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How to use the MouseListener interface

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• The MouseListener Interface
  • The MouseListener interface contains call-back functions to respond to the following mouse events:

    Mouse click (pressed and released very rapidly) Mouse pressed Mouse released Mouse enters a graphical component Mouse leaves a graphical component

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  • The definition of the MouseListener interface is as follows:

    interface MouseListener { void mouseClicked(MouseEvent e); // Called when Mouse clicked void mouseEntered(MouseEvent e); // Called when Mouse enters component void mouseExited(MouseEvent e); // Called when Mouse exits component void mousePressed(MouseEvent e); // Called when Mouse was pressed void mouseReleased(MouseEvent e); // Called when Mouse released }

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  • A very simple implementation of the MouseListener interface:

    class myMouseListener implements MouseListener { public void mouseClicked(MouseEvent event) { System.out.println("Mouse clicked @ position x = " + event.getX() + " y = " + event.getY()); } public void mouseEntered(MouseEvent event) { System.out.println("Mouse entered. x = " + event.getX() + " y = " + event.getY()); } public void mouseExited(MouseEvent event) { System.out.println("Mouse exited. x = " + event.getX() + " y = " + event.getY()); } public void mousePressed(MouseEvent event) { System.out.println("Mouse pressed. x = " + event.getX() + " y = " + event.getY()); } public void mouseReleased(MouseEvent event) { System.out.println("Mouse released. x = " + event.getX() + " y = " + event.getY()); } }

    Note:

    The implementation class (myMouseListener) contains the call-back methods for events !! In other words: The call-back method is invoked when an the corresponding event is detected ! For this to happen, we must: Register the call-back implementation call with the GUI component !!! (This is discussed next)

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• Register the call back class:     make a GUI component execute the call-back methods in response to events
  • Fact:

    Every GUI component has (inherited) a number of "register a call back class" methods The generic syntax to invoke a "register a call back class" method is: GUI_Component x; CallBackClass c = new CallBackClass(); x.addSomeEventsListener( c ); // this will register the CallBackClass c // for some class of events

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  • The is one call back class registering method for each call back interface class

    Examples:

    Callback Interface	Callback Registering method name
    MouseListener	addMouseListener()
    ActionListener	addActionListener()
    and so on...	and so on...

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  • Contrete example: registering the call back class myMouseListener that we have written above as a MouseListener callback interface:

    JFrame f = new JFrame("Active frame !"); myMouseListener x = new myMouseListener(); f.addMouseListener( x ); Or shorter: JFrame f = new JFrame("Active frame !"); f.addMouseListener( new myMouseListener() );

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• Our first working Example: Making a JFrame respond to Mouse events
  • First: make a JFrame - that's an old cow...

    public class MouseSpyAppl { public static void main(String[] args) { JFrame f = new JFrame("Mouse Test"); f.setSize(300, 300); f.setVisible(true); } }

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  • Next: define the call-back interface class by implementing the MouseListener interface:

    public static class myMouseListener implements MouseListener { public void mouseClicked(MouseEvent event) { System.out.println("Mouse clicked @ position x = " + event.getX() + " y = " + event.getY()); } public void mouseEntered(MouseEvent event) { System.out.println("Mouse entered. x = " + event.getX() + " y = " + event.getY()); } public void mouseExited(MouseEvent event) { System.out.println("Mouse exited. x = " + event.getX() + " y = " + event.getY()); } public void mousePressed(MouseEvent event) { System.out.println("Mouse pressed. x = " + event.getX() + " y = " + event.getY()); } public void mouseReleased(MouseEvent event) { System.out.println("Mouse released. x = " + event.getX() + " y = " + event.getY()); } }

    • NOTE:
      In case you need a refresher on Java interface: click here

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  • Finally: register the call back interface class with the GUI component (in our example: the JFrame)

    import javax.swing.*; import java.awt.*; import java.awt.event.*; // You need to import this class ! public class MouseSpyAppl { public static void main(String[] args) { JFrame f = new JFrame("Mouse Test"); // +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ // Register the CallBack Interface class (below) // +++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++ f.addMouseListener(new myMouseListener() ); f.setSize(300, 300); f.setVisible(true); } }

  • THAT'S IT, FOLKS !!!

    (You can teach GUI programming to high scholers nowadays...)

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  • Example Program: (Demo above code)                        
    • Prog file: /home/cs377001/demo/GUI/MouseSpyAppl.java

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  • NOTE: The class that you have registered to be executed when an event happens is called a call-back class