Class 19: Intro to GUIs

What we did

We built our way up from the simplest GUI - a blank window - to a GUI that responded to button-clicks: in particular, a GUI with a button that incremented a visible counter with every click. Below is a version with an addition I made while talking to a student after class. It has a decrement as well as an increment button. I give that here because it cleared up a question for that student, so maybe it'll clear things up for you.

import javax.swing.*;
import java.awt.event.*;
public class Ex1 extends JFrame
{
  //-- Data Members -------------------------------//
  private JLabel l;
  private int c;

  //-- Constructor --------------------------------//
  public Ex1()
  {
    // Basic initialization
    c = 0;
    setSize(400,100);
    setLocation(30,30);
    setTitle("Java GUI Demo From Class");
    addWindowListener(new CloseWindowListener());

    // Create Panel
    JPanel p = new JPanel();
    add(p);

    // Add text
    l = new JLabel("count is " + c);
    p.add(l);

    // Add a button & create a listener and add it to the button
    JButton b1 = new JButton("decrement");
    p.add(b1);
    b1.addActionListener(new ButtonDecActionListener());

    // Add a button & create a listener and add it to the button
    JButton b2 = new JButton("increment");
    p.add(b2);
    b2.addActionListener(new ButtonIncActionListener());

    // Show the frame
    setVisible(true);
  }

  //-- main function ------------------------------//
  public static void main(String[] args) { JFrame f = new Ex1();  }

  //-- listeners ----------------------------------//
  class ButtonIncActionListener implements ActionListener
  {
    public void actionPerformed(ActionEvent e) { l.setText("count is " + ++c); }
  }

  class ButtonDecActionListener implements ActionListener
  {
    public void actionPerformed(ActionEvent e) { l.setText("count is " + --c); }
  }
  
  class CloseWindowListener implements WindowListener
  {
    public void windowActivated(WindowEvent e)   { }
    public void windowClosed(WindowEvent e)      { }
    public void windowClosing(WindowEvent e)     { System.exit(0); }
    public void windowDeactivated(WindowEvent e) { }
    public void windowDeiconified(WindowEvent e) { }
    public void windowIconified(WindowEvent e)   { }
    public void windowOpened(WindowEvent e)      { }
  }
}