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POWhy won't my HashSet allow me to add two of the same instance, if their equals() says they're false?
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  1. POWhy won't my HashSet allow me to add two of the same instance, if their equals() says they're false?
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    copied!<p><a href="http://docs.oracle.com/javase/6/docs/api/java/util/HashSet.html#add%28E%29" rel="nofollow">The documentation for <code>HashSet.add</code></a> says</p> <blockquote> <p>Adds the specified element to this set if it is not already present. More formally, adds the specified element e to this set if this set contains no element e2 such that (e==null ? e2==null : e.equals(e2)). If this set already contains the element, the call leaves the set unchanged and returns false.</p> </blockquote> <p>Since my code below will return false for <code>e.equals(e2)</code>, I'd expect it to let me add the same instance twice. But the set only contains my instance once. Can someone explain why?</p> <pre><code>package com.sandbox; import java.util.HashSet; import java.util.Set; public class Sandbox { public static void main(String[] args) { Set&lt;A&gt; as = new HashSet&lt;A&gt;(); A oneInstance = new A(); System.out.println(oneInstance.equals(oneInstance)); //this prints false as.add(oneInstance); as.add(oneInstance); System.out.println(as.size()); //this prints 1, I'd expect it to print 2 since the System.out printed false } private static class A { private Integer key; @Override public boolean equals(Object o) { if (!(o instanceof A)) { return false; } A a = (A) o; if (this.key == null || a.key == null) { return false; //the key is null, it should return false } if (key != null ? !key.equals(a.key) : a.key != null) { return false; } return true; } @Override public int hashCode() { return key != null ? key.hashCode() : 0; } } } </code></pre>
 

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