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27 <head>
28 <title>Value-based Classes</title>
29 </head>
30 <body>
31 <h1 id="ValueBased">{@index "Value-based Classes"}</h1>
32
33 Some classes, such as <code>java.lang.Integer</code> and
34 <code>java.time.LocalDate</code>, are <em>value-based</em>.
35 A value-based class has the following properties:
36 <ul>
37 <li>the class declares only final instance fields (though these may contain references
38 to mutable objects);</li>
39 <li>the class's implementations of <code>equals</code>, <code>hashCode</code>,
40 and <code>toString</code> compute their results solely from the values
41 of the class's instance fields (and the members of the objects they
42 reference), not from the instance's identity;</li>
43 <li>the class's methods treat instances as <em>freely substitutable</em>
44 when equal, meaning that interchanging any two instances <code>x</code> and
45 <code>y</code> that are equal according to <code>equals()</code> produces no
46 visible change in the behavior of the class's methods;</li>
47 <li>the class performs no synchronization using an instance's monitor;</li>
48 <li>the class does not declare (or has deprecated any) accessible constructors;</li>
49 <li>the class does not provide any instance creation mechanism that promises
50 a unique identity on each method call—in particular, any factory
51 method's contract must allow for the possibility that if two independently-produced
52 instances are equal according to <code>equals()</code>, they may also be
53 equal according to <code>==</code>;</li>
54 <li>the class is final, and extends either <code>Object</code> or a hierarchy of
55 abstract classes that declare no instance fields or instance initializers
56 and whose constructors are empty.</li>
57 </ul>
58
59 <p>When two instances of a value-based class are equal (according to `equals`), a program
60 should not attempt to distinguish between their identities, whether directly via reference
61 equality or indirectly via an appeal to synchronization, identity hashing,
62 serialization, or any other identity-sensitive mechanism.</p>
63
64 <p>Synchronization on instances of value-based classes is strongly discouraged,
65 because the programmer cannot guarantee exclusive ownership of the
66 associated monitor.</p>
67
68 <p>Identity-related behavior of value-based classes may change in a future release.
69 For example, synchronization may fail.</p>
70
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