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    <blockquote> <p>How do Object.hashCode() and System.identityHashCode() work at the back end? </p> </blockquote> <p>Assuming that it hasn't been overridden, the <code>Object.hashCode()</code> method simply calls <code>System.identityHashCode(this)</code>.</p> <p>The exact behavior of <code>System.identityHashCode(Object)</code> depends on the JVM implementation. (The actual implementation on recent Hotspot JVMs is rather clever, but I digress.)</p> <blockquote> <p>Does <code>identityHashCode()</code> return the reference of the object? </p> </blockquote> <p>No. It returns an <code>int</code>, and an <code>int</code> cannot hold a reference. (Duh!)</p> <p>That integer returned by <code>identityHashCode</code> may be related to the (a) machine address for the object, or it may not be<sup>1</sup>. The value returned by <code>identityHashCode()</code> <em>is guaranteed not to change</em> for the lifetime of the object. This means that of the GC relocates an object (after an <code>identityHashCode()</code> call) then it cannot use the new object address as the identity hashcode.</p> <blockquote> <p>Does hashCode() depend on the <code>?</code> of the object <code>? ==</code> operator how to work in back end.</p> </blockquote> <p>This doesn't make sense. There is no <code>? ==</code> or <code>?==</code> operator in Java.</p> <blockquote> <p>What is the difference between hashCode() and identityHashCode()?</p> </blockquote> <p>This is partly explained above. Other differences include:</p> <ul> <li><p>The <code>hashcode()</code> method is a non-final instance method, and should be overridden in any class where the <code>equals(Object)</code> is overridden. By contrast, <code>identityHashCode(Object)</code> is a <code>static</code> method and therefore cannot be overridden.</p></li> <li><p>The <code>identityHashCode(Object)</code> method gives you a identifier for an object which can (in theory) be used for other things than hashing and hash tables. (Unfortunately, it is not a <em>unique</em> identifier, but it <em>is</em> guaranteed to never change for the lifetime of the object.)</p></li> </ul> <hr> <p><sup>1 - For current generation JVMs, it is not related to the memory address at all. See @bestsss's answer.</sup></p>
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