Basic MouseListener Question

Hi, I have a really basic question that I can't find the answer to online. Would anyone mind explaining where my understanding is lacking.

Below is some pseudo code explaining an example of implementing a mouse listener that doesn't work:

Code:

---

class MainApplet extends JApplet
  init() { 
    add(new MyJComponent())
  }


class MyJComponent extends JComponent implements MouseListener
  public void mouseClicked(MouseEvent e) {
    System.out.println("hello")
  }
  public void paint(g)
    ...

---

Since a component in the applet has a mouse listener I would expect the component to know to watch for mouse events automatically, however it seems that I need to do this:

Code:

---

class MainApplet extends JApplet implements MouseListener
  init() { 
    MyJComponent c = new MyJComponent()
    c.addMouseListener(this)
    add(c)
  }
  public void mouseClicked(MouseEvent e) {
    System.out.println("hello")
  }

class MyJComponent extends JComponent
  public void paint(g)
    ...

---

placing the handling of all mouse events in the hands of the applet. I'm sure I must be doing something wrong.

Many Thanks,

James

Just creating a MouseListener is not enough. You also need to add that mouselistener to the component via addMouseListener. Myself, I would avoid having either the MainApplet or the MyJComponent classes implement MouseListener and instead would use an inner class -- if it is a real small listener than an anonymous inner class, if bigger than a non-anonymous inner class, or if bigger still than the mouse listener could be in its own stand alone class.

For instance an example of both a private inner and an anonymous inner class:

Code:

---

import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;
import javax.swing.JComponent;

public class MyJComponent extends JComponent
{
  public MyJComponent()
  {
    addMouseListener(new MyMouseListener()); // add inner private class
   
    addMouseListener(new MouseAdapter() // add anon inner class
    {
      public void mousePressed(MouseEvent e)
      {
        String point = "[" + e.getPoint().x + ", " + e.getPoint().y + "]";
        System.out.println("From Anon Inner Class, Press at: " + point);
      }     
    });
  }
 
  // inner private class
  private class MyMouseListener extends MouseAdapter
  {
    public void mousePressed(MouseEvent e)
    {
      String point = "[" + e.getPoint().x + ", " + e.getPoint().y + "]";
      System.out.println("From Inner Private Class, Press at: " + point);
    }
  }
 
}

---

I see that addMouseListener().

I believe the problem lies with your component. Just extending JComponent doesn't do much in terms of taking up space on the display. Your component will only receive mouse events when the mouse is over it. Try changing you component to a JPanel and setting preferredSize and a backgroundColor. This will make it take up space, and you will be able to see where it is. JLabel would work well, also.

Abuse reported on stuv602

Thanks for your reply. I have tried an example and it appeared to work until I tried using it in the current application! I think I've nailed down where my problem is and its that Jcomponents can't have objects in them that have listeners?

Code:

---

public class MyComponent extends JComponent {
        public MyComponent() {
                JButton btn = new JButton();
                btn.addMouseListener(new MouseAdapter() // add anon inner class
            {
              public void mousePressed(MouseEvent e)
              {
                String point = "[" + e.getPoint().x + ", " + e.getPoint().y + "]";
                System.out.println("From Anon Inner Class, Press at: " + point);
              }     
            });
                add(btn);
        }
}

---

If I add this component to an applet then the button does not appear, unless I implement the paint method. If I implement the paint method then the action listener still doesn't work. Does that make sense?

hi. thanks for your reply about the jpanel. I did try this with the example above and it did resolve the problem. However my JComponent is simply going to be a Line2D instance and therefore wrapping it in a panel leave a huge border around the outside of each line. Is it possible to make the JPanel exactly the same dimensions as the line (e.g. 2 pixels wide)?

Quote:

---

that Jcomponents can't have objects in them that have listeners

---

This is not true. Your problem must be somewhere else. But on a separate note, why would you add a mouse listener to a button and not an action listener?

edit: likely you haven't set the layout of your JComponent yet. try setting it to flow layout

sample code:

Code:

---

import java.awt.FlowLayout;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.awt.event.MouseAdapter;
import java.awt.event.MouseEvent;

import javax.swing.JButton;
import javax.swing.JComponent;

public class MyJComponent extends JComponent
{
  public MyJComponent()
  {
    setLayout(new FlowLayout());
    JButton button = new JButton("My Button");
    add(button);
    button.addActionListener(new ActionListener()
    {
      public void actionPerformed(ActionEvent e)
      {
        System.out.println("button pressed");
      }
    });
   
    addMouseListener(new MouseAdapter() // add anon inner class
    {
      public void mousePressed(MouseEvent e)
      {
        String point = "[" + e.getPoint().x + ", " + e.getPoint().y + "]";
        System.out.println("From Anon Inner Private Class, Press at: " + point);
      }     
    });
  }
}

---

Code:

---

import java.awt.Dimension;

import javax.swing.JFrame;
import javax.swing.JPanel;
import javax.swing.JComponent;

public class MyJComponentTest
{
  private JPanel mainPanel = new JPanel();

  public MyJComponentTest()
  {
    MyJComponent jComp = new MyJComponent();
    jComp.setPreferredSize(new Dimension(300, 300));
    mainPanel.add(jComp);
  }

  public JComponent getComponent()
  {
    return mainPanel;
  }

  private static void createAndShowUI()
  {
    JFrame frame = new JFrame("MyJComponentTest");
    frame.getContentPane().add(new MyJComponentTest().getComponent());
    frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    frame.pack();
    frame.setLocationRelativeTo(null);
    frame.setVisible(true);
  }

  public static void main(String[] args)
  {
    java.awt.EventQueue.invokeLater(new Runnable()
    {
      public void run()
      {
        createAndShowUI();
      }
    });
  }
}

---

Quote:

---

Originally Posted by jshailes

hi. thanks for your reply about the jpanel. I did try this with the example above and it did resolve the problem. However my JComponent is simply going to be a Line2D instance and therefore wrapping it in a panel leave a huge border around the outside of each line. Is it possible to make the JPanel exactly the same dimensions as the line (e.g. 2 pixels wide)?

---

You can set the preferred size of the jpanel or jcomponent to whatever size you'd like, though this seems a bit funny here. What are you trying to achieve with a very skinny jpanel here? Perhaps there's a better way to solve this then to force some kluge.

Not all components can hold other components. Rather than using something very basic like JComponent, choose something a little more advanced. I suggested JPanel just because you wanted to use a mouse listener. For something like a JButton, you would use an action listener. For a simple demo app, all you really need is a JFrame as a container and one component to display in it and to generate events.

> Not all components can hold other components.
If we're talking about Swing components here, all Swing components inherit from JComponent, which subclasses Container and all inherit the add(...) method.

Granted that all AWT components can't hold other components.

Never even looked at SWT/GWT so I can't comment on those.

db