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copied!<p><strong>Note</strong>: Originally I posted C# code in this answer for purposes of illustration, since C# allows you to pass <code>int</code> parameters by reference with the <code>ref</code> keyword. I've decided to update it with actual legal Java code using the first <a href="http://commons.apache.org/lang/api-2.4/org/apache/commons/lang/mutable/MutableInt.html" rel="noreferrer"><code>MutableInt</code></a> class I found on Google to sort of approximate what <code>ref</code> does in C#. I can't really tell if that helps or hurts the answer. I will say that I personally haven't done all that much Java development; so for all I know there could be much more idiomatic ways to illustrate this point.</p> <hr> <p>Perhaps if we write out a method to do the equivalent of what <code>x++</code> does it will make this clearer.</p> <pre><code>public MutableInt postIncrement(MutableInt x) { int valueBeforeIncrement = x.intValue(); x.add(1); return new MutableInt(valueBeforeIncrement); } </code></pre> <p>Right? Increment the value passed and return the original value: that's the definition of the postincrement operator.</p> <p>Now, let's see how this behavior plays out in your example code:</p> <pre><code>MutableInt x = new MutableInt(); x = postIncrement(x); </code></pre> <p><code>postIncrement(x)</code> does what? Increments <code>x</code>, yes. And then <strong>returns what <code>x</code> <em>was</em> before the increment</strong>. This return value then gets assigned to <code>x</code>.</p> <p>So the order of values assigned to <code>x</code> is 0, then 1, then 0.</p> <p>This might be clearer still if we re-write the above:</p> <pre><code>MutableInt x = new MutableInt(); // x is 0. MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0. x = temp; // Now x is 0 again. </code></pre> <p>Your fixation on the fact that when you replace <code>x</code> on the left side of the above assignment with <code>y</code>, "you can see that it first increments x, and later attributes it to y" strikes me as confused. It is not <code>x</code> that is being assigned to <code>y</code>; it is <strong>the value formerly assigned to <code>x</code></strong>. Really, injecting <code>y</code> makes things no different from the scenario above; we've simply got:</p> <pre><code>MutableInt x = new MutableInt(); // x is 0. MutableInt y = new MutableInt(); // y is 0. MutableInt temp = postIncrement(x); // Now x is 1, and temp is 0. y = temp; // y is still 0. </code></pre> <p>So it's clear: <code>x = x++</code> effectively does not change the value of x. It always causes x to have the values x<sub>0</sub>, then x<sub>0</sub> + 1, and then x<sub>0</sub> again.</p> <hr> <p><strong>Update</strong>: Incidentally, lest you doubt that <code>x</code> ever gets assigned to 1 "between" the increment operation and the assignment in the example above, I've thrown together a quick demo to illustrate that this intermediate value does indeed "exist," though it will never be "seen" on the executing thread.</p> <p>The demo calls <code>x = x++;</code> in a loop while a separate thread continuously prints the value of <code>x</code> to the console.</p> <pre><code>public class Main { public static volatile int x = 0; public static void main(String[] args) { LoopingThread t = new LoopingThread(); System.out.println("Starting background thread..."); t.start(); while (true) { x = x++; } } } class LoopingThread extends Thread { public @Override void run() { while (true) { System.out.println(Main.x); } } } </code></pre> <p>Below is an excerpt of the above program's output. Notice the irregular occurrence of both 1s and 0s.</p> <pre> Starting background thread... 0 0 1 1 0 0 0 0 0 0 0 0 0 0 1 0 1 </pre>

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