15.6.1 Left-Hand Operand First

The left-hand operand of a binary operator appears to be fully evaluated before any part of the right-hand operand is evaluated. For example, if the left-hand operand contains an assignment to a variable and the right-hand operand contains a reference to that same variable, then the value produced by the reference will reflect the fact that the assignment occurred first.

Thus:

class Test {
	public static void main(String[] args) {
		int i = 2;
		int j = (i=3) * i;
		System.out.println(j);
	}
}

prints:

9

It is not permitted for it to print 6 instead of 9.

If the operator is a compound-assignment operator (§15.25.2), then evaluation of the left-hand operand includes both remembering the variable that the left-hand operand denotes and fetching and saving that variable's value for use in the implied combining operation. So, for example, the test program:

class Test {
	public static void main(String[] args) {
		int a = 9;
		a += (a = 3);									// first example
		System.out.println(a);
		int b = 9;
		b = b + (b = 3);									// second example
		System.out.println(b);
	}
}

prints:

12
12

because the two assignment statements both fetch and remember the value of the left-hand operand, which is 9, before the right-hand operand of the addition is evaluated, thereby setting the variable to 3. It is not permitted for either example to produce the result 6. Note that both of these examples have unspecified behavior in C, according to the ANSI/ISO standard.

If evaluation of the left-hand operand of a binary operator completes abruptly, no part of the right-hand operand appears to have been evaluated.

Thus, the test program:

class Test {

	public static void main(String[] args) {

		int j = 1;

		try {
			int i = forgetIt() / (j = 2);
		} catch (Exception e) {
			System.out.println(e);
			System.out.println("Now j = " + j);
		}
	}

	static int forgetIt() throws Exception {
		throw new Exception("I'm outta here!");
	}
}

prints:

java.lang.Exception: I'm outta here!
Now j = 1

because the left-hand operand forgetIt() of the operator / throws an exception before the right-hand operand and its embedded assignment of 2 to j occurs.