String-Based State Machine

A state machine is an object that exists in one of several named states at any time. Transitions between states happen only when certain methods are called — and only some transitions are valid from each state.

The simplest AP CSA implementation: a String field stores the current state name; each transition method checks the current state, decides whether the transition is allowed, and updates the field.

On the AP exam, state machines appear in lock/unlock FRQs, traffic-light problems, and account status problems.

Door class

public class Door {
    private String state;   // "open", "closed", or "locked"

    public Door() { state = "closed"; }

    public void open() {
        if (state.equals("closed")) {
            state = "open";
        }
        // can't open from locked or already-open
    }

    public void close() {
        if (state.equals("open")) {
            state = "closed";
        }
    }

    public void lock() {
        if (state.equals("closed")) {
            state = "locked";
        }
    }

    public void unlock() {
        if (state.equals("locked")) {
            state = "closed";
        }
    }

    public String getState() { return state; }
}

Transition table (in code)

• closed → open via open()
• open → closed via close()
• closed → locked via lock()
• locked → closed via unlock()

Every other call is ignored. Trying to open a locked door silently fails — the state stays "locked".

String comparison uses .equals()

Never compare state strings with == — that compares references, not contents. Always .equals().