// a op b == c
class Equation {
    final Operand a, b, c;
    final Operation op;
    Equation(Operand a, Operand b, Operand c, Operation op) {
        this.a = a;
        this.b = b;
        this.c = c;
        this.op = op;
    }
    static Equation createRandom(Random r) {
        Operand[] abc = new Operand[3];
        int missingIndex = r.newInt(3);
        for (int i = 0; i < abc.length; i++) {
            abc[i] = i == missingIndex
                ? new Missing()
                : new Operand(r.nextInt());
        }
        return new Equation(abc[0], abc[1], abc[2], new PlusOperation());
    }
}