class Calculator {

  private double accumulator;
  private double display;
  private char operation;


  public Calculator() {
    accumulator = 0.0;
    display = 0.0;
    operation = '=';
  }


  public void add() {
    display = accumulator;
    operation = '+';
  }


  public void subtract() {
    display = accumulator;
    operation = '-';
  }


  public void multiply() {
    display = accumulator;
    operation = '*';
  }


  public void divide() {
    display = accumulator;
    operation = '/';
  }


  public void equals() {
    display = accumulator;
    operation = '=';
  }


  public void enterNumber(double input) {
    if ('='  ==  operation)
      accumulator = input;

    if ('+'  ==  operation)
      accumulator = accumulator + input;

    if ('-'  ==  operation)
      accumulator = accumulator - input;

    if ('*'  ==  operation)
      accumulator = accumulator * input;

    if ('/'  ==  operation)
      accumulator = accumulator / input;

    operation = '=';
    display = input;
  }


  public void display() {
    System.out.println (display);
  }


  public static void main(String[] args) {
    Calculator c = new Calculator();

    c.enterNumber(4);
    c.display();
    c.add();
    c.display();
    c.enterNumber(5);
    c.display();
    c.add();
    c.display();
    c.enterNumber(6);
    c.display();
    c.equals();
    c.display();
  }
}