1 Lecture 25 Listening to buttons and mice Quotes by Tony Hoare There are two ways of constructing a software design: (1) make it so simple that there are obviously no deficiencies and (2) make it so complicated that there are no obvious deficiencies. Inside every large program is a small program trying to get out. The unavoidable price of reliability is simplicity. I was eventually persuaded of the need to design programming notations so as to maximize the number of errors that cannot be made, or if made, can be reliably detected at compile time. You cannot teach beginners top-down program- ming because they don't know which end is up.

2 Lecture 25 Listening to buttons and mice Reading material is the same as the previous lecture. The programs we demo today are on the website. Download them and experiment with them! Look at java.awt and javax.swing as examples of how one designs a large system of classes. This is not the only way to do GUIs. This is how Java does it.

3 Review of lecture 23: Lecture 24. We: (1) Looked at different components, like JButton, JPanel. (2) Saw three different layout managers: 1.FlowLayout 2.BorderLayout 3.BoxLayout They are used to lay out components as they are added to a container. (3) Saw how to add components to a container. End with pack() and show().

4 Basic Hierarachy of GUI classes A Component generally has a position and size, can be painted, can receive input events. Canvas: rectangular component with methods for painting it. Subclass it and override method paint() Component: abstract Container Window Frame JFrame JComponent JPanel JButton JCheckBox JComboBox JLabel JRadioBox JTextArea JTextField JFileChooser Canvas Dialog JDialog

5 Container: superclass of all Components that can hold other components. Components are generally added to a Container c: c.add(new JButton(“yes”)); c.add(new JButton(“no”), “north”); Basic top-level containers: JWindow: top-level window with no border JFrame: top-level window with border, menubar JDialog: top-level window used for a dialog JPanel, primary use: as a container of other components. Allows one to organize objects into a unit, often to simplify layouts. See this on the next slides. JPanel, secondary use: paint on it with graphics commands --lines, circles, text, etc. (instead of using class Canvas). Container WindowJComponent JPanelJFrameJDialog

6 Listening to a button public class ButtonDemo1 extends JFrame implements ActionListener { private JButton westButton= new JButton("west"); private JButton eastButton= new JButton("east"); /** Constructor: invisible frame: title t, 2 buttons */ public ButtonDemo1(String t) { super(t); Container cp= getContentPane(); cp.add(westButton,BorderLayout.WEST); cp.add(eastButton,BorderLayout.EAST); westButton.setEnabled(false); eastButton.setEnabled(true); westButton.addActionListener(this); eastButton.addActionListener(this); pack(); } public void actionPerformed(ActionEvent e) { boolean b= eastButton.isEnabled(); eastButton.setEnabled(!b); westButton.setEnabled(b); } 1. write procedure actionPerformed 2. implement ActionListener 3. Register this object with the button

7 public class ButtonDemo2 extends JFrame implements ActionListener { private JButton westButton= new JButton("west"); private JButton eastButton= new JButton("east"); /** Constructor: inv frame with title t, two buttons */ public ButtonDemo2(String t) { super(t); Container cp= getContentPane(); cp.add(westButton,BorderLayout.WEST); cp.add(eastButton,BorderLayout.EAST); westButton.setEnabled(false); eastButton.setEnabled(true); westButton.addActionListener(this); eastButton.addActionListener(new BeListener()); } public void actionPerformed(ActionEvent e) { boolean b= eastButton.isEnabled(); eastButton.setEnabled(!b); westButton.setEnabled(b); } private class BeListener implements ActionListener { public void actionPerformed(ActionEvent e) { boolean b= eastButton.isEnabled(); eastButton.setEnabled(!b); westButton.setEnabled(b); } listener for eastButton listener for westButton, in its own inner class different listeners for different buttons

8 Processing mouse clicks /** A red or green square, possibly with a pink disk */ public class Square extends JPanel { public static final int HEIGHT= 50; // height and public static final int WIDTH= 50; // width, square private int x, y; // Coordinates of square on board private bool hasDisk= false; // = "square has pink disk" public Square(int x, int y) { // a square at (x,y) this.x= x; this.y= y; setPreferredSize(new Dimension(WIDTH,HEIGHT)); this.addMouseListener(new MouseEvents()); } public void paint(Graphics g) { // paint this square if ((x+y)%2 == 0) { g.setColor(Color.green); } else { g.setColor(Color.red); } g.fillRect(0,0,WIDTH-1,HEIGHT-1); if (hasDisk) { g.setColor(Color.pink); g.fillOval(7,7,WIDTH-14,HEIGHT-14); } g.setColor(Color.black); g.drawRect(0,0,WIDTH-1,HEIGHT-1); g.drawString("("+x+", "+y+")", 10, 5+HEIGHT/2); } // continued on next slide //

9 // Class Square (continued) // /** Complement the "has pink disk" property */ public void complementDisk() { hasDisk= !hasDisk; repaint(); // Don’t call paint(Graphics) directly } // instead, call inherited method repaint() /** Remove pink disk (if present) */ public void clearDisk() { hasDisk= false; repaint(); } /** Contains methods that process mouse events */ public class MouseEvents extends MouseInputAdapter { /** Complement the "has pink disk" property */ public void mouseClicked(MouseEvent e) { complementDisk(); } } API class MouseInputAdapter has several methods for processing mouse clicks, mouse presses, mouse releases, etc. We override only one of them here. That’s how we get the program to listen to mouse events.

10 /** Demo use of a mouse listener. Instance is a JFrame that is a 2x2 grid of squares, each of class Square, and a button with title "reset". Clicking an empty square draws a pink disk on it. Clicking again removes it. Clicking button "reset" removes all pink disks. This class listens to clicks of the button. Clicks on a square are listened to in class Square. */ public class MouseDemo extends JFrame { Box bl= new Box(BoxLayout.Y_AXIS); Square b00= new Square(0,0); Square b01= new Square(0,1); Box br= new Box(BoxLayout.Y_AXIS); Square b10= new Square(1,0); Square b11= new Square(1,1); JButton jb= new JButton("reset"); Box b= new Box(BoxLayout.X_AXIS); // continued on next page //

11 // class MouseDemo continued from last page */ /** Constructor: an invisible JFrame with title t */ public MouseDemo(String t) { super(t); bl.add(b00); bl.add(b01); br.add(b10); br.add(b11); b.add(bl); b.add(br); Container cp= getContentPane(); cp.add(b, BorderLayout.CENTER); cp.add(jb, BorderLayout.SOUTH); jb.addActionListener(new ButtonListener()); pack(); setResizable(false); // don’t let user resize!!! } /** Respond to mouse click on button jb */ public class ButtonListener implements ActionListener { public void actionPerformed(ActionEvent e) { b00.clearDisk(); b01.clearDisk(); b10.clearDisk(); b11.clearDisk(); } // An inner class so that it can refer to fields b00, etc.