Expressions

expressions.java

public interface Expression {
    int evaluate();
}

public class Constant implements Expression {
    private final int value;

    Constant(int value) { this.value = value;  }

    public int evaluate() { return value;  }
}

public abstract class BinaryExpression implements Expression {
    private final Expression lhs;
    private final Expression rhs;

    BinaryExpression(Expression lhs, Expression rhs) {
        this.lhs = lhs;
        this.rhs = rhs;
    }

    public int evaluate() {
        return op(lhs.evaluate(),rhs.evaluate());
    }

    protected abstract int op(int lhs, int rhs);
}