2.5 Compound Boolean Expressions

Combining conditions with && and ||

Compound boolean expressions combine simple conditions using logical operators: && (AND, true only if both sides are true), || (OR, true if at least one side is true), and ! (NOT, reverses a boolean value). These let you test multiple conditions in one expression, like `(age >= 13 && age <= 19)` to check for teenagers.

Short-circuit evaluation

Java uses short-circuit evaluation: for `&&`, if the left operand is false, the right is never evaluated (result is already false). For `||`, if the left operand is true, the right is skipped. This can prevent errors, like avoiding a division by zero or a null reference check: `(x != 0 && 10/x > 1)`.

De Morgan's Laws

De Morgan's Laws let you rewrite negated compound expressions: `!(a && b)` is equivalent to `!a || !b`, and `!(a || b)` is equivalent to `!a && !b`. This is useful for simplifying complex conditions or avoiding double negatives in code, making logic easier to read and debug.

Key terms

&& operator

Logical AND; true only if both operands are true.

|| operator

Logical OR; true if at least one operand is true.

! operator

Logical NOT; reverses a boolean value (true ↔ false).

Short-circuit evaluation

Java stops evaluating a compound expression as soon as the result is known, skipping unnecessary checks.

De Morgan's Laws

!(a && b) = !a || !b, and !(a || b) = !a && !b — used to simplify negated compound expressions.

Try an AP-style question

int x = 0;
if (x != 0 && 10 / x > 1)
    System.out.print("yes");
else
    System.out.print("no");

What is printed?

1. no
2. An ArithmeticException
3. Nothing
4. yes