src/java.base/share/classes/java/lang/doc-files/ValueBased.html - master - githubmirror/jdk17u-dev

src/java.base/share/classes/java/lang/doc-files/ValueBased.html
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Dan Smith
8257845: Integrate JEP 390
09 дек 2020, 02:04
09 дек 2020, 02:0448d8650
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  <title>Value-based Classes</title>
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<h1 id="ValueBased">{@index "Value-based Classes"}</h1>

Some classes, such as <code>java.lang.Integer</code> and
<code>java.time.LocalDate</code>, are <em>value-based</em>.
A value-based class has the following properties:
<ul>
    <li>the class declares only final instance fields (though these may contain references
        to mutable objects);</li>
    <li>the class's implementations of <code>equals</code>, <code>hashCode</code>,
        and <code>toString</code> compute their results solely from the values
        of the class's instance fields (and the members of the objects they
        reference), not from the instance's identity;</li>
    <li>the class's methods treat instances as <em>freely substitutable</em>
        when equal, meaning that interchanging any two instances <code>x</code> and
        <code>y</code> that are equal according to <code>equals()</code> produces no
        visible change in the behavior of the class's methods;</li>
    <li>the class performs no synchronization using an instance's monitor;</li>
    <li>the class does not declare (or has deprecated any) accessible constructors;</li>
    <li>the class does not provide any instance creation mechanism that promises
        a unique identity on each method call&mdash;in particular, any factory
        method's contract must allow for the possibility that if two independently-produced
        instances are equal according to <code>equals()</code>, they may also be
        equal according to <code>==</code>;</li>
    <li>the class is final, and extends either <code>Object</code> or a hierarchy of
        abstract classes that declare no instance fields or instance initializers
        and whose constructors are empty.</li>
</ul>

<p>When two instances of a value-based class are equal (according to `equals`), a program
    should not attempt to distinguish between their identities, whether directly via reference
    equality or indirectly via an appeal to synchronization, identity hashing,
    serialization, or any other identity-sensitive mechanism.</p>

<p>Synchronization on instances of value-based classes is strongly discouraged,
    because the programmer cannot guarantee exclusive ownership of the
    associated monitor.</p>

<p>Identity-related behavior of value-based classes may change in a future release.
    For example, synchronization may fail.</p>

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