parameters. Stores formal
332 parameter names of constructors and methods in the generated class file so
333 that the method `java.lang.reflect.Executable.getParameters` from the
334 Reflection API can retrieve them.
335
336 <a id="option-proc">`-proc:`\[`none`, `only`, `full`\]</a>
337 : Controls whether annotation processing and compilation are done.
338
339 - `-proc:none` means that compilation takes place without annotation
340 processing
341
342 - `-proc:only` means that only annotation processing is done,
343 without any subsequent compilation.
344
345 - `-proc:full` means annotation processing and compilation are done.
346
347 If this option is not used, annotation processing and compilation
348 are done if at least one other option is used to explicitly
349 configure annotation processing.
350
351 <a id="option-processor">`-processor` *class1*\[`,`*class2*`,`*class3*...\]</a>
352 : Names of the annotation processors to run. This bypasses the default
353 discovery process.
354
355 <a id="option-processor-module-path">`--processor-module-path` *path*</a>
356 : Specifies the module path used for finding annotation processors.
357
358 <a id="option-processor-path">`--processor-path` *path* or `-processorpath` *path*</a>
359 : Specifies where to find annotation processors. If this option is not used,
360 then the class path is searched for processors.
361
362 <a id="option-profile">`-profile` *profile*</a>
363 : Checks that the API used is available in the specified profile.
364 This option is deprecated and may be removed in a future release.
365
366 **Note:** This can only be used when compiling for releases prior to JDK 9.
367 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
368 or [`-target`](#option-target) for details.
369
370 <a id="option-release">`--release` *release*</a>
371 : Compiles source code according to the rules of the Java programming language
372 for the specified Java SE release, generating class files which target
373 that release.
374 Source code is compiled against the combined Java SE and JDK API for the
375 specified release.
376
377 The supported values of *release* are the current Java SE release and a
378 limited number of previous releases, detailed in the command-line help.
379
380 For the current release, the Java SE API consists of the `java.*`,
381 `javax.*`, and `org.*` packages that are exported by the Java SE modules in
382 the release; the JDK API consists of the `com.*` and `jdk.*` packages that
383 are exported by the JDK modules in the release, plus the `javax.*` packages
384 that are exported by standard, but non-Java SE, modules in the release.
385
386 For previous releases, the Java SE API and the JDK API are as defined in
387 that release.
388
389 **Note:** When using `--release`, you cannot also use the [`--source`](#option-source)/`-source` or
390 [`--target`](#option-target)/`-target` options.
391
392 **Note:** When using `--release` to specify a release that supports the Java
393 Platform Module System, the `--add-exports` option cannot be used to
394 enlarge the set of packages exported by the Java SE, JDK, and standard
395 modules in the specified release.
396
397 <a id="option-s">`-s` *directory*</a>
398 : Specifies the directory used to place the generated source files. If a
399 class is part of a package, then the compiler puts the source file in a
400 subdirectory that reflects the module name (if appropriate) and package name.
401 The directory, and any necessary subdirectories, will be created if they
402 do not already exist.
403
404 Except when compiling code for multiple modules, the contents of the
405 source output directory will be organized in a package hierarchy.
406 When compiling code for multiple modules, the contents of the source output directory will be
407 organized in a module hierarchy, with the contents of each
408 module in a separate subdirectory, each organized as a package
409 hierarchy.
410
411 <a id="option-source">`--source` *release* or `-source` *release*</a>
412 : Compiles source code according to the rules of the Java programming language
413 for the specified Java SE release.
414 The supported values of *release* are the current Java SE release and a
415 limited number of previous releases, detailed in the command-line help.
416
417 If the option is not specified, the default is to compile source code
418 according to the rules of the Java programming language for the current
419 Java SE release.
420
421 <a id="option-source-path">`--source-path` *path* or `-sourcepath` *path*</a>
422 : Specifies where to find source files.
423 Except when compiling multiple modules together, this is the source code path
424 used to search for class or interface definitions.
425
426 **Note:** Classes found through the class path might be recompiled when their source
427 files are also found. See [Searching for Module, Package and Type Declarations].
428
429 <a id="option-system">`--system` *jdk* \| `none`</a>
430 : Overrides the location of system modules.
431
432 <a id="option-target">`--target` *release* or `-target` *release*</a>
433 : Generates `class` files suitable for the specified Java SE release.
434 The supported values of *release* are the current Java SE release and a
435 limited number of previous releases, detailed in the command-line help.
436
437 **Note:** The target release must be equal to or higher than the source release.
438 (See [`--source`](#option-source).)
439
440 <a id="option-upgrade-module-path">`--upgrade-module-path` *path*</a>
441 : Overrides the location of upgradeable modules.
442
443 <a id="option-verbose">`-verbose`</a>
444 : Outputs messages about what the compiler is doing. Messages include
445 information about each class loaded and each source file compiled.
446
447 <a id="option-version">`--version` or `-version`</a>
448 : Prints version information.
449
450 <a id="option-Werror">`-Werror`</a>
451 : Terminates compilation when warnings occur.
452
453 ### Extra Options
454
455 <a id="option-add-exports">`--add-exports` *module*`/`*package*`=`*other-module*(`,`*other-module*)\*</a>
456 : Specifies a package to be considered as exported from its defining module
457 to additional modules or to all unnamed modules when the value of
458 *other-module* is `ALL-UNNAMED`.
459
460 <a id="option-add-reads">`--add-reads` *module*`=`*other-module*(`,`*other-module*)\*</a>
461 : Specifies additional modules to be considered as required by a given
462 module.
463
464 <a id="option-default-module-for-created-files">`--default-module-for-created-files` *module-name*</a>
465 : Specifies the fallback target module for files created by annotation
466 processors, if none is specified or inferred.
467
468 <a id="option-disable-line-doc-comments">`--disable-line-doc-comments`</a>
469 : Disables support for documentation comments with lines beginning ///.
470
471 <a id="option-Djava.endorsed.dirs">`-Djava.endorsed.dirs=`*dirs*</a>
472 : Overrides the location of the endorsed standards path.
473
474 **Note:** This can only be used when compiling for releases prior to JDK 9.
475 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
476 or [`-target`](#option-target) for details.
477
478 <a id="option-Djava.ext.dirs">`-Djava.ext.dirs=`*dirs*</a>
479 : Overrides the location of installed extensions.
480
481 **Note:** This can only be used when compiling for releases prior to JDK 9.
482 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
483 or [`-target`](#option-target) for details.
484
485 <a id="option-patch-module">`--patch-module` *module*`=`*path*</a>
486 : Overrides or augments a module with classes and resources in JAR files or
487 directories.
488
489 <a id="option-Xbootclasspath">`-Xbootclasspath:`*path*</a>
490 : Overrides the location of the bootstrap class files.
491
492 **Note:** This can only be used when compiling for releases prior to JDK 9.
493 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
494 or [`-target`](#option-target) for details.
495
496 <a id="option-Xbootclasspath-a">`-Xbootclasspath/a:`*path*</a>
497 : Adds a suffix to the bootstrap class path.
498
499 **Note:** This can only be used when compiling for releases prior to JDK 9.
500 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
501 or [`-target`](#option-target) for details.
502
503 <a id="option-Xbootclasspath-p">`-Xbootclasspath/p:`*path*</a>
504 : Adds a prefix to the bootstrap class path.
505
506 **Note:** This can only be used when compiling for releases prior to JDK 9.
507 As applicable, see the descriptions in [`--release`](#option-release), [`-source`](#option-source),
508 or [`-target`](#option-target) for details.
509
510 <a id="option-Xdiags">`-Xdiags:`\[`compact`, `verbose`\]</a>
511 : Selects a diagnostic mode.
512
513 <a id="option-Xdoclint">`-Xdoclint`</a>
514 : Enables recommended checks for problems in documentation comments.
515
516 <a id="option-Xdoclint-custom">`-Xdoclint:`(`all`|`none`|\[`-`\]*group*)\[`/`*access*\]</a>
517 : Enables or disables specific groups of checks in documentation comments.
518
519 *group* can have one of the following values:
520 `accessibility`, `html`, `missing`, `reference`, `syntax`.
521
522 The variable *access* specifies the minimum visibility level of classes and
523 members that the `-Xdoclint` option checks. It can have one of the
524 following values (in order of most to least visible):
525 `public`, `protected`, `package`, `private`.
526
527 The default *access* level is `private`.
528
529 When prefixed by `doclint:`, the *group* names and `all` can be used with `@SuppressWarnings`
530 to suppress warnings about documentation comments in parts of the code being compiled.
531
532 For more information about these groups of checks, see the [DocLint](javadoc.html#doclint)
533 section of the `javadoc` command documentation.
534 The `-Xdoclint` option is disabled by default in the `javac` command.
535
536 For example, the following option checks classes and members (with all
537 groups of checks) that have the access level of protected and higher (which
538 includes protected and public):
539
540 > `-Xdoclint:all/protected`
541
542 The following option enables all groups of checks for all access levels,
543 except it will not check for HTML errors for classes and members that have the
544 access level of package and higher (which includes package, protected and
545 public):
546
547 > `-Xdoclint:all,-html/package`
548
549 <a id="option-Xdoclint-package">`-Xdoclint/package:`\[`-`\]*packages*(`,`\[`-`\]*package*)\*</a>
550 : Enables or disables checks in specific packages. Each *package* is either
551 the qualified name of a package or a package name prefix followed by `.*`,
552 which expands to all sub-packages of the given package. Each *package* can
553 be prefixed with a hyphen (`-`) to disable checks for a specified package
554 or packages.
555
556 For more information, see the [DocLint](javadoc.html#doclint)
557 section of the `javadoc` command documentation.
558
559 <a id="option-Xlint">`-Xlint`</a>
560 : Enables all recommended warnings. In this release, enabling all available
561 warnings is recommended.
562
563 <a id="option-Xlint-custom">`-Xlint:`\[`-`\]*key*(`,`\[`-`\]*key*)\*</a>
564 : Enables and/or disables warning categories using the one or more of the keys described
565 below separated by commas. The keys `all` and `none` enable or disable all categories
566 (respectively); other keys enable the corresponding category, or disable it if preceded
567 by a hyphen (`-`).
568
569 Supported values for *key* are:
570
571 - `all`: Enables all warning categories.
572
573 - `auxiliaryclass`: Warns about an auxiliary class that is hidden in a
574 source file, and is used from other files.
575
576 - `cast`: Warns about the use of unnecessary casts.
577
578 - `classfile`: Warns about the issues related to classfile contents.
579
580 - `dangling-doc-comments`: Warns about extra or misplaced documentation
581 comments near the beginning of a declaration.
582
583 - `deprecation`: Warns about the use of deprecated items.
584
585 - `dep-ann`: Warns about the items marked as deprecated in `javadoc` but
586 without the `@Deprecated` annotation.
587
588 - `divzero`: Warns about the division by the constant integer 0.
589
590 - `empty`: Warns about an empty statement after `if`.
591
592 - `exports`: Warns about the issues regarding module exports.
593
594 - `fallthrough`: Warns about the falling through from one case of a
595 switch statement to the next.
596
597 - `finally`: Warns about `finally` clauses that do not terminate normally.
598
599 - `identity`: Warns about use of a value-based class where an identity
600 class is expected
601
602 - `incubating`: Warns about the use of incubating modules.
603
604 - `initialization`: Warns about code in identity classes that wouldn't be
605 allowed in early construction due to a `this` dependency.
606
607 - `lossy-conversions`: Warns about possible lossy conversions
608 in compound assignment.
609
610 - `missing-explicit-ctor`: Warns about missing explicit constructors in
611 public and protected classes in exported packages.
612
613 - `module`: Warns about the module system-related issues.
614
615 - `opens`: Warns about the issues related to module opens.
616
617 - `options`: Warns about the issues relating to use of command line
618 options.
619
620 - `output-file-clash`: Warns if any output file is overwritten during compilation.
621 This can occur, for example, on case-insensitive filesystems.
622
623 - `overloads`: Warns about the issues related to method overloads.
624
625 - `overrides`: Warns about the issues related to method overrides.
626
627 - `path`: Warns about the invalid path elements on the command line.
628
629 - `preview`: Warns about the use of preview language features.
630
631 - `processing`: Warns about the issues related to annotation processing.
632
633 - `rawtypes`: Warns about the use of raw types.
634
635 - `removal`: Warns about the use of an API that has been marked for
636 removal.
637
638 - `restricted`: Warns about the use of restricted methods.
639
640 - `requires-automatic`: Warns developers about the use of automatic
641 modules in requires clauses.
642
643 - `requires-transitive-automatic`: Warns about automatic modules in
644 requires transitive.
645
646 - `serial`: Warns about the serializable classes that do not provide a
647 serial version ID. Also warns about access to non-public members from a
648 serializable element.
649
650 - `static`: Warns about the accessing a static member using an instance.
651
652 - `strictfp`: Warns about unnecessary use of the `strictfp` modifier.
653
654 - `synchronization`: Warns about synchronization attempts on instances
655 of value-based classes. This key is a deprecated alias for `identity`,
656 which has the same uses and effects. Users are encouraged to use the
657 `identity` category for all future and existing uses of `synchronization`.
658
659 - `text-blocks`: Warns about inconsistent white space characters in text
660 block indentation.
661
662 - `this-escape`: Warns about constructors leaking `this` prior to subclass initialization.
663
664 - `try`: Warns about the issues relating to the use of try blocks (that
665 is, try-with-resources).
666
667 - `unchecked`: Warns about the unchecked operations.
668
669 - `varargs`: Warns about the potentially unsafe `vararg` methods.
670
671 - `none`: Disables all warning categories.
672
673 With the exception of `all` and `none`, the keys can be used with
674 the `@SuppressWarnings` annotation to suppress warnings in a part
675 of the source code being compiled.
676
677 See [Examples of Using -Xlint keys].
678
679 <a id="option-Xmaxerrs">`-Xmaxerrs` *number*</a>
680 : Sets the maximum number of errors to print.
681
682 <a id="option-Xmaxwarns">`-Xmaxwarns` *number*</a>
683 : Sets the maximum number of warnings to print.
684
685 <a id="option-Xpkginfo">`-Xpkginfo:`\[`always`, `legacy`, `nonempty`\]</a>
686 : Specifies when and how the `javac` command generates `package-info.class`
687 files from `package-info.java` files using one of the following options:
688
689 `always`
690 : Generates a `package-info.class` file for every `package-info.java`
691 file. This option may be useful if you use a build system such as Ant,
692 which checks that each `.java` file has a corresponding `.class` file.
693
694 `legacy`
695 : Generates a `package-info.class` file only if `package-info.java`
696 contains annotations. This option does not generate a
697 `package-info.class` file if `package-info.java` contains only
698 comments.
699
700 **Note:** A `package-info.class` file might be generated but be empty if all the
701 annotations in the `package-info.java` file have
702 `RetentionPolicy.SOURCE`.
703
704 `nonempty`
705 : Generates a `package-info.class` file only if `package-info.java`
706 contains annotations with `RetentionPolicy.CLASS` or
707 `RetentionPolicy.RUNTIME`.
708
709 <a id="option-Xplugin">`-Xplugin:`*name* *args*</a>
710 : Specifies the name and optional arguments for a plug-in to be run.
711 If *args* are provided, *name* and *args* should be quoted or otherwise
712 escape the whitespace characters between the name and all the arguments.
713 For details on the API for a plugin, see the API documentation for
714 [jdk.compiler/com.sun.source.util.Plugin](../../api/jdk.compiler/com/sun/source/util/Plugin.html).
715
716 <a id="option-Xprefer">`-Xprefer:`\[`source`, `newer`\]</a>
717 : Specifies which file to read when both a source file and class file are
718 found for an implicitly compiled class using one of the following options.
719 See [Searching for Module, Package and Type Declarations].
720
721 - `-Xprefer:newer`: Reads the newer of the source or class files for a
722 type (default).
723
724 - `-Xprefer:source` : Reads the source file. Use `-Xprefer:source` when
725 you want to be sure that any annotation processors can access
726 annotations declared with a retention policy of `SOURCE`.
727
728 <a id="option-Xprint">`-Xprint`</a>
729 : Prints a textual representation of specified types for debugging purposes.
730 This does not perform annotation processing or compilation. The format of
731 the output could change.
732
733 <a id="option-XprintProcessorInfo">`-XprintProcessorInfo`</a>
734 : Prints information about which annotations a processor is asked to process.
735
736 <a id="option-XprintRounds">`-XprintRounds`</a>
737 : Prints information about initial and subsequent annotation processing
738 rounds.
739
740 <a id="option-Xstdout">`-Xstdout` *filename*</a>
741 : Sends compiler messages to the named file. By default, compiler messages go
742 to `System.err`.
743
744 ## Environment Variables
745
746 ### CLASSPATH
747
748 If the [`--class-path`](#option-class-path) option or any of its alternate forms are not specified,
749 the class path will default to the value of the `CLASSPATH` environment
750 variable if it is set.
751 However, it is recommended that this environment variable should _not_ be set,
752 and that the `--class-path` option should be used to provide an explicit
753 value for the class path when one is required.
754
755 ### JDK\_JAVAC\_OPTIONS
756
757 The content of the `JDK_JAVAC_OPTIONS` environment variable, separated by
758 white-spaces ( ) or white-space characters (`\n`, `\t`, `\r`, or `\f`) is
759 prepended to the command line arguments passed to `javac` as a list of
760 arguments.
761
762 The encoding requirement for the environment variable is the same as the
763 `javac` command line on the system. `JDK_JAVAC_OPTIONS` environment variable
764 content is treated in the same manner as that specified in the command line.
765
766 Single quotes (`'`) or double quotes (`"`) can be used to enclose arguments
767 that contain whitespace characters. All content between the open quote and the
768 first matching close quote are preserved by simply removing the pair of quotes.
769 In case a matching quote is not found, the launcher will abort with an error
770 message. `@`*files* are supported as they are specified in the command line.
771 However, as in `@`*files*, use of a wildcard is not supported.
772
773 **Examples of quoting arguments containing white spaces:**
774
775 > `export JDK_JAVAC_OPTIONS='@"C:\white spaces\argfile"'`
776
777 > `export JDK_JAVAC_OPTIONS='"@C:\white spaces\argfile"'`
778
779 > `export JDK_JAVAC_OPTIONS='@C:\"white spaces"\argfile'`
780
781 ## Command-Line Argument Files
782
783 An argument file can include command-line options and source file names in any
784 combination. The arguments within a file can be separated by spaces or new line
785 characters. If a file name contains embedded spaces, then put the whole file
786 name in double quotation marks.
787
788 File names within an argument file are relative to the current directory, not
789 to the location of the argument file. Wildcards (`*`) are not allowed in these
790 lists (such as for specifying `*.java`). Use of the at sign (`@`) to
791 recursively interpret files is not supported. The `-J` options are not supported
792 because they're passed to the launcher, which does not support argument files.
793
794 When executing the `javac` command, pass in the path and name of each argument
795 file with the at sign (`@`) leading character. When the `javac` command
796 encounters an argument beginning with the at sign (`@`), it expands the
797 contents of that file into the argument list.
798
799
800 ### Examples of Using javac @filename
801
802 Single Argument File
803 : You could use a single argument file named `argfile` to hold all `javac`
804 arguments:
805
806 > `javac @argfile`
807
808 This argument file could contain the contents of both files shown in the
809 following **Two Argument Files** example.
810
811 Two Argument Files
812 : You can create two argument files: one for the `javac` options and the
813 other for the source file names. Note that the following lists have no
814 line-continuation characters.
815
816 Create a file named `options` that contains the following:
817
818 **Linux and macOS:**
819
820 ```
821 -d classes
822 -g
823 -sourcepath /java/pubs/ws/1.3/src/share/classes
824 ```
825
826 **Windows:**
827
828 ```
829 -d classes
830 -g
831 -sourcepath C:\java\pubs\ws\1.3\src\share\classes
832 ```
833
834 Create a file named `sources` that contains the following:
835
836 ```
837 MyClass1.java
838 MyClass2.java
839 MyClass3.java
840 ```
841
842 Then, run the `javac` command as follows:
843
844 > `javac @options @sources`
845
846 Argument Files with Paths
847 : The argument files can have paths, but any file names inside the files are
848 relative to the current working directory (not `path1` or `path2`):
849
850 > `javac @path1/options @path2/sources`
851
852 ## Arrangement of Source Code
853
854 In the Java language, classes and interfaces can be organized into
855 packages, and packages can be organized into modules. `javac` expects
856 that the physical arrangement of source files in directories of the
857 file system will mirror the organization of classes into packages, and
858 packages into modules.
859
860 It is a widely adopted convention that module names and package names
861 begin with a lower-case letter, and that class names begin with an
862 upper-case letter.
863
864 ### Arrangement of Source Code for a Package
865
866 When classes and interfaces are organized into a package, the package
867 is represented as a directory, and any subpackages are represented as
868 subdirectories.
869
870 For example:
871
872 - The package `p` is represented as a directory called `p`.
873
874 - The package `p.q` -- that is, the subpackage `q` of package `p` --
875 is represented as the subdirectory `q` of directory `p`. The
876 directory tree representing package `p.q` is therefore `p\q`
877 on Windows, and `p/q` on other systems.
878
879 - The package `p.q.r` is represented as the directory tree `p\q\r`
880 (on Windows) or `p/q/r` (on other systems).
881
882 Within a directory or subdirectory, `.java` files represent classes
883 and interfaces in the corresponding package or subpackage.
884
885 For example:
886
887 - The class `X` declared in package `p` is represented by the file
888 `X.java` in the `p` directory.
889
890 - The class `Y` declared in package `p.q` is represented by the file
891 `Y.java` in the `q` subdirectory of directory `p`.
892
893 - The class `Z` declared in package `p.q.r` is represented by the file
894 `Z.java` in the `r` subdirectory of `p\q` (on Windows) or `p/q`
895 (on other systems).
896
897 In some situations, it is convenient to split the code into
898 separate directories, each structured as described above, and
899 the aggregate list of directories specified to `javac`.
900
901 ### Arrangement of Source Code for a Module
902
903 In the Java language, a module is a set of packages designed for
904 reuse. In addition to `.java` files for classes and interfaces, each
905 module has a source file called `module-info.java` which:
906
907 1. declares the module's name;
908
909 2. lists the packages exported by the module (to allow reuse by other
910 modules);
911
912 3. lists other modules required by the module (to reuse their exported
913 packages).
914
915 When packages are organized into a module, the module is represented
916 by one or more directories representing the packages in the module,
917 one of which contains the `module-info.java` file. It may be convenient,
918 but it is not required, to use a single directory, named after the module,
919 to contain the `module-info.java` file alongside the directory tree which
920 represents the packages in the module (i.e., the _package hierarchy_
921 described above). The exact arrangement of source code for a module
922 is typically dictated by the conventions adopted by a development
923 environment (IDE) or build system.
924
925 For example:
926
927 - The module `a.b.c` may be represented by the directory `a.b.c`, on all
928 systems.
929
930 - The module's declaration is represented by the file
931 `module-info.java` in the `a.b.c` directory.
932
933 - If the module contains package `p.q.r`, then the `a.b.c` directory
934 contains the directory tree `p\q\r` (on Windows) or `p/q/r`
935 (on other systems).
936
937 The development environment may prescribe some directory hierarchy
938 between the directory named for the module and the source files to
939 be read by `javac`.
940
941 For example:
942
943 - The module `a.b.c` may be represented by the directory `a.b.c`
944
945 - The module's declaration and the module's packages may be in
946 some subdirectory of `a.b.c`, such as `src\main\java` (on Windows)
947 or `src/main/java` (on other systems).
948
949
950 ## Configuring a Compilation
951
952 This section describes how to configure `javac` to perform a basic compilation.
953
954 See [Configuring the Module System] for additional details for use when compiling
955 for a release of the platform that supports modules.
956
957 ### Source Files
958
959 * Specify the source files to be compiled on the command line.
960
961 If there are no compilation errors, the corresponding class files will
962 be placed in the [output directory].
963
964 Some systems may limit the amount you can put on a command line;
965 to work around those limits, you can use [argument files](#command-line-argument-files).
966
967 When compiling code for modules, you can also specify source files indirectly,
968 by using the [`--module`](#option-module) or `-m` option.
969
970 ### Output Directory
971
972 * Use the [`-d`](#option-d) option to specify an output directory in which to put the compiled class files.
973
974 This will normally be organized in a [package hierarchy](#package-hierarchy),
975 unless you are compiling source code from multiple modules, in which case it will be
976 organized as a [module hierarchy](#module-hierarchy).
977
978 When the compilation has been completed, if you are compiling one or more modules,
979 you can place the output directory on the module path for the Java [launcher](java.html);
980 otherwise, you can place the place the output directory on the class path
981 for the Java launcher.
982
983 ### Precompiled Code
984
985 The code to be compiled may refer to libraries beyond what is provided by the platform.
986 If so, you must place these libraries on the class path or module path.
987 If the library code is not in a module, place it on the class path;
988 if it is in a module, place it on the module path.
989
990 * Use the [`--class-path`](#option-class-path) option to specify libraries to be placed on the class path.
991 Locations on the class path should be organized in a [package hierarchy](#package-hierarchy).
992 You can also use alternate forms of the option: `-classpath` or `-cp`.
993
994 * Use the [`--module-path`](#option-module-path) option to specify libraries to be placed on the module path.
995 Locations on the module path should either be modules or directories of modules.
996 You can also use an alternate form of the option: `-p`.
997
998 See [Configuring the Module System] for details on how to modify the default
999 configuration of library modules.
1000
1001 **Note**: the options for the class path and module path are not mutually
1002 exclusive, although it is not common to specify the class path when compiling
1003 code for one or more modules.
1004
1005 ### Additional Source Files
1006
1007 The code to be compiled may refer to types in additional source files that are not
1008 specified on the command line.
1009 If so, you must put those source files on either the source path or module path.
1010 You can only specify one of these options: if you are not compiling code for a module,
1011 or if you are only compiling code for a single module, use the source path;
1012 if you are compiling code for multiple modules, use the module source path.
1013
1014 * Use the [`--source-path`](#option-source-path) option to specify the locations of additional source
1015 files that may be read by javac.
1016 Locations on the source path should be organized in a [package hierarchy](#package-hierarchy).
1017 You can also use an alternate form of the option: `-sourcepath`.
1018
1019 * Use the [`--module-source-path`](#option-module-source-path) option one or more times to specify the location
1020 of additional source files in different modules that may be read by javac,
1021 or when compiling source files in multiple modules.
1022 You can either specify the locations for each module [individually](#module-specific-form),
1023 or you can organize the source files so that you can specify the locations all
1024 [together](#module-pattern-form). For more details, see [The Module Source Path Option].
1025
1026 If you want to be able to refer to types in additional source files but do not
1027 want them to be compiled, use the [`-implicit`](#option-implicit) option.
1028
1029 **Note**: if you are compiling code for multiple modules, you must always specify
1030 a module source path, and all source files specified on the command line must be
1031 in one of the directories on the module source path, or in a subdirectory thereof.
1032
1033
1034 ### Example of Compiling Multiple Source Files
1035
1036 This example compiles the `Aloha.java`, `GutenTag.java`, `Hello.java`, and
1037 `Hi.java` source files in the `greetings` package.
1038
1039 **Linux and macOS:**
1040
1041 ```
1042 % javac greetings/*.java
1043 % ls greetings
1044 Aloha.class GutenTag.class Hello.class Hi.class
1045 Aloha.java GutenTag.java Hello.java Hi.java
1046 ```
1047
1048 **Windows:**
1049
1050 ```
1051 C:\>javac greetings\*.java
1052 C:\>dir greetings
1053 Aloha.class GutenTag.class Hello.class Hi.class
1054 Aloha.java GutenTag.java Hello.java Hi.java
1055 ```
1056
1057 ### Example of Specifying a User Class Path
1058
1059 After changing one of the source files in the previous example, recompile it:
1060
1061 **Linux and macOS:**
1062
1063 ```
1064 pwd
1065 /examples
1066 javac greetings/Hi.java
1067 ```
1068
1069 **Windows:**
1070
1071 ```
1072 C:\>cd
1073 \examples
1074 C:\>javac greetings\Hi.java
1075 ```
1076
1077 Because `greetings.Hi` refers to other classes in the `greetings` package, the
1078 compiler needs to find these other classes. The previous example works because
1079 the default user class path is the directory that contains the package
1080 directory. If you want to recompile this file without concern for which
1081 directory you are in, then add the examples directory to the user class path by
1082 setting `CLASSPATH`. This example uses the `-classpath` option.
1083
1084 **Linux and macOS:**
1085
1086 > `javac -classpath /examples /examples/greetings/Hi.java`
1087
1088 **Windows:**
1089
1090 > `C:\>javac -classpath \examples \examples\greetings\Hi.java`
1091
1092 If you change `greetings.Hi` to use a banner utility, then that utility also
1093 needs to be accessible through the user class path.
1094
1095 **Linux and macOS:**
1096
1097 ```
1098 javac -classpath /examples:/lib/Banners.jar \
1099 /examples/greetings/Hi.java
1100 ```
1101
1102 **Windows:**
1103
1104 ```
1105 C:\>javac -classpath \examples;\lib\Banners.jar ^
1106 \examples\greetings\Hi.java
1107 ```
1108
1109 To execute a class in the `greetings` package, the program needs access to the
1110 `greetings` package, and to the classes that the `greetings` classes use.
1111
1112 **Linux and macOS:**
1113
1114 > `java -classpath /examples:/lib/Banners.jar greetings.Hi`
1115
1116 **Windows:**
1117
1118 > `C:\>java -classpath \examples;\lib\Banners.jar greetings.Hi`
1119
1120 ## Configuring the Module System
1121
1122 If you want to include additional modules in your compilation, use the
1123 [`--add-modules`](#option-add-modules) option.
1124 This may be necessary when you are compiling code that is not in a module,
1125 or which is in an automatic module, and the code refers to API in the additional
1126 modules.
1127
1128 If you want to restrict the set of modules in your compilation, use the
1129 [`--limit-modules`](#option-limit-modules) option.
1130 This may be useful if you want to ensure that the code you are compiling
1131 is capable of running on a system with a limited set of modules installed.
1132
1133 If you want to break encapsulation and specify that additional packages
1134 should be considered as exported from a module, use the [`--add-exports`](#option-add-exports) option.
1135 This may be useful when performing white-box testing; relying on access
1136 to internal API in production code is strongly discouraged.
1137
1138 If you want to specify that additional packages
1139 should be considered as required by a module, use the [`--add-reads`](#option-add-reads) option.
1140 This may be useful when performing white-box testing; relying on access
1141 to internal API in production code is strongly discouraged.
1142
1143 You can patch additional content into any module using the
1144 [`--patch-module`](#option-patch-module) option. See [Patching a Module] for more details.
1145
1146 ## Searching for Module, Package and Type Declarations
1147
1148 To compile a source file, the compiler often needs information about a module
1149 or type, but the declaration is not in the source files specified on the command
1150 line.
1151
1152 `javac` needs type information for every class or interface used,
1153 extended, or implemented in the source file. This includes classes and
1154 interfaces not explicitly mentioned in the source file, but that provide
1155 information through inheritance.
1156
1157 For example, when you create a subclass of `java.awt.Window`, you are also
1158 using the ancestor classes of `Window`: `java.awt.Container`,
1159 `java.awt.Component`, and `java.lang.Object`.
1160
1161 When compiling code for a module, the compiler also needs to have available
1162 the declaration of that module.
1163
1164 A successful search may produce a class file, a source file, or both. If
1165 both are found, then you can use the [`-Xprefer`](#option-Xprefer) option to instruct the compiler
1166 which to use.
1167
1168 If a search finds and uses a source file, then by default `javac`
1169 compiles that source file. This behavior can be altered with
1170 [`-implicit`](#option-implicit).
1171
1172 The compiler might not discover the need for some type information until after
1173 annotation processing completes. When the type information is found in a source
1174 file and no [`-implicit`](#option-implicit) option is specified, the compiler gives a warning that
1175 the file is being compiled without being subject to annotation processing. To
1176 disable the warning, either specify the file on the command line (so that it
1177 will be subject to annotation processing) or use the [`-implicit`](#option-implicit) option to
1178 specify whether or not class files should be generated for such source files.
1179
1180 The way that `javac` locates the declarations of those types
1181 depends on whether the reference exists within code for a module or not.
1182
1183 ### Searching Package Oriented Paths
1184
1185 When searching for a source or class file on a path composed of package oriented
1186 locations, `javac` will check each location on the path in turn for the
1187 possible presence of the file. The first occurrence of a particular file
1188 shadows (hides) any subsequent occurrences of like-named files. This shadowing
1189 does not affect any search for any files with a different name. This can be
1190 convenient when searching for source files, which may be grouped in different
1191 locations, such as shared code, platform-specific code and generated code.
1192 It can also be useful when injecting alternate versions of a class file into
1193 a package, to debugging or other instrumentation reasons. But, it can also
1194 be dangerous, such as when putting incompatible different versions of a library
1195 on the class path.
1196
1197 ### Searching Module Oriented Paths
1198
1199 Prior to scanning any module paths for any package or type declarations,
1200 `javac` will lazily scan the following paths and locations to determine
1201 the modules that will be used in the compilation.
1202
1203 * The module source path (see the [`--module-source-path`](#option-module-source-path) option)
1204 * The path for upgradeable modules (see the [`--upgrade-module-path`](#option-upgrade-module-path) option)
1205 * The system modules (see the [`--system`](#option-system) option)
1206 * The user module path ( see the [`--module-path`](#option-module-path) option)
1207
1208 For any module, the first occurrence of the module during the scan completely
1209 shadows (hides) any subsequent appearance of a like-named module. While locating
1210 the modules, `javac` is able to determine the packages exported by the module
1211 and to associate with each module a package oriented path for the contents of
1212 the module. For any previously compiled module, this path will typically be a
1213 single entry for either a directory or a file that provides an internal
1214 directory-like hierarchy, such as a JAR file. Thus, when searching for a type
1215 that is in a package that is known to be exported by a module, `javac` can
1216 locate the declaration directly and efficiently.
1217
1218 ### Searching for the Declaration of a Module
1219
1220 If the module has been previously compiled, the module declaration is
1221 located in a file named `module-info.class` in the root of the package hierarchy
1222 for the content of the module.
1223
1224 If the module is one of those currently being compiled, the module declaration
1225 will be either the file named `module-info.class` in the root of the
1226 package hierarchy for the module in the class output directory, or the
1227 file named `module-info.java` in one of the locations on the source path
1228 or one the module source path for the module.
1229
1230 ### Searching for the Declaration of a Type When the Reference is not in a Module
1231
1232 When searching for a type that is referenced in code that is not in a module,
1233 `javac` will look in the following places:
1234
1235 * The platform classes (or the types in exported packages of the platform modules)
1236 (This is for compiled class files only.)
1237
1238 * Types in exported packages of any modules on the module path, if applicable.
1239 (This is for compiled class files only.)
1240
1241 * Types in packages on the class path and/or source path:
1242
1243 * If both are specified, `javac` looks for compiled class files on the class path
1244 and for source files on the source path.
1245
1246 * If the class path is specified, but not source path, `javac` looks for both
1247 compiled class files and source files on the class path.
1248
1249 * If the class path is not specified, it defaults to the current directory.
1250
1251 When looking for a type on the class path and/or source path, if both a compiled
1252 class file and a source file are found, the most recently modified file will
1253 be used by default.
1254 If the source file is newer, it will be compiled and will may override any
1255 previously compiled version of the file. You can use the [`-Xprefer`](#option-Xprefer) option
1256 to override the default behavior.
1257
1258 ### Searching for the Declaration of a Type When the Reference is in a Module
1259
1260 When searching for a type that is referenced in code in a module,
1261 `javac` will examine the declaration of the enclosing module to determine
1262 if the type is in a package that is exported from another module that is
1263 readable by the enclosing module.
1264 If so, `javac` will simply and directly go to the definition of that module
1265 to find the definition of the required type.
1266 Unless the module is another of the modules being compiled, `javac` will
1267 only look for compiled class files files. In other words, `javac` will
1268 not look for source files in platform modules or modules on the module path.
1269
1270 If the type being referenced is not in some other readable module,
1271 `javac` will examine the module being compiled to try and find the
1272 declaration of the type.
1273 `javac` will look for the declaration of the type as follows:
1274
1275 * Source files specified on the command line or on the source path or
1276 module source path.
1277
1278 * Previously compiled files in the output directory.
1279
1280
1281 ## Directory Hierarchies
1282
1283 `javac` generally assumes that source files and compiled class files will be
1284 organized in a file system directory hierarchy or in a type of file that
1285 supports in an internal directory hierarchy, such as a JAR file.
1286 Three different kinds of hierarchy are supported: a _package hierarchy_,
1287 a _module hierarchy_, and a _module source hierarchy_.
1288
1289 While `javac` is fairly relaxed about the organization of source code,
1290 beyond the expectation that source will be organized in one or package
1291 hierarchies, and can generally accommodate organizations prescribed by
1292 development environments and build tools, Java tools in general,
1293 and `javac` and the Java launcher in particular, are more stringent
1294 regarding the organization of compiled class files, and will be
1295 organized in package hierarchies or module hierarchies, as appropriate.
1296
1297 The location of these hierarchies are specified to `javac` with command-line
1298 options, whose names typically end in "path", like [`--source-path`](#option-source-path) or
1299 [`--class-path`](#option-class-path). Also as a general rule, path options whose name includes the
1300 word `module`, like [`--module-path`](#option-module-path), are used to specify module hierarchies,
1301 although some module-related path options allow a package hierarchy to be
1302 specified on a per-module basis. All other path options are used to specify
1303 package hierarchies.
1304
1305 ### Package Hierarchy
1306
1307 In a package hierarchy, directories and subdirectories are used
1308 to represent the component parts of the package name, with the source
1309 file or compiled class file for a type being stored as a file with an
1310 extension of `.java` or `.class` in the most nested directory.
1311
1312 For example, in a package hierarchy, the source file for a class
1313 `com.example.MyClass` will be stored in the file _com/example/MyClass.java_
1314
1315 ### Module Hierarchy
1316
1317 In a module hierarchy, the first level of directories are named
1318 for the modules in the hierarchy; within each of those directories
1319 the contents of the module are organized in package hierarchies.
1320
1321 For example, in a module hierarchy, the compiled class file for a type called
1322 `com.example.MyClass` in a module called `my.library` will be stored in
1323 _my.library/com/example/MyClass.class_.
1324
1325 The various output directories used by `javac` (the class output directory,
1326 the source output directory, and native header output directory)
1327 will all be organized in a module hierarchy when multiple modules are being compiled.
1328
1329 ### Module Source Hierarchy
1330
1331 Although the source for each individual module should always be
1332 organized in a package hierarchy, it may be convenient to group
1333 those hierarchies into a module source hierarchy. This is similar
1334 to a module hierarchy, except that there may be intervening directories
1335 between the directory for the module and the directory that is
1336 the root of the package hierarchy for the source code of the module.
1337
1338 For example, in a module source hierarchy, the source file for a type called
1339 `com.example.MyClass` in a module called `my.library` may be stored in a
1340 file such as
1341 _my.library/src/main/java/com/example/MyClass.java_.
1342
1343 ## The Module Source Path Option
1344
1345 The [`--module-source-path`](#option-module-source-path) option has two forms: a _module-specific form_,
1346 in which a package path is given for each module containing code to be compiled,
1347 and a _module-pattern_ form, in which the source path for each module is specified
1348 by a pattern.
1349 The module-specific form is generally simpler to use when only a small number of
1350 modules are involved; the module-pattern form may be more convenient when the
1351 number of modules is large and the modules are organized in a regular manner that
1352 can be described by a pattern.
1353
1354 Multiple instances of the `--module-source-path` option may be given, each one
1355 using either the module-pattern form or the module-specific form, subject to the
1356 following limitations:
1357
1358 * the module-pattern form may be used at most once
1359 * the module-specific form may be used at most once for any given module
1360
1361 If the module-specific form is used for any module, the associated search path
1362 overrides any path that might otherwise have been inferred from the module-pattern form.
1363
1364 ### Module-specific form
1365
1366 The module-specific form allows an explicit search path to be given for any specific module.
1367 This form is:
1368
1369 * `--module-source-path` *module-name*`=`*file-path* (*path-separator* *file-path*)*
1370
1371 The path separator character is `;` on Windows, and `:` otherwise.
1372
1373 **Note:** this is similar to the form used for the [`--patch-module`](#option-patch-module) option.
1374
1375 ### Module-pattern form
1376
1377 The module-pattern form allows a concise specification of the module source path
1378 for any number of modules organized in regular manner.
1379
1380 * `--module-source-path` *pattern*
1381
1382 The pattern is defined by the following rules, which are applied in order:
1383
1384 * The argument is considered to be a series of segments separated by the path
1385 separator character (`;` on Windows, and `:` otherwise).
1386
1387 * Each segment containing curly braces of the form
1388
1389 string1{alt1 ( ,alt2 )* } string2
1390
1391 is considered to be replaced by a series of segments formed by "expanding" the braces:
1392
1393 string1 alt1 string2
1394 string1 alt2 string2
1395 and so on...
1396
1397 The braces may be nested.
1398
1399 This rule is applied for all such usages of braces.
1400
1401 * Each segment must have at most one asterisk (`*`).
1402 If a segment does not contain an asterisk, it is considered to be as though the
1403 file separator character and an asterisk are appended.
1404
1405 For any module _M_, the source path for that module is formed from the series
1406 of segments obtained by substituting the module name _M_ for the asterisk in
1407 each segment.
1408
1409 **Note**: in this context, the asterisk is just used as a special marker, to
1410 denote the position in the path of the module name. It should not be confused
1411 with the use of `*` as a file name wildcard character, as found on most
1412 operating systems.
1413
1414 ## Patching Modules
1415
1416 javac allows any content, whether in source or compiled form, to be patched
1417 into any module using the [`--patch-module`](#option-patch-module) option.
1418 You may want to do this to compile alternative implementations of a class
1419 to be patched at runtime into a JVM, or to inject additional classes into
1420 the module, such as when testing.
1421
1422 The form of the option is:
1423
1424 * `--patch-module` *module-name*`=`*file-path* (*path-separator* *file-path* )*
1425
1426 The path separator character is `;` on Windows, and `:` otherwise.
1427 The paths given for the module must specify the root of a
1428 package hierarchy for the contents of the module
1429
1430 The option may be given at most once for any given module.
1431 Any content on the path will hide any like-named content later in the path
1432 and in the patched module.
1433
1434 When patching source code into more than one module, the [`--module-source-path`](#option-module-source-path)
1435 must also be used, so that the output directory is organized in a module hierarchy,
1436 and capable of holding the compiled class files for the modules being compiled.
1437
1438 ## Annotation Processing
1439
1440 The `javac` command provides direct support for annotation processing.
1441
1442 The API for annotation processors is defined in the
1443 `javax.annotation.processing` and `javax.lang.model` packages and subpackages.
1444
1445 ### How Annotation Processing Works
1446
1447 Annotation processing is requested by using an option to configure
1448 annotation processing, such as [`-processor`](#option-processor),
1449 [`--processor-path`](#option-processor-path),
1450 [`--processor-module-path`](#option-processor-module-path) or by
1451 explicitly enabling processing with the [`-proc:full`](#option-proc)
1452 or [`-proc:only`](#option-proc) options. Annotation processing is
1453 disabled using the [`-proc:none`](#option-proc) option.
1454
1455 If annotation processing is requested, the compiler searches for any
1456 annotation processors that are available.
1457
1458 The search path can be specified with the
1459 [`-processorpath`](#option-processor-path) option. If no path is
1460 specified, then the user class path is used. Processors are located by
1461 means of service provider-configuration files named
1462 `META-INF/services/javax.annotation.processing.Processor` on the
1463 search path. Such files should contain the names of any
1464 annotationation processors to be used, listed one per
1465 line. Alternatively, processors can be specified explicitly, using the
1466 [`-processor`](#option-processor) option.
1467
1468 After scanning the source files and classes on the command line to determine
1469 what annotations are present, the compiler queries the processors to determine
1470 what annotations they process. When a match is found, the processor is called.
1471 A processor can claim the annotations it processes, in which case no further
1472 attempt is made to find any processors for those annotations. After all of the
1473 annotations are claimed, the compiler does not search for additional
1474 processors.
1475
1476 If any processors generate new source files, then another round of annotation
1477 processing occurs: Any newly generated source files are scanned, and the
1478 annotations processed as before. Any processors called on previous rounds are
1479 also called on all subsequent rounds. This continues until no new source files
1480 are generated.
1481
1482 After a round occurs where no new source files are generated, the annotation
1483 processors are called one last time, to give them a chance to complete any
1484 remaining work. Finally, unless the [`-proc:only`](#option-proc) option is used, the compiler
1485 compiles the original and all generated source files.
1486
1487 If you use an annotation processor that generates additional source
1488 files to be included in the compilation, you can specify a default
1489 module to be used for the newly generated files, for use when a
1490 module declaration is not also generated. In this case, use the
1491 [`--default-module-for-created-files`](#option-default-module-for-created-files) option.
1492
1493 ### Compilation Environment and Runtime Environment.
1494
1495 The declarations in source files and previously compiled class files are analyzed
1496 by `javac` in a _compilation environment_ that is distinct from the
1497 _runtime environment_ used to execute `javac` itself. Although there is a
1498 deliberate similarity between many `javac` options and like-named options for the
1499 Java [launcher](java.html), such as `--class-path`, `--module-path` and so
1500 on, it is important to understand that in general the `javac` options just affect
1501 the environment in which the source files are compiled, and do not affect
1502 the operation of `javac` itself.
1503
1504 The distinction between the compilation environment and runtime environment
1505 is significant when it comes to using annotation processors.
1506 Although annotations processors process elements (declarations) that exist
1507 in the compilation environment, the annotation processor itself is executed
1508 in the runtime environment. If an annotation processor has dependencies on
1509 libraries that are not in modules, the libraries can be placed, along with the
1510 annotation processor itself, on the processor path.
1511 (See the [`--processor-path`](#option-processor-path) option.)
1512 If the annotation processor and its dependencies are in modules, you should
1513 use the processor module path instead.
1514 (See the [`--processor-module-path`](#option-processor-module-path) option.)
1515 When those are insufficient, it may be necessary to provide further
1516 configuration of the runtime environment. This can be done in two ways:
1517
1518 1. If `javac` is invoked from the command line, options can be passed to the
1519 underlying runtime by prefixing the option with [`-J`](#option-J).
1520
1521 2. You can start an instance of a Java Virtual Machine directly and use
1522 command line options and API to configure an environment in which
1523 `javac` can be invoked via one of its [APIs].
1524
1525 ## Compiling for Earlier Releases of the Platform
1526
1527 `javac` can compile code that is to be used on other releases of the platform,
1528 using either the [`--release`](#option-release) option, or the [`--source`](#option-source)/`-source` and
1529 [`--target`](#option-target)/`-target` options, together with additional options to specify the
1530 platform classes.
1531
1532 Depending on the desired platform release, there are some restrictions on some
1533 of the options that can be used.
1534
1535 * When compiling for JDK 8 and earlier releases, you cannot use any option
1536 that is intended for use with the module system.
1537 This includes all of the following options:
1538
1539 * [`--module-source-path`](#option-module-source-path),
1540 [`--upgrade-module-path`](#option-upgrade-module-path),
1541 [`--system`](#option-system),
1542 [`--module-path`](#option-module-path),
1543 [`--add-modules`](#option-add-modules),
1544 [`--add-exports`](#option-add-exports),
1545 `--add-opens`,
1546 [`--add-reads`](#option-add-reads),
1547 [`--limit-modules`](#option-limit-modules),
1548 [`--patch-module`](#option-patch-module)
1549
1550 If you use the `--source`/`-source` or `--target`/`-target` options,
1551 you should also set the appropriate platform classes using the
1552 boot class path family of options.
1553
1554 * When compiling for JDK 9 and later releases, you cannot use any option
1555 that is intended to configure the boot class path.
1556 This includes all of the following options:
1557
1558 * [`-Xbootclasspath/p:`](#option-Xbootclasspath-p),
1559 [`-Xbootclasspath`](#option-Xbootclasspath),
1560 [`-Xbootclasspath/a:`](#option-Xbootclasspath-a),
1561 [`-endorseddirs`](#option-endorseddirs),
1562 [`-Djava.endorsed.dirs`](#option-Djava.endorsed.dirs),
1563 [`-extdirs`](#option-extdirs),
1564 [`-Djava.ext.dirs`](#option-Djava.ext.dirs),
1565 [`-profile`](#option-profile)
1566
1567 If you use the `--source`/`-source` or `--target`/`-target` options,
1568 you should also set the appropriate platform classes using the `--system`
1569 option to give the location of an appropriate installed release of JDK.
1570
1571 When using the `--release` option, only the supported documented API for that
1572 release may be used; you cannot use any options to break encapsulation to
1573 access any internal classes.
1574
1575 ## APIs
1576
1577 The `javac` compiler can be invoked using an API in three different ways:
1578
1579 The [Java Compiler API](../../api/java.compiler/javax/tools/JavaCompiler.html)
1580 : This provides the most flexible way to invoke the compiler,
1581 including the ability to compile source files provided in
1582 memory buffers or other non-standard file systems.
1583
1584 The [ToolProvider API](../../api/java.base/java/util/spi/ToolProvider.html)
1585 : A `ToolProvider` for `javac` can be obtained by calling
1586 `ToolProvider.findFirst("javac")`. This returns an object
1587 with the equivalent functionality of the command-line tool.
1588
1589 **Note**: This API should not be confused with the like-named
1590 API in the [`javax.tools`](../../api/java.compiler/javax/tools/ToolProvider.html)
1591 package.
1592
1593 The `javac` [Legacy API](../../api/jdk.compiler/com/sun/tools/javac/Main.html)
1594 : This API is retained for backward compatibility only.
1595 All new code should use either the Java Compiler API or the ToolProvider API.
1596
1597 **Note:** All other classes and methods found in a package with names that start with
1598 `com.sun.tools.javac` (subpackages of `com.sun.tools.javac`) are strictly
1599 internal and subject to change at any time.
1600
1601 ## Examples of Using -Xlint keys
1602
1603 `cast`
1604 : Warns about unnecessary and redundant casts, for example:
1605
1606 > `String s = (String) "Hello!"`
1607
1608 `classfile`
1609 : Warns about issues related to class file contents.
1610
1611 `deprecation`
1612 : Warns about the use of deprecated items. For example:
1613
1614 ```
1615 java.util.Date myDate = new java.util.Date();
1616 int currentDay = myDate.getDay();
1617 ```
1618
1619 The method `java.util.Date.getDay` has been deprecated since JDK 1.1.
1620
1621 `dep-ann`
1622 : Warns about items that are documented with the `@deprecated` Javadoc
1623 comment, but do not have the `@Deprecated` annotation, for example:
1624
1625 ```
1626 /**
1627 * @deprecated As of Java SE 7, replaced by {@link #newMethod()}
1628 */
1629 public static void deprecatedMethod() { }
1630 public static void newMethod() { }
1631 ```
1632
1633 `divzero`
1634 : Warns about division by the constant integer 0, for example:
1635
1636 > `int divideByZero = 42 / 0;`
1637
1638 `empty`
1639 : Warns about empty statements after `if`statements, for example:
1640
1641 ```
1642 class E {
1643 void m() {
1644 if (true) ;
1645 }
1646 }
1647 ```
1648
1649 `fallthrough`
1650 : Checks the switch blocks for fall-through cases and provides a warning
1651 message for any that are found. Fall-through cases are cases in a switch
1652 block, other than the last case in the block, whose code does not include a
1653 `break` statement, allowing code execution to fall through from that case to
1654 the next case. For example, the code following the case 1 label in this
1655 switch block does not end with a `break` statement:
1656
1657 ```
1658 switch (x) {
1659 case 1:
1660 System.out.println("1");
1661 // No break statement here.
1662 case 2:
1663 System.out.println("2");
1664 }
1665 ```
1666
1667 If the `-Xlint:fallthrough` option was used when compiling this code, then
1668 the compiler emits a warning about possible fall-through into case, with
1669 the line number of the case in question.
1670
1671 `finally`
1672 : Warns about `finally` clauses that cannot be completed normally, for
1673 example:
1674
1675 ```
1676 public static int m() {
1677 try {
1678 throw new NullPointerException();
1679 } catch (NullPointerException e) {
1680 System.err.println("Caught NullPointerException.");
1681 return 1;
1682 } finally {
1683 return 0;
1684 }
1685 }
1686 ```
1687
1688 The compiler generates a warning for the `finally` block in this example.
1689 When the `int` method is called, it returns a value of 0. A `finally` block
1690 executes when the `try` block exits. In this example, when control is
1691 transferred to the `catch` block, the `int` method exits. However, the
1692 `finally` block must execute, so it's executed, even though control was
1693 transferred outside the method.
1694
1695 `options`
1696 : Warns about issues that related to the use of command-line options. See
1697 [Compiling for Earlier Releases of the Platform].
1698
1699 `overrides`
1700 : Warns about issues related to method overrides. For example, consider the
1701 following two classes:
1702
1703 ```
1704 public class ClassWithVarargsMethod {
1705 void varargsMethod(String... s) { }
1706 }
1707
1708 public class ClassWithOverridingMethod extends ClassWithVarargsMethod {
1709 @Override
1710 void varargsMethod(String[] s) { }
1711 }
1712 ```
1713
1714 The compiler generates a warning similar to the following:.
1715
1716 ```
1717 warning: [override] varargsMethod(String[]) in ClassWithOverridingMethod
1718 overrides varargsMethod(String...) in ClassWithVarargsMethod; overriding
1719 method is missing '...'
1720 ```
1721
1722 When the compiler encounters a `varargs` method, it translates the
1723 `varargs` formal parameter into an array. In the method
1724 `ClassWithVarargsMethod.varargsMethod`, the compiler translates the
1725 `varargs` formal parameter `String... s` to the formal parameter
1726 `String[] s`, an array that matches the formal parameter of the method
1727 `ClassWithOverridingMethod.varargsMethod`. Consequently, this example
1728 compiles.
1729
1730 `path`
1731 : Warns about invalid path elements and nonexistent path directories on the
1732 command line (with regard to the class path, the source path, and other
1733 paths). Such warnings cannot be suppressed with the `@SuppressWarnings`
1734 annotation. For example:
1735
1736 - **Linux and macOS:**
1737 `javac -Xlint:path -classpath /nonexistentpath Example.java`
1738
1739 - **Windows:**
1740 `javac -Xlint:path -classpath C:\nonexistentpath Example.java`
1741
1742 `processing`
1743 : Warns about issues related to annotation processing. The compiler generates
1744 this warning when you have a class that has an annotation, and you use an
1745 annotation processor that cannot handle that type of annotation. For example,
1746 the following is a simple annotation processor:
1747
1748 **Source file AnnoProc.java**:
1749
1750 ```
1751 import java.util.*;
1752 import javax.annotation.processing.*;
1753 import javax.lang.model.*;
1754 import javax.lang.model.element.*;
1755
1756 @SupportedAnnotationTypes("NotAnno")
1757 public class AnnoProc extends AbstractProcessor {
1758 public boolean process(Set<? extends TypeElement> elems, RoundEnvironment renv){
1759 return true;
1760 }
1761
1762 public SourceVersion getSupportedSourceVersion() {
1763 return SourceVersion.latest();
1764 }
1765 }
1766 ```
1767
1768 **Source file AnnosWithoutProcessors.java**:
1769
1770 ```
1771 @interface Anno { }
1772
1773 @Anno
1774 class AnnosWithoutProcessors { }
1775 ```
1776
1777 The following commands compile the annotation processor `AnnoProc`, then
1778 run this annotation processor against the source file
1779 `AnnosWithoutProcessors.java`:
1780
1781 ```
1782 javac AnnoProc.java
1783 javac -cp . -Xlint:processing -processor AnnoProc -proc:only AnnosWithoutProcessors.java
1784 ```
1785
1786 When the compiler runs the annotation processor against the source file
1787 `AnnosWithoutProcessors.java`, it generates the following warning:
1788
1789 ```
1790 warning: [processing] No processor claimed any of these annotations: Anno
1791 ```
1792
1793 To resolve this issue, you can rename the annotation defined and used in
1794 the class `AnnosWithoutProcessors` from `Anno` to `NotAnno`.
1795
1796 `rawtypes`
1797 : Warns about unchecked operations on raw types. The following statement
1798 generates a `rawtypes` warning:
1799
1800 > `void countElements(List l) { ... }`
1801
1802 The following example does not generate a `rawtypes` warning:
1803
1804 > `void countElements(List<?> l) { ... }`
1805
1806 `List` is a raw type. However, `List<?>` is an unbounded wildcard
1807 parameterized type. Because `List` is a parameterized interface, always
1808 specify its type argument. In this example, the `List` formal argument is
1809 specified with an unbounded wildcard (`?`) as its formal type parameter,
1810 which means that the `countElements` method can accept any instantiation of
1811 the `List` interface.
1812
1813 `serial`
1814 : Warns about missing `serialVersionUID` definitions on serializable classes.
1815 For example:
1816
1817 ```
1818 public class PersistentTime implements Serializable
1819 {
1820 private Date time;
1821
1822 public PersistentTime() {
1823 time = Calendar.getInstance().getTime();
1824 }
1825
1826 public Date getTime() {
1827 return time;
1828 }
1829 }
1830 ```
1831
1832 The compiler generates the following warning:
1833
1834 ```
1835 warning: [serial] serializable class PersistentTime has no definition of
1836 serialVersionUID
1837 ```
1838
1839 If a serializable class does not explicitly declare a field named
1840 `serialVersionUID`, then the serialization runtime environment calculates a
1841 default `serialVersionUID` value for that class based on various aspects of
1842 the class, as described in the Java Object Serialization Specification.
1843 However, it's strongly recommended that all serializable classes explicitly
1844 declare `serialVersionUID` values because the default process of computing
1845 `serialVersionUID` values is highly sensitive to class details that can
1846 vary depending on compiler implementations. As a result, this might cause
1847 an unexpected `InvalidClassExceptions` during deserialization. To guarantee
1848 a consistent `serialVersionUID` value across different Java compiler
1849 implementations, a serializable class must declare an explicit
1850 `serialVersionUID` value.
1851
1852 `static`
1853 : Warns about issues relating to the use of static variables, for example:
1854
1855 ```
1856 class XLintStatic {
1857 static void m1() { }
1858 void m2() { this.m1(); }
1859 }
1860 ```
1861
1862 The compiler generates the following warning:
1863
1864 ```
1865 warning: [static] static method should be qualified by type name,
1866 XLintStatic, instead of by an expression
1867 ```
1868
1869 To resolve this issue, you can call the `static` method `m1` as follows:
1870
1871 > `XLintStatic.m1();`
1872
1873 Alternately, you can remove the `static` keyword from the declaration of
1874 the method `m1`.
1875
1876 `this-escape`
1877 : Warns about constructors leaking
1878 `this` prior to subclass initialization.
1879 For example, this class:
1880
1881 ```
1882 public class MyClass {
1883 public MyClass() {
1884 System.out.println(this.hashCode());
1885 }
1886 }
1887 ```
1888
1889 generates the following warning:
1890
1891 ```
1892 MyClass.java:3: warning: [this-escape] possible 'this' escape
1893 before subclass is fully initialized
1894 System.out.println(this.hashCode());
1895 ^
1896 ```
1897
1898 A 'this' escape warning is generated when a constructor does something
1899 that might result in a subclass method being invoked before the
1900 constructor returns.
1901 In such cases the subclass method would be operating on an incompletely
1902 initialized instance.
1903 In the above example, a subclass of `MyClass` that overrides
1904 `hashCode()` to incorporate its own fields would likely produce
1905 an incorrect result when invoked as shown.
1906
1907 Warnings are only generated if a subclass could exist that is outside
1908 of the current module (or package, if no module) being compiled.
1909 So, for example, constructors in final and non-public classes do not
1910 generate warnings.
1911
1912 `try`
1913 : Warns about issues relating to the use of `try` blocks, including
1914 try-with-resources statements. For example, a warning is generated for the
1915 following statement because the resource `ac` declared in the `try` block
1916 is not used:
1917
1918 ```
1919 try ( AutoCloseable ac = getResource() ) { // do nothing}
1920 ```
1921
1922 `unchecked`
1923 : Gives more detail for unchecked conversion warnings that are mandated by
1924 the Java Language Specification, for example:
1925
1926 ```
1927 List l = new ArrayList<Number>();
1928 List<String> ls = l; // unchecked warning
1929 ```
1930
1931 During type erasure, the types `ArrayList<Number>` and `List<String>`
1932 become `ArrayList` and `List`, respectively.
1933
1934 The `ls` command has the parameterized type `List<String>`. When the `List`
1935 referenced by `l` is assigned to `ls`, the compiler generates an unchecked
1936 warning. At compile time, the compiler and JVM cannot determine whether `l`
1937 refers to a `List<String>` type. In this case, `l` does not refer to a
1938 `List<String>` type. As a result, heap pollution occurs.
1939
1940 A heap pollution situation occurs when the `List` object `l`, whose static
1941 type is `List<Number>`, is assigned to another `List` object, `ls`, that
1942 has a different static type, `List<String>`. However, the compiler still
1943 allows this assignment. It must allow this assignment to preserve backward
1944 compatibility with releases of Java SE that do not support generics. Because
1945 of type erasure, `List<Number>` and `List<String>` both become `List`.
1946 Consequently, the compiler allows the assignment of the object `l`, which
1947 has a raw type of `List`, to the object `ls`.
1948
1949 `varargs`
1950 : Warns about unsafe use of variable arguments (`varargs`) methods, in
1951 particular, those that contain non-reifiable arguments, for example:
1952
1953 ```
1954 public class ArrayBuilder {
1955 public static <T> void addToList (List<T> listArg, T... elements) {
1956 for (T x : elements) {
1957 listArg.add(x);
1958 }
1959 }
1960 }
1961 ```
1962
1963 A non-reifiable type is a type whose type information is not fully available
1964 at runtime.
1965
1966 The compiler generates the following warning for the definition of the
1967 method `ArrayBuilder.addToList`:
1968
1969 ```
1970 warning: [varargs] Possible heap pollution from parameterized vararg type T
1971 ```
1972
1973 When the compiler encounters a varargs method, it translates the `varargs`
1974 formal parameter into an array. However, the Java programming language
1975 does not permit the creation of arrays of parameterized types. In the method
1976 `ArrayBuilder.addToList`, the compiler translates the `varargs` formal
1977 parameter `T...` elements to the formal parameter `T[]` elements, an array.
1978 However, because of type erasure, the compiler converts the `varargs`
1979 formal parameter to `Object[]` elements. Consequently, there's a
1980 possibility of heap pollution.