public class Accum {
    private int value = 0; // hidden
    public Accum() {}
    public void incr() { value ++; }
    public void twice() { value *= 2; }
    public int get() { return value; }
    public String toString() { return ""+value; }
    @Override
    public boolean equals(Object o) {
        if(o instanceof Accum) { // Safe to Cast
            Accum a = (Accum)o; // Cast
            return a.value == value; }
        return false; }
    public static void main(String[] args) {
        Accum m = new Accum();
        m.incr(); m.incr(); m.twice();
        System.out.println("m="+m);
        // something you think is true
        assert m.equals(new Accum()); }}