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POWhy are these == but not `equals()`?
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copied!<p>I'm a bit confused about the way Java treats <code>==</code> and <code>equals()</code> when it comes to <code>int</code>, <code>Integer</code> and other types of numbers. For example:</p> <pre><code>Integer X = 9000; int x = 9000; Short Y = 9000; short y = 9000; List<Boolean> results = new ArrayList<Boolean>(); // results.add(X == Y); DOES NOT COMPILE 1) results.add(Y == 9000); // 2) results.add(X == y); // 3) results.add(X.equals(x)); // 4) results.add(X.equals(Y)); // 5) results.add(X.equals(y)); // 6) System.out.println(results); </code></pre> <p>outputs (maybe you should make your guess first):</p> <pre><code>[true, true, true, false, false] </code></pre> <ol> <li>That <code>X == Y</code> does not compile is to be expected, being different objects. </li> <li>I'm a little surprised that <code>Y == 9</code> is <code>true</code>, given that 9 is by default an <code>int</code>, and given that 1) didn't even compile. Note that you can't put an <code>int</code> into a method expecting a <code>Short</code>, yet here they are equal.</li> <li>This is surprising for the same reason as two, but it seems worse.</li> <li>Not surprising, as <code>x</code> is autoboxed to and <code>Integer</code>.</li> <li>Not surprising, as objects in different classes should not be <code>equal()</code>.</li> <li>What?? <code>X == y</code> is <code>true</code> but <code>X.equals(y)</code> is <code>false</code>? Shouldn't <code>==</code> always be stricter than <code>equals()</code>?</li> </ol> <p>I'd appreciate it if anyone can help me make sense of this. For what reason do == and equals() behave this way?</p> <p><strong>Edit:</strong> I have changed 9 to 9000 to show that this behavior is not related to the any unusual ways that the integers from -128 to 127 behave.</p> <p><strong>2<sup>nd</sup> Edit:</strong> OK, if you think you understand this stuff, you should consider the following, just to make sure:</p> <pre><code>Integer X = 9000; Integer Z = 9000; short y = 9000; List<Boolean> results = new ArrayList<Boolean>(); results.add(X == Z); // 1) results.add(X == y); // 2) results.add(X.equals(Z)); // 3) results.add(X.equals(y)); // 4) System.out.println(results); </code></pre> <p>outputs:</p> <pre><code>[false, true, true, false] </code></pre> <p>The reason, as best as I understand it:</p> <ol> <li>Different instance, so different.</li> <li><code>X</code> unboxed, then same value, so equal.</li> <li>Same value, so equal.</li> <li><code>y</code> cannot be boxed to an <code>Integer</code> so cannot be equal.</li> </ol>

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