[Leostraza]

you mean class for every method(+-*/) ? why?

This is what he means:

Code

public int add(int a, int b) { return a + b; }

That's a good idea..

If you have a gui, there isn't a need for Switch/case just create buttonhandlers for each button, inside those handlers do the method. (add(1, 2), etc.)

Code

public class Calculator extends JFrame {
  public Calculator() {
    setTitle("Calculator");
    //... blah blah blah
    addButton.addEventListener(new AddButtonHandler());
  }

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

  private class AddButtonHandler implements ActionListener {
    public void actionPerformed(ActionEvent e) {
      textField.setText(Integer.toString(add(number1, number2)));
    }
  }
}

Was bored, you could do something like that..
Maybe add the number to a stack (on the textField) which has a list of operations?