/** 
  This program is a simple calculator that does addition and substraction.
  Do try to enhance the program by :
 (1) Add in division and multiplication features.
 (2) Using Button arrays for better efficiency.
 (3) Improve error handling.
**/
import javax.swing.*;
import java.awt.*;
import java.awt.event.*;
  public class mySimpleCal extends JFrame implements ActionListener{

    private JTextField jtext = new JTextField("");
    private JButton b1 = new JButton("1");
    private JButton b2 = new JButton("2");
    private JButton b3 = new JButton("3");
    private JButton b4 = new JButton("4");
    private JButton b5 = new JButton("5");
    private JButton b6 = new JButton("6");
    private JButton b7 = new JButton("7");
    private JButton b8 = new JButton("8");
    private JButton b9 = new JButton("9");
    private JButton b0 = new JButton("0");
    private JButton badd = new JButton("+");
    private JButton bsub = new JButton("-");
    private JButton beq = new JButton("=");
    private JButton bclr = new JButton("C");
    private int lastResult = 0;
    private String process; 
    private boolean next = false;
    
		public mySimpleCal(){
		Container container = getContentPane();

    /** creating the text panel for calculator **/
    JPanel p1 = new JPanel();
		p1.setLayout(new GridLayout(1,1));
    p1.add(jtext);

    /** creating buttons pad for calculator **/
    JPanel p2 = new JPanel();
    p2.setLayout(new GridLayout(4,3,1,1));

 		p2.add(b1);
 		p2.add(b2);
 		p2.add(b3);
 		p2.add(b4);
 		p2.add(b5);
 		p2.add(b6);
 		p2.add(b7);
 		p2.add(b8);
 		p2.add(b9);
        p2.add(badd);
        p2.add(b0);
        p2.add(bsub);

    //Create a listener for each button
   b1.addActionListener(this);
   b2.addActionListener(this);
   b3.addActionListener(this);
   b4.addActionListener(this);
   b5.addActionListener(this);
   b6.addActionListener(this);
   b7.addActionListener(this);
   b8.addActionListener(this);
   b9.addActionListener(this);
   b0.addActionListener(this);
   badd.addActionListener(this);
   bsub.addActionListener(this);
   beq.addActionListener(this);
   bclr.addActionListener(this);

/** creating result button **/
    JPanel p3 = new JPanel();
    p3.setLayout(new GridLayout(1,2));
    p3.add(beq);
    p3.add(bclr);
  
    container.add(p1, BorderLayout.NORTH);
    container.add(p2, BorderLayout.CENTER);
    container.add(p3, BorderLayout.SOUTH);
		}

	public static void main(String[] args) {
		mySimpleCal frame = new mySimpleCal();
		frame.setTitle("My Simple Calculator");
		//frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
		frame.setSize(200,200);
		frame.setVisible(true);
	}

  /** This method will be invoked when a button is clicked **/
    public void actionPerformed(ActionEvent e)
    {
     if (next == true) 
     {
        jtext.setText("");
        next = false;
     }
      if (e.getSource()==b1)
        jtext.setText(jtext.getText() + "1");
      else if (e.getSource()==b2)
        jtext.setText(jtext.getText() + "2");
      else if (e.getSource()==b3)
        jtext.setText(jtext.getText() + "3");
      else if (e.getSource()==b4)
        jtext.setText(jtext.getText() + "4");
      else if (e.getSource()==b5)
        jtext.setText(jtext.getText() + "5");
      else if (e.getSource()==b6)
        jtext.setText(jtext.getText() + "6");
      else if (e.getSource()==b7)
        jtext.setText(jtext.getText() + "7");
      else if (e.getSource()==b8)
        jtext.setText(jtext.getText() + "8");
      else if (e.getSource()==b9)
        jtext.setText(jtext.getText() + "9");
      else if (e.getSource()==b0)
      {
        /** Disallow preceding zero **/
        if (jtext.getText().equals("") == false)
          jtext.setText(jtext.getText() + "0");
      }
      else if (e.getSource()==badd)
      {
        process = "+";
        String s = jtext.getText();
        lastResult = Integer.parseInt(s);
        next = true;
      } 
      else if (e.getSource()==bsub)
      {
        process = "-";
        String s = jtext.getText();
        lastResult = Integer.parseInt(s);
        next = true;
      }
      else if (e.getSource()==beq)
      {
        String s = jtext.getText();
        int i =  Integer.parseInt(s);
        if (process.equals("+") == true)
            lastResult = lastResult + i;
        if (process.equals("-") == true)
            lastResult = lastResult - i;            
        jtext.setText("" + lastResult);
        process = "";
        lastResult = 0;
      }
      else if (e.getSource()==bclr)
      {
        process = "";
        next = false;
        jtext.setText("");
        lastResult = 0;
      }
    }
}