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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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