Object Oriented Programming Java 1 GUI example taken from “Computing Concepts with Java 2” by Cay Horstmann GUI Programming

Object Oriented Programming Java 2 Frame Windows A Frame window has a border and a title bar A Frame window has an addWindowListener method. We can use this method to add listeners to our frame window.

Object Oriented Programming Java 3 An example import javax.swing.*; import java.awt.event.*; public class InternetFrameExample { public static void main(String[] args) { JFrame f = new InternetFrame("Example"); f.setTitle("Internet browser"); f.show(); } javax means Java standard extension Tell the window manager to display the frame.

Object Oriented Programming Java 4 class InternetFrame extends JFrame { public InternetFrame(String s){ setSize(300,300); WindowCloser listener = new WindowCloser(); addWindowListener(listener); } private class WindowCloser extends WindowAdapter { public void windowClosing(WindowEvent e) { System.exit(0); } windowOpened() widowClosed() windowClosing() : Add the handler

Object Oriented Programming Java 5

6 Adding User Interface Components to a Frame Do not draw directly on the surface of a frame. Frames have been designed to arrange user interface components. User interface components are such things as buttons, menus, scroll bars, and so on. If you want to draw on a frame, draw on a separate component and then add that component to the frame. The Swing UI toolkit provides the Jpanel class for this purpose.

Object Oriented Programming Java 7 Drawing on a JPanel To draw on a Jpanel you override the paintComponent method. Make sure that from within your paintComponent method you call super.paintComponent(…) so that the superclass method paintComponent has a chance to erase the existing contents, redraw the borders and decorations, etc.

Object Oriented Programming Java 8 Adding the Panel to the JFrame The surface of a Swing frame is covered with four panes. Each of these four has a purpose. We are interested in getting access to the JFrame’s content pane. So, call the getContentPane on the JFrame. This call returns a Container object. Add your Panel object to the content pane Container.

Object Oriented Programming Java 9 Adding a Panel to a JFrame x = getContentPane (a contentPane holds components for display). x refers to a Container (may contain other components) c = a Panel or some other component. Add c to x using x’s layout manager (a content pane uses border layout) JFrame getContentPane()

Object Oriented Programming Java 10 Adding a JTextfield to the JFrame JFrame getContentPane() class MyFrame extends JFRAME { private JTextField textField; public MyFrame() { Container cp = getContentPane(); textField = new JTextField(); cp.add(textField, “South”); cp

Object Oriented Programming Java 11 We may want to add a listener to the TextField JFrame getContentPane() Class MyFrame extends JFRAME { private JTextField textField; public myFrame() { Container cp = getContentPane(); textField = new JTextField(); cp.add(textField, “South”); textField.addActionListener( new TextFieldListener()); cp

Object Oriented Programming Java 12 Output first! -- user enters number of eggs and we draw them

Object Oriented Programming Java 13 Strategy Think about What do we want on the screen? What events should we listen for? What should we do when those events occur? What processing will we do when user input arrives? What object has responsibilities for what activities? Think about The ‘has-a’ relationship,e.g., the Jframe’s ContentPane “has-a” Panel and a TextField. The ‘is-a’ relationship,e.g., The TextFieldListener ‘is-an’ actionListener.

Object Oriented Programming Java 14 // Example // Eggs.java import java.awt.Container; import java.awt.Graphics; import java.awt.Graphics2D; import java.awt.event.ActionEvent; import java.awt.event.ActionListener; import java.awt.event.WindowAdapter; import java.awt.event.WindowEvent; import java.awt.geom.Ellipse2D; import java.util.Random; import javax.swing.JFrame; import javax.swing.JPanel; import javax.swing.JTextField; We need classes from the awt, util, and swing packages.

Object Oriented Programming Java 15 public class Eggs { public static void main(String[] args) { EggFrame frame = new EggFrame(); frame.setTitle("Enter number of eggs"); frame.show(); } This thread is done after creating a frame and starting up the frame thread. A frame now exists and is running.

Object Oriented Programming Java 16 The EggFrame Constructor Set the size of the frame Add a listener to listen for the stop event Create a JPanel object to draw on and a JTextField object to interact with the user via the keyboard Add a listener for the JTextField Add the Jpanel and the JTextField to the contentPane container.

Object Oriented Programming Java 17 class EggFrame extends JFrame { private JTextField textField; private EggPanel panel; public EggFrame() { final int DEFAULT_FRAME_WIDTH = 300; final int DEFAULT_FRAME_HEIGHT = 300; setSize(DEFAULT_FRAME_WIDTH, DEFAULT_FRAME_HEIGHT); addWindowListener(new WindowCloser()); panel = new EggPanel(); textField = new JTextField(); textField.addActionListener(new TextFieldListener()); Container contentPane = getContentPane(); contentPane.add(panel, "Center"); contentPane.add(textField, "South"); } A listener for the jframe window A textField listener As before

Object Oriented Programming Java 18 The constructor will be called from our main thread. The other thread operates asynchronously. What do we mean by asynchronous execution? Who is running the show? Don’t programs run sequentially? We have to think differently. Event driven programming

Object Oriented Programming Java 19 The TextField The TextField object will call us when it detects an event. We don’t ‘read the input’. We set up a babysitter to respond. The TextField object sends us an event object.

Object Oriented Programming Java 20 // Use an inner class to listen on the text field private class TextFieldListener implements ActionListener { public void actionPerformed(ActionEvent event) { String input = textField.getText(); panel.setEggCount(Integer.parseInt(input)); textField.setText(""); } We do two things when we have a textfield event. 1)Get the data 2)Tell the panel the number of eggs to display

Object Oriented Programming Java 21 // Use an inner class to listen on the text field private class TextFieldListener implements ActionListener { public void actionPerformed(ActionEvent event) { String input = textField.getText(); panel.setEggCount(Integer.parseInt(input)); textField.setText(""); } Note how handy the inner class is. Both panel and textField are available. What if the listener were not an object of an inner class. how would it get access to these variables? We can’t just pass the variables on the call because we don’t make the call.

Object Oriented Programming Java 22 private class WindowCloser extends WindowAdapter { public void windowClosing(WindowEvent event) { System.exit(0); } This is how we respond when the close signal is received. system.exit(0) stops the java virtual machine. 0 means normal termination.

Object Oriented Programming Java 23 How about the panel object What are the panel object’s responsibilities? get input? _____ repaint itself? _____ keep track of the egg count? _____ hold the data it needs to repaint itself? _____ Do we want our panel to inherit properties and methods from any existing classes? _____

Object Oriented Programming Java 24 How about the panel object What are the panel object’s responsibilities? get input? No, that’s the TextField’s job. repaint itself? Sure, that’s its main job. keep track of the egg count? Yes, better here where it’s needed hold the data it needs to repaint itself? Yes Do we want our panel to inherit properties from any existing classes? Sure, we want to re-use existing code whenever possible.

Object Oriented Programming Java 25 How about the panel object When should the panel object repaint itself? What will the panel need to repaint itself? Who actually calls the paintComponent method?

Object Oriented Programming Java 26 How about the panel object When should the panel object repaint itself? When a new input arrives from the user. When the egg count changes. What will the panel need to repaint itself? A graphics objectto draw on. Who actually calls the paintComponent method? While we have to provide a paintComponent method we don’t call it directly. It’s called by the Java run-time environment after we make a call on repaint.

Object Oriented Programming Java 27 class EggPanel extends JPanel { public void paintComponent(Graphics g) { super.paintComponent(g); Graphics2D g2 = (Graphics2D)g; // draw eggCount ellipses with random centers Random generator = new Random(); for (int i = 0; i < eggCount; i++) { double x = getWidth() * generator.nextDouble(); double y = getHeight() * generator.nextDouble(); Ellipse2D.Double egg = new Ellipse2D.Double(x, y, EGG_WIDTH, EGG_HEIGHT); g2.draw(egg); }

Object Oriented Programming Java 28 public void setEggCount(int count) { eggCount = count; repaint(); } private int eggCount; private static final double EGG_WIDTH = 30; private static final double EGG_HEIGHT = 50; }