Object state and conventions

A class can own objects of classes you wrote. A tiny Timer inside a Stopwatch:

public class Stopwatch {
    private int minutes;
    private int seconds;

    public Stopwatch() {
        minutes = 0;
        seconds = 0;
    }

    public void tick() {              // one second passes
        seconds++;
        if (seconds >= 60) {
            seconds = 0;
            minutes++;
        }
    }

    public String toString() {        // AP-style format: M:SS
        return minutes + ":" + String.format("%02d", seconds);
    }
}

Two conventions the exam relies on:

• toString() — every class can define it; concatenation ("" + stopwatch) calls it automatically. AP FRQ answers frequently end with building a String and returning it.
• Constructors calling constructors — this(...) as the first statement delegates; overloads stay DRY:

public Point(int x, int y) {
    this.x = x;
    this.y = y;
}

public Point() {
    this(0, 0);       // reuse the main constructor
}

State transitions are the heart of trace questions: after t.tick(); t.tick(); t.tick(); from 0:59, the state is 1:02 — the carry (seconds >= 60) is where students lose points by ticking only one field.