jdk/src/share/classes/java/lang/doc-files/ValueBased.html

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  <title>Value-based Classes</title>
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<h2 id="ValueBased">Value-based Classes</h2>

Some classes, such as <code>java.util.Optional</code> and
<code>java.time.LocalDateTime</code>, are <em>value-based</em>.  Instances of a
value-based class:
<ul>
    <li>are final and immutable (though may contain references to mutable
        objects);</li>
    <li>have implementations of <code>equals</code>,
        <code>hashCode</code>, and <code>toString</code> which are computed
        solely from the instance's state and not from its identity or the state
        of any other object or variable;</li>
    <li>make no use of identity-sensitive operations such as reference
        equality (<code>==</code>) between instances, identity hash code of
        instances, or synchronization on an instances's intrinsic lock;</li>
    <li>are considered equal solely based on <code>equals()</code>, not
        based on reference equality (<code>==</code>);</li>
    <li>do not have accessible constructors, but are instead instantiated
        through factory methods which make no committment as to the identity
        of returned instances;</li>
    <li>are <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> in any computation or method
        invocation should produce no visible change in behavior.
    </li>
</ul>

<p>A program may produce unpredictable results if it attempts to distinguish two
    references to equal values of a value-based class, whether directly via reference
    equality or indirectly via an appeal to synchronization, identity hashing,
    serialization, or any other identity-sensitive mechanism.  Use of such
    identity-sensitive operations on instances of value-based classes may have
    unpredictable effects and should be avoided.</p>
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