Core Java

Event Handling

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Understand how a program can react to user interface events

Basics of Event Handling

• Event source (button, window, ...) produces events.
• Event object (ActionEvent, WindowEvent, ...) carries information about an event.
• Event listener contains code that reacts to events.
• Add event listener to event source:

  ActionListener listener = new MyListener();
  JButton button = new JButton("OK");
  button.addActionListener(listener);

• Listener object implements appropriate listener interface:

  class MyListener implements ActionListener
  {
     public void actionPerformed(ActionEvent event)
     {
        reaction to button click      
     }
  }
  

Handling Button Clicks

• Create a JButton object:

  JButton yellowButton = new JButton("Yellow");
  JButton blueButton = new JButton(new ImageIcon("blue-ball.gif"));
  

• Add the buttons to a JPanel, and add the panel to the frame:

  JPanel buttonPanel = new JPanel();
  buttonPanel.add(yellowButton);
  buttonPanel.add(blueButton);
  frame.add(buttonPanel);
  

• Add a listener to each button:

  class ColorAction implements ActionListener { . . . }
  . . .
  yellowButton.addActionListener(new ColorAction(Color.YELLOW));
  blueButton.addActionListener(new ColorAction(Color.BLUE));
  

button

Specifying Listeners Concisely

• Use lambda expressions:

  exitButton.addActionListener(event -> System.exit(0));
  

• If you need actions with parameters, use a helper method instead of a listener class:

  public void makeButton(String name, Color backgroundColor)
  {
     JButton button = new JButton(name);
     buttonPanel.add(button);
     button.addActionListener(
        event -> buttonPanel.setBackground(backgroundColor));
  }

• Then call:

  makeButton("yellow", Color.YELLOW);
  makeButton("blue", Color.BLUE);
  

• Older code used anonymous inner classes:

  exitButton.addActionListener(new ActionListener()
     {
        public void actionPerformed(ActionEvent event) { System.exit(0); }
     });
  

plaf

Adapter Classes

• Windows send out events in multiple situations:
  • When the window is opened or closed
  • When a window is minimized or restored
  • When a window becomes inactive or active
  • When the user wants to close the window
• WindowListener interface has a separate method for each of them:

  void windowOpened(WindowEvent e);
  void windowClosing(WindowEvent e);
  void windowClosed(WindowEvent e);
  . . .
  

• Suppose you just want to intercept windowClosing.
  • Extend WindowAdapter instead of implementing WindowListener:

    class Terminator extends WindowAdapter
    {
       public void windowClosing(WindowEvent e)
       {
          if (user agrees) System.exit(0);
       }
    }
    

  • Install listener:

    frame.addWindowListener(new Terminator());

Provide user interface actions that can be activated in multiple ways

Actions

• Programs commonly provide multiple ways for selecting an action: menu, toolbar button, keystroke.
• Action interface combines listener code and action properties.
• Mechanism to disable or enable action:

  yellowAction.setEnabled(false);
  

• Properties with keys:
  • NAME
  • SMALL_ICON
  • SHORT_DESCRIPTION (for tooltips)
  • MNEMONIC_KEY (for underlined characters in menus)

  yellowAction.putValue(Action.SHORT_DESCRIPTION, "Set panel color to yellow");
  

Keyboard Commands

• Produce Keystroke object:

  KeyStroke ctrlYKey = KeyStroke.getKeyStroke("ctrl Y");
  

• Normally, keystrokes are sent to the component that has focus.
• To override, use input map that maps keys to action map keys:

  InputMap imap = panel.getInputMap(JComponent.WHEN_ANCESTOR_OF_FOCUSED_COMPONENT);
  imap.put(ctrlYKey, "panel.yellow");

• Map the action map key to the action:

  ActionMap amap = panel.getActionMap();
  amap.put("panel.yellow", yellowAction);
  

action

Handle mouse events

Mouse Events

• MouseListener reports on mouse events in a component.
• When the mouse button is clicked, three listener methods are called:

  void mousePressed(MouseEvent event)
  void mouseReleased(MouseEvent event)
  void mouseClicked(MouseEvent event)
  

• MouseAdapter implements all methods to do nothing.
• MouseEvent object has methods describing the event:

  int getX()
  int getY()
  int getClickCount()
  

• To test for right mouse click, use:

  if ((event.getModifiersEx() & InputEvent.BUTTON3_DOWN_MASK) != 0)

• MouseMotionListener tracks movement with mouseMoved and mouseDragged methods.
• Component can change cursor shape:

  comp.setCursor(Cursor.getPredefinedCursor(Cursor.CROSSHAIR_CURSOR));

mouse

Understand the AWT event hierarchy

AWT Event Classes

• Semantic events are meaningful to application programmers:
  • ActionEvent (button, menu, ...)
  • AdjustmentEvent (scrollbar)
  • ItemEvent (list box)
• Low-level events come from input sources:
  • KeyEvent
  • MouseEvent
  • MouseWheelEvent
  • FocusEvent
  • WindowEvent

AWT Listener Classes

• ActionListener (button, menu item, combo box, text field, timer)
• AdjustmentListener (scroll bar)
• ItemListener (combo box)
• FocusListener (component)
• KeyListener (component)
• MouseListener, MouseMotionListener, MouseWheelListener (component)
• WindowListener, WindowFocusListener, WindowStateListener (window)
• Many more, some very technical: PopupMenuListener, TreeWillExpandListener