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   8 <h2 id="ValueBased">Value-based Classes</h2>
   9 
  10 Some classes, such as <code>java.util.Optional</code> and
  11 <code>java.time.LocalDateTime</code>, are <em>value-based</em>.  Instances of a
  12 value-based class:
  13 <ul>
  14     <li>are final and immutable (though may contain references to mutable
  15         objects);</li>
  16     <li>have implementations of <code>equals</code>,
  17         <code>hashCode</code>, and <code>toString</code> which are computed
  18         solely from the instance's state and not from its identity or the state
  19         of any other object or variable;</li>
  20     <li>make no use of identity-sensitive operations such as reference
  21         equality (<code>==</code>) between instances, identity hash code of
  22         instances, or synchronization on an instances's intrinsic lock;</li>
  23     <li>are considered equal solely based on <code>equals()</code>, not
  24         based on reference equality (<code>==</code>);</li>
  25     <li>do not have accessible constructors, but are instead instantiated
  26         through factory methods which make no committment as to the identity
  27         of returned instances;</li>
  28     <li>are <em>freely substitutable</em> when equal, meaning that interchanging
  29         any two instances <code>x</code> and <code>y</code> that are equal
  30         according to <code>equals()</code> in any computation or method
  31         invocation should produce no visible change in behavior.
  32     </li>
  33 </ul>
  34 
  35 <p>A program may produce unpredictable results if it attempts to distinguish two
  36     references to equal values of a value-based class, whether directly via reference
  37     equality or indirectly via an appeal to synchronization, identity hashing,
  38     serialization, or any other identity-sensitive mechanism.  Use of such
  39     identity-sensitive operations on instances of value-based classes may have
  40     unpredictable effects and should be avoided.</p>
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