public class Calculator 
{
  private double leftOperand, rightOperand;

  public Calculator()
  {
    leftOperand = 0;
    rightOperand = 0;
  }

  public double add()
  {
    return leftOperand + rightOperand;
  }

  public double subtract()
  {
    return leftOperand - rightOperand;
  }

  public void setLeftOperand(double operand)
  {
    leftOperand = operand;
  }

  public void setRightOperand(double operand)
  {
    rightOperand = operand;
  }

}