Short-Circuit Evaluation

Java evaluates conditions left to right, and it stops early whenever it already knows the answer. For &&, both sides must be true, so the moment the left side is false, the whole expression is false no matter what the right side says. Java skips the right side entirely. For ||, only one side needs to be true, so the moment the left side is true, Java skips the right side too, since the result is already decided.

This is not just an efficiency trick. The skipped operand never runs at all, which means any side effect inside it, a method call, an increment, an array access, simply does not happen. That matters most when the right side depends on the left side being safe to evaluate. Checking arr ≠ null before arr.length > 0 works because a false first check blocks Java from ever touching arr.length. Reverse the order and Java checks arr.length first, which throws an exception immediately if arr is null, before the null check ever gets a chance to help.

This connects directly to trace tables and loop conditions covered nearby: a loop guard like i < n && someCall() can silently skip someCall() forever once i < n goes false, which explains loops that seem to stop updating a variable partway through. Reading conditions as a left-to-right gate, not a pair of independent checks, is the same skill used to trace boolean expressions and predict exactly which branches and side effects execute.