1.4 Compound Assignment Operators

What compound assignment does

Compound assignment operators combine an arithmetic operation with assignment in one step. For example, `x += 5;` means `x = x + 5;`. Other operators include `-=`, `*=`, `/=`, and `%=`. They make code shorter and reduce repetition when updating a variable's own value.

Order of operations matters

Java evaluates the right side fully before applying the operator to the variable. `x *= y + 2;` is equivalent to `x = x * (y + 2);`, NOT `x = x * y + 2;`. The whole right-hand expression acts as a single operand before combining with the original variable.

Type casting with compound operators

Compound assignment can implicitly cast the result back to the variable's original type. If `int x = 5;` then `x /= 2;` performs integer division, giving `x = 2` (not 2.5), since `x` stays an `int`. This differs from writing it as two separate statements if types mismatch.

Key terms

Compound assignment operator

An operator that combines an arithmetic operation with assignment, e.g., `+=`, `-=`, `*=`, `/=`, `%=`.

x += 5;

Equivalent to `x = x + 5;` — adds 5 to x and stores the result back in x.

x *= y + 2;

Equivalent to `x = x * (y + 2);` — the right side is evaluated as one expression first.

Integer division with /=

If the variable is an int, `/=` performs integer division and truncates any decimal part.

%= operator

Performs modulo (remainder) division and assigns the result back to the variable, e.g., `x %= 4;`.

Try an AP-style question

int x = 5;
x *= 2;
x -= 3;

What is x?

1. 7
2. 13
3. 4
4. 10