Custom Cell Renderer for JList

Question

I'm getting some strange behaviour from my custom cell renderer.
import java.awt.*;
import javax.swing.*;
import javax.swing.border.*;
public class TestRenderer implements ListCellRenderer {
  private JPanel jpCell = new JPanel();
  public Component getListCellRendererComponent
      (JList list, Object value, int index, boolean isSelected,
       boolean cellHasFocus) {
    jpCell.add(new JLabel("Render"));
    return jpCell;
import javax.swing.*;
import java.awt.*;
public class TestPanel extends JFrame {
     public TestPanel() {
          JList jlst = new JList(new String[]{"Value", "Value2", "Value3"});
          jlst.setCellRenderer(new TestRenderer());
          JPanel panel = new JPanel();
          panel.add(jlst);
          add(panel);
     public static void main(String[] args) {
          TestPanel frame = new TestPanel();
          frame.setSize(300, 300);
          frame.setLocationRelativeTo(null);
          frame.setDefaultCloseOperation(EXIT_ON_CLOSE);
          frame.setVisible(true);
}As you will see the renderer displays the string several times in each cell depending on which layout manager I use. However, if I replace the JPanel with a JLabel and set the String as text on the label the String is only printed once per cell. I can't see to find the reason for this.
Edited by: 811488 on 18-Nov-2010 09:44
Edited by: 811488 on 18-Nov-2010 09:45
Edited by: 811488 on 18-Nov-2010 09:45

Answer 1

So getListCellRendererComponent returns a component whose paintComponent method is called with the Graphics object of the JList Yes, except that the paint(...) method is called. There is a difference. paintComponent() only paints the component. In the case of a JPanel you would get a boring gray colored component. paint() will paint the component and its children and the Border of the component as well. So you get a much more exiting component.
Read the section from the Swng tutorial on [url http://download.oracle.com/javase/tutorial/uiswing/painting/index.html]Custom Painting for more information.
So the state of the cell is not the state of the cell Renderer Component meaning the image rendered onto the cell only reflects the state of the Cell Renderer Component at the time the cell was rendered.Yes which is why this method should be efficient because repainting is consistently being done for all the cells. For example every time row selection changes multiple rows need to be repainted. Think of this same concept when you use a JTable which also contains multiple columns for each row.
That is why you should not be adding/removing components in the rendering code.
It makes sense except that if this was the case the first version of the Render that I posted should have been rendered first with one "Render" then two then three, shouldn't it?Yes, except that you can't control when or how often the rendering is done. Add the following to the renderer:
System.out.println(index + " : " + jpCell.getComponentCount());You will see that rendering is done multiple times. The JList tries to determine its preferred width. To do this it loops through all the items in the list and invokes the renderer to get the width of the largest renderer. Well the largest width is the last renderer because 3 labels have been added to the panel. So that width becomes the preferred width of the list. Then the GUI is displayed and the list is painted. Every time the renderer is called an new label is added, but after the 3rd label there is no room to paint the label so it is truncated.
Change your code to the following:
//add(panel);
add(jlst);Now change the width of the frame to see what happens.
Given all the help you have received, I expect to see many "helpfull answers" selected as well as a "correct answer" (if you want help in the future that is).