Assignment Statements and Swapping Two Values

Java's = does one job: evaluate the right side completely, then copy that result into the variable on the left. That is why x = x + 1 is normal code instead of nonsense. The right side, x + 1, is computed using x's current value, say 5, giving 6. Only after that computation finishes does Java overwrite x with 6. Nothing is claimed to be equal to anything; a value is just moved into a box.

This order of operations explains why a = b; b = a; does not swap two variables. Say a is 5 and b is 8. The line a = b; evaluates b, which is 8, and stores it into a. Now a is 8 and the original 5 is gone forever. The next line, b = a;, evaluates a, which is already 8, and stores it into b. Both variables end up holding 8, and the original 5 vanished with no way to recover it. To actually swap, you need a third variable to hold one value temporarily: int temp = a; a = b; b = temp; Now temp rescues a's original value before a gets overwritten, so b can safely grab it on the last line.

This is the moment to stop reading = as the algebra symbol and start reading it as 'store into.' Every assignment statement you write for the rest of the course, including ones involving objects and references, follows this same two-step rhythm: evaluate the whole right side first, then overwrite the left.