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    <p>The operator being discussed here is called the postfix increment operator (<a href="http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.14.2" rel="noreferrer">JLS 15.14.2</a>). It is specified to behave as follows:</p> <blockquote> <ol> <li>At run time, if evaluation of the operand expression completes abruptly, then the postfix increment expression completes abruptly for the same reason and no incrementation occurs.</li> <li>Otherwise, the value 1 is added to the value of the variable and the sum is stored back into the variable. <ol> <li>Before the addition, binary numeric promotion (§5.6.2) is performed on the value 1 and the value of the variable.</li> <li>If necessary, the sum is narrowed by a narrowing primitive conversion (§5.1.3) and/or subjected to boxing conversion (§5.1.7) to the type of the variable before it is stored.</li> </ol></li> <li><em>The value of the postfix increment expression is <strong>the value</strong> of the variable before the new value is stored</em>.</li> </ol> </blockquote> <p>The last point is the key for this question: the reason why you can't do <code>arr[i]++ = v;</code> is the same exact reason why you can't do <code>x++ = v;</code>; the postfix increment expression returns a <em>value</em>, not a <em>variable</em>.</p> <p>From <a href="http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.1" rel="noreferrer">JLS 15.1 Evaluation, Denotation and Result</a>:</p> <blockquote> <p>When an expression in a program is evaluated (executed), the result denotes one of three things:</p> <ul> <li>A variable [...] (in C, this would be called an lvalue)</li> <li>A value [...]</li> <li>Nothing (the expression is said to be void) </li> </ul> </blockquote> <p>An assignment needs a <em>variable</em> on the left hand side, and a value is NOT a variable, and that's why you can't do <code>x++ = v;</code>.</p> <p>From <a href="http://java.sun.com/docs/books/jls/third_edition/html/expressions.html#15.26" rel="noreferrer">JLS 15.26 Assignment Operators</a>:</p> <blockquote> <p><em>The result of the first operand of an assignment operator must be a variable, or a compile-time error occurs</em>. This operand may be a named variable [...], or it may be a computed variable, as can result from a field [...] or an array access. [...]</p> </blockquote> <p>The following snippet shows erroneous attempts to assign to a <em>value</em>, going from rather subtle to more obvious:</p> <pre><code>int v = 42; int x = 0; x = v; // OKAY! x++ = v; // illegal! (x + 0) = v; // illegal! (x * 1) = v; // illegal! 42 = v; // illegal! // Error message: "The left-hand side of an assignment must be a variable" </code></pre> <p>Note that you <em>can</em> use the postfix increment operator <em>somewhere</em> on the left hand side of an assignment operator, as long as the final result is a variable.</p> <pre><code>int[] arr = new int[3]; int i = 0; arr[i++] = 2; arr[i++] = 3; arr[i++] = 5; System.out.println(Arrays.toString(arr)); // prints "[2, 3, 5]" </code></pre>
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