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+<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">
+<html lang="en">
+<head>
+  <title>Value-based Classes</title>
+  <link rel="stylesheet" type="text/css" href="../../../stylesheet.css" title="Style">
+</head>
+<body>
+<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>
+</body>
+</html>
