Why is paint()/paintComponent() never called?

Question

For the last two days I have tried to understand how Java handles graphics, but have failed miserably at just that. My main problem is understanding exactly how and when paint() (or the newer paintComponent() ) is/should be called.

In the following code I made to see when things are created, the paintComponent() is never called, unless I manually add a call to it myself or calls to JFrame.paintAll()/JFrame.paintComponents().

I renamed the paint() method to paintComponent() in hoping that would fix my problem of it never being called (even at repaint()), but no luck.

package jpanelpaint;

import java.awt.*;
import javax.imageio.*;
import javax.swing.*;
import java.io.*;
import java.util.ArrayList;

public class ImageLoadTest extends JComponent {
 ArrayList<Image> list;

 public ImageLoadTest() {
  list = new ArrayList<Image>();

  try { //create the images (a deck of 4 cards)
   for(String name : createImageFileNames(4)){
    System.err.println(name);
    list.add(ImageIO.read(new File(name)));
   }
  } catch (IOException e) {  }
 }

    protected void paintComponent(Graphics g) {
     int yOffset=0;
  System.err.println("ImageLoadTest.paintComponent()");
     for(Image img : list) {
      g.drawImage(img, 0, yOffset,  null);
      yOffset+=20;
     }
    }

 public static void main(String args[]) throws InterruptedException {
  JFrame frame = new JFrame("Empty JFrame");
  frame.setSize(new Dimension(1000, 500));
  frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);

  frame.setVisible(true);

  Thread.sleep(1000);
  frame.setTitle("Loading images");
  ImageLoadTest ilt = new ImageLoadTest();
  frame.add(ilt);
  //update the screen
  //DOESN'T WORK. only works if I call frame.paintAll(frame.getGraphics()) 
  ilt.repaint();
  frame.repaint();

  Thread.sleep(1000);
  frame.setTitle("Setting background");
  ilt.setBackground(Color.BLACK);
  //update the screen - DOESN'T WORK even if I call paintAll ..
  ilt.repaint();
  frame.repaint();

            //have to call one of these to get anything to display  
//  ilt.paintComponent(frame.getGraphics()); //works
  frame.paintComponents(frame.getGraphics()); //works
 }

 //PRIVATE HELPER FUNCTIONS

 private String[] createImageFileNames(int count){
  String[] fileNames = new String[count];
  for(int i=0; i < count; i++)
   fileNames[i] = "Cards" + File.separator + (i+1) + ".bmp";  
  return fileNames;
 }
}

Answer 1

One of the reasons the paintComponent() doesn't get invoked in the original code is because the component has a "zero size" and the RepaintManger is smart enough not to try and paint something with no size.

The reason the reordering of the code works is because when you add the component to the frame and then make the frame visible the layout manager is invoked to layout the component. By default a frame uses a BorderLayout and by default a component is added to the center of the BorderLayout which happens give all the space available to the component so it gets painted.

However, you change the layout manager of the content pane to be a FlowLayout, you would still have a problem because a FlowLayout respects the preferred size of the component which is zero.

So what you really need to do is assign a preferred size to you your component so layout managers can do their job.