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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} and
34 {@code java.time.LocalDate}, are <em>value-based</em>.
35 The compiler and runtime enforce the value based properties below if it is declared
36 as {@code value class} and preview features are enabled.
37 A value-based class has the following properties:
38 <ul>
39 <li>the class declares only final instance fields (though these may contain references
40 to mutable objects);</li>
41 <li>the class's implementations of {@code equals}, {@code hashCode},
42 and {@code toString} compute their results solely from the values
43 of the class's instance fields (and the members of the objects they
44 reference), not from the instance's identity;</li>
45 <li>the class's methods treat instances as <em>freely substitutable</em>
46 when equal, meaning that interchanging any two instances {@code x} and
47 {@code y} that are equal according to {@code equals()} produces no
48 visible change in the behavior of the class's methods;</li>
49 <li>the class performs no synchronization using an instance's monitor;</li>
50 <li>the class does not declare (or discourages use of) accessible constructors;</li>
51 <li>the class does not provide any instance creation mechanism that promises
52 a unique identity on each method call—in particular, any factory
53 method's contract must allow for the possibility that if two independently-produced
54 instances are equal according to {@code equals()}, they may also be
55 equal according to {@code ==};</li>
56 <li>the class is final, and extends either {@code Object} or a hierarchy of
57 abstract value classes.</li>
58 </ul>
59
60 <p>When two instances of a value-based class are equal (according to `equals`), a program
61 should not attempt to distinguish between their identities, whether directly via reference
62 equality {@code ==} or indirectly via an appeal to synchronization, identity hashing,
63 serialization, or any other identity-sensitive mechanism.</p>
64
65 <p>Synchronization on instances of value-based classes is strongly discouraged,
66 because the programmer cannot guarantee exclusive ownership of the
67 associated monitor.</p>
68
69 <p>Identity-related behavior of value-based classes may change when implemented as a value class.
70 </p>
71 <ul>
72 <li>The class may choose to allocate/cache instances differently.
73 <li>The use of the value class for synchronization or with
74 {@linkplain java.lang.ref.Reference object references} result in {@link IdentityException}.
75 </ul>
76
77 </body>
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