public class Calculator {
  public Formula newFormula() {
    return new Formula();
  }

  public static enum Operation {
    SUM, SUB, MULT, DIV, POW;
  }

  public static class Formula {
    protected Double a, b, result;

    protected Formula() {}

    public Formula addOperand(double operand) {
      if (a == null) {
        a = operand;
      } else if (b == null) {
        b = operand;
      } else {
        throw new IllegalStateException("Formula is full of operands");
      }
      return this;
    }

    public Formula calculate(Operation op) {
      if (a == null || b == null)
        throw new IllegalStateException("Not enough operands!");
      switch (op) {
        case SUM:
          result = a + b;
          break;
        case SUB:
          result = a - b;
          break;
        case MULT:
          result = a * b;
          break;
        case DIV:
          result = a / b;
          break;
        case POW:
          result = Math.pow(a, b);
          break;
      }
      return this;
    }

    public double result() {
      if (result == null)
        throw new IllegalStateException("Formula is not computed!");
      return result;
    }
  }
}

Calculator calc = new Calculator();
System.out.println(
  calc.newFormula()
    .addOperand(5)
    .addOperand(15)
    .calculate(Calculator.Operation.MULT)
    .result()
);

public interface Ints {
  int sum(int arg0, int arg1);
  int mult(int arg0, int arg1);
  int pow(int a, int b);
}

public static void main(String[] args) {
  Ints intsCalc = new IntsCalculator();
  System.out.println(intsCalc.sum(2, 2));
  System.out.println(intsCalc.sum(10, 22));
  System.out.println(intsCalc.pow(2, 10));
}

public class IntsCalculator implements Ints {
  protected final Calculator target;

  public IntsCalculator() { this.target = new Calculator(); }

  @Override
  public int sum(int arg0, int arg1) {
    //считаем через target
  }

  @Override
  public int mult(int arg0, int arg1) {
    //считаем через target
  }

  @Override
  public int pow(int a, int b) {
    //считаем через target
  }
}

public class Main {
  public static void main(String[] args) {
    Ints calc = new IntsCalculator();
    //демонстрация
  }
}