Java Notes

Mouse Listeners

Listening within the panel itself

It is common to have a panel listen to its own events. For example,

class DrawingPanel extends JPanel implements MouseListener{

   public DrawingPanel() { // Constructor
      this.addMouseListener(this);
      . . .
   }
   
   public void paintComponent(Graphics g) {
      . . .
   }
   . . .
   public void mousePressed(MouseEvent e) {. . .}
   public void mouseReleased(MouseEvent e) {. . .}
   public void mouseClicked(MouseEvent e) {. . .}
   . . .
}

It can communicate changes with the outside by (1) making it a subclass, (2) supplying getter methods, or (3) supplying a "model" object to the constructor.

Listening from outside the panel

You may create a panel and want the listeners outside it because it is more convenient to interact with them that way. If you only have one such panel, then you can implement the mouse listener interfaces in your non-panel class and write all of the listener methods. For example,

public class MyClass implements MouseListener {
   . . .
   DrawingPanel drawing = new DrawingPanel();
   drawing.addMouseListener(this);
   . . .
   
   public void mousePressed(MouseEvent e) {. . .}
   public void mouseReleased(MouseEvent e) {. . .}
   public void mouseClicked(MouseEvent e) {. . .}
   . . .
}

class DrawingPanel extends JPanel {
   public void paintComponent(Graphics g) {
      . . .
   }
   . . .
}
   

This requires setter methods in the DrawingPanel class so that what is drawn can be changed. Or a constructor for DrawingPanel could be passed an object for the "model" that would allow it to get values needed by paintComponent.

As above with anonymous listeners

If you only want to listen for one kind of event, it's easy to use the MouseAdapter or MouseMotionAdapter classes to create an anonymous listener. For example, to listen for mouse clicks,

p.addMouseListener(new MouseAdapter() {
   public void mouseClicked(MouseEvent e) {
      x = e.getX();
      y = e.getY();
   }
});