Event Handling

Chapter Goals

• To understand the Java event model
• To install action and mouse event listeners
• To accept input from buttons, text fields, and the mouse

Events, Event Sources, and Event Listeners

• User interface events include key presses, mouse moves, button clicks, and so on
• Most programs don't want to be flooded by boring events
• A program can indicate that it only cares about certain specific events
• Event listener:
  • Notified when event happens
  • Belongs to a class that is provided by the application programmer
  • Its methods describe the actions to be taken when an event occurs
  • A program indicates which events it needs to receive by installing event listener objects
• Event source:
  • Event sources report on events
  • When an event occurs, the event source notifies all event listeners

Events, Event Sources, and Event Listeners

• Example: Use JButton components for buttons; attach an ActionListener to each button
• ActionListener interface:

  public interface ActionListener
  {
     void actionPerformed(ActionEvent event);
  }

• Need to supply a class whose actionPerformed method contains instructions to be executed when button is clicked
• event parameter contains details about the event, such as the time at which it occurred
• Construct an object of the listener and add it to the button:

  ActionListener listener = new ClickListener();
  button.addActionListener(listener);

File ClickListener.java

File ButtonTester.java

Output

Self Check

1. Which objects are the event source and the event listener in the ButtonTester program?
2. Why is it legal to assign a ClickListener object to a variable of type ActionListener?

Answers

1. The button object is the event source. The listener object is the event listener.
2. The ClickListener class implements the ActionListener interface.

Building Applications with Buttons

• Example: investment viewer program; whenever button is clicked, interest is added, and new balance is displayed
  
• Construct an object of the JButton class:
  JButton button = new JButton("Add Interest");
• We need a user interface component that displays a message:
  JLabel label = new JLabel("balance=" + account.getBalance());
• Use a JPanel container to group multiple user interface components together:
  JPanel panel = new JPanel();
  panel.add(button);
  panel.add(label);
  frame.add(panel);

Building Applications with Buttons

• Listener class adds interest and displays the new balance:

  class AddInterestListener implements ActionListener
  {
     public void actionPerformed(ActionEvent event)
     {
        double interest = account.getBalance() * INTEREST_RATE / 100;
        account.deposit(interest);
        label.setText("balance=" + account.getBalance());
     }
  }

• Add AddInterestListener as inner class so it can have access to surrounding final variables (account and label)

File InvestmentViewer1.java

Self Check

3. How do you place the "balance = . . ." message to the left of the "Add Interest" button?
4. Why was it not necessary to declare the button variable as final?

Answers

3. First add label to the panel, then add button.
4. The actionPerformed method does not access that variable.

Processing Text Input

• Use JTextField components to provide space for user input
  final int FIELD_WIDTH = 10; // In characters
  final JTextField rateField = new JTextField(FIELD_WIDTH);
• Place a JLabel next to each text field
  JLabel rateLabel = new JLabel("Interest Rate: ");
• Supply a button that the user can press to indicate that the input is ready for processing
  
• The button's actionPerformed method reads the user input from the text fields (use getText)

  class AddInterestListener implements ActionListener
  {
     public void actionPerformed(ActionEvent event)
     {
        double rate = Double.parseDouble(rateField.getText());
        . . .
     }
  }

File InvestmentViewer2.java

Self Check

5. What happens if you omit the first JLabel object?
6. If a text field holds an integer, what expression do you use to read its contents?

Answers

5. Then the text field is not labeled, and the user will not know its purpose.
6. Integer.parseInt(textField.getText())

Mouse Events

• Use a mouse listener to capture mouse events
• Implement the MouseListener interface:

  public interface MouseListener
  {
     void mousePressed(MouseEvent event);
     // Called when a mouse button has been pressed on a component
     void mouseReleased(MouseEvent event);
     // Called when a mouse button has been released on a component
     void mouseClicked(MouseEvent event);
     // Called when the mouse has been clicked on a component
     void mouseEntered(MouseEvent event);
     // Called when the mouse enters a component
     void mouseExited(MouseEvent event);
     // Called when the mouse exits a component
  }

• mousePressed, mouseReleased: called when a mouse button is pressed or released
• mouseClicked: if button is pressed and released in quick succession, and mouse hasn't moved
• mouseEntered, mouseExited: mouse has entered or exited the component's area

Mouse Events

• Add a mouse listener to a component by calling the addMouseListener method:

  public class MyMouseListener implements MouseListener
  {
     // Implements five methods
  }
  MouseListener listener = new MyMouseListener();
  component.addMouseListener(listener);

• Sample program: enhance RectangleComponentViewer program of Chapter 5; when user clicks on rectangle component, move the rectangle

File RectangleComponent.java

Mouse Events

• Call repaint when you modify the shapes that paintComponent draws

  box.setLocation(x, y);
  repaint();

• Mouse listener: if the mouse is pressed, listener moves the rectangle to the mouse location

  class MousePressListener implements MouseListener
  {
     public void mousePressed(MouseEvent event)
     {
        int x = event.getX();
        int y = event.getY();
        component.moveTo(x, y);
     }
     // Do-nothing methods
     public void mouseReleased(MouseEvent event) {}
     public void mouseClicked(MouseEvent event) {}
     public void mouseEntered(MouseEvent event) {}
     public void mouseExited(MouseEvent event) {}
  }

• All five methods of the interface must be implemented; unused methods can be empty

RectangleComponentViewer Program Output

File RectangleComponentViewer.java

Self Check

7. What would happen if you omitted the call to repaint in the moveTo method?
8. Why must the MousePressListener class supply five methods?

Answers

7. The rectangle would only be painted at the new location when the component is repainted for some other reason, for example, when the frame is resized.
8. It implements the MouseListener interface, which has five methods.