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<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
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<html lang="en">
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<head>
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<title>Value-based Classes</title>
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<link rel="stylesheet" type="text/css" href="../../../stylesheet.css" title="Style">
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</head>
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<body>
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<h2 id="ValueBased">Value-based Classes</h2>
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Some classes, such as <code>java.util.Optional</code> and
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<code>java.time.LocalDateTime</code>, are <em>value-based</em>. Instances of a
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value-based class:
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<ul>
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<li>are final and immutable (though may contain references to mutable
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objects);</li>
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<li>have implementations of <code>equals</code>,
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<code>hashCode</code>, and <code>toString</code> which are computed
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solely from the instance's state and not from its identity or the state
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of any other object or variable;</li>
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<li>make no use of identity-sensitive operations such as reference
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equality (<code>==</code>) between instances, identity hash code of
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instances, or synchronization on an instances's intrinsic lock;</li>
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<li>are considered equal solely based on <code>equals()</code>, not
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based on reference equality (<code>==</code>);</li>
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<li>do not have accessible constructors, but are instead instantiated
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through factory methods which make no committment as to the identity
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of returned instances;</li>
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<li>are <em>freely substitutable</em> when equal, meaning that interchanging
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any two instances <code>x</code> and <code>y</code> that are equal
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according to <code>equals()</code> in any computation or method
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invocation should produce no visible change in behavior.
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</li>
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</ul>
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<p>A program may produce unpredictable results if it attempts to distinguish two
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references to equal values of a value-based class, whether directly via reference
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equality or indirectly via an appeal to synchronization, identity hashing,
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serialization, or any other identity-sensitive mechanism. Use of such
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identity-sensitive operations on instances of value-based classes may have
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unpredictable effects and should be avoided.</p>
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</body>
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</html>
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