Object state machines

State machines over objects — the second integrated pattern: an object's methods change its state, and later calls read that state.

public static class Register {
    private int cents;

    public Register() { cents = 0; }

    public void add(int c) { cents += c; }

    public boolean spend(int c) {
        if (c > cents) { return false; }
        cents -= c;
        return true;
    }

    public int balance() { return cents; }
}

Trace questions ask: after this SEQUENCE of calls, what is the state? The discipline is literal — a two-column trace table, one row per call, updating every field. Sequence matters: spend(500) before any add fails; after enough adds it succeeds. Method order is content.

The exam's object-tracing items ("What is the value of...") are all this pattern with different nouns. Trace, don't guess.