private interface Add {
    int calculate(int a, int b);
}

private interface Subtract {
    int calculate(int a, int b);
}

private class Calculator implements Add, Subtract {
    public int calculate(int a, int b) {
        return a + b;
        // return a - b;
    }
}

interface Calculator{
    public int calculate(BiFunction<Integer, Integer, Integer> operation, int a, int b);
}

class CalculatorImpl implements Calculator{
    public int calculate(BiFunction<Integer, Integer, Integer> operation, int a, int b) {
        return operation.apply(a, b);
    }
}

Calculator calc = new CalculatorImpl();
System.out.println(calc.calculate((a, b) -> a + b, 10, 20)); //output 30