1.6 - Compound Assignment Operators

Compound assignment operators let you combine an arithmetic operation with an assignment in one step: +=, -=, *=, /=, and %=. They take the value on the left, apply the operation with the value on the right, and store the result back into the variable on the left.


int score = 10;
score += 5; // same as score = score + 5, now 15
score *= 2; // now 30


int n = 17;
n -= 2; // n = 15
n /= 4; // n = 3, int division
n %= 2; // n = 1

The whole right side gets evaluated first, and then the operation is applied with that result:


int x = 4;
x *= 2 + 3; // x = x * (2 + 3) = 4 * 5 = 20, not 4 * 2 + 3 = 11

The post-increment (++) and post-decrement (--) operators add or subtract 1 from a variable’s value.


int count = 0;
count++; // count is now 1
count--; // count is now 0 again

So x++, x += 1, and x = x + 1 all do the same thing.

Note: prefix form (++x) and using increment/decrement inside other expressions (like arr[x++]) are both outside the scope of this course.

Here’s the kind of trace you’ll see on the exam. Track each variable line by line:


int x = 7;
int y = 3;
x += y; // x = 10
y *= 2; // y = 6
x %= y; // x = 10 % 6 = 4
y--;    // y = 5
x /= 2; // x = 2
System.out.println(x + " " + y);

Output:

2 5

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