Rendering Image in JPanel

Question

Hi,
I am completely new to java imaging and I am planning to build a small image viewer application (something similar to the comic book reader).
After some search and following the discussion here [http://forum.java.sun.com/thread.jspa?forumID=20&threadID=362429|http://forum.java.sun.com/thread.jspa?forumID=20&threadID=362429] , I have main app consists of JFrame with SplitPane which consists a ScrollPane. In separate class, I have JPanel class. I am trying to add the JPanel in ScrollPane when application starts. JPane is responsible for fetching and rendering image but something went wrong and it is giving me a NullPointer Exceptions. So far I was able to figure out that it is causing by the "paintComponent" method but no idea why. I am looking for a nice suggestions and guidance on this topic (especially something like why it didn't work and various ways to do things). "For Dummies" style suggestions are highly appreciated.
Follwing are my two classes:
===============================
=======================
Separate JPanel Class
=======================
* JPanelCanvas.java
* Created on July 17, 2008, 4:16 PM
package org.zeenet.bookreader;
* @author  Deepak K. Shrestha
import java.awt.*;
import java.awt.image.*;
import javax.imageio.*;
import java.io.*;
public class JPanelCanvas extends javax.swing.JPanel {
    BufferedImage prevImg, currImg, nextImg = null;
    Graphics g;
    /** Creates new form JPanelCanvas */
    public JPanelCanvas() {
        initComponents();
        getImage();
        setSize();
        paintComponent(g); //<<<<-- so far this is causing the trouble <<<<
    //fetch image from file
    public void getImage() {
       try {
           currImg = ImageIO.read(new File("D:\\TEMPORARY\\logo.JPG"));
       } catch (IOException e) {
    public void setSize() {
        if (currImg == null) {
             this.setPreferredSize(new Dimension (100,100));
        } else {
           this.setPreferredSize(new Dimension(currImg.getWidth(null), currImg.getHeight(null)));
    public void paintComponent(Graphics g) {
        super.paintComponent(g);
        g.drawImage(currImg, 0, 0, null);
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                         
    private void initComponents() {
        javax.swing.GroupLayout layout = new javax.swing.GroupLayout(this);
        this.setLayout(layout);
        layout.setHorizontalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 400, Short.MAX_VALUE)
        layout.setVerticalGroup(
            layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
            .addGap(0, 300, Short.MAX_VALUE)
    }// </editor-fold>                       
    // Variables declaration - do not modify                    
    // End of variables declaration                  
}==========================
Main application
==========================
* JFrameReader.java
* Created on July 16, 2008, 3:16 PM
package org.zeenet.bookreader;
import javax.swing.*;
* @author  Deepak K. Shrestha
public class JFrameReader extends javax.swing.JFrame {
    /** Creates new form JFrameReader */
    public JFrameReader() {
        initComponents();
        setExtendedState(JFrameReader.MAXIMIZED_BOTH); //maximize application
        jScrollPaneCanvas.add(new JPanelCanvas()); //<<<<---attached the JPanel here <<<<<<<<<
    /** This method is called from within the constructor to
     * initialize the form.
     * WARNING: Do NOT modify this code. The content of this method is
     * always regenerated by the Form Editor.
    @SuppressWarnings("unchecked")
    // <editor-fold defaultstate="collapsed" desc="Generated Code">                         
    private void initComponents() {
        jLayeredPane1 = new javax.swing.JLayeredPane();
        jSplitPane1 = new javax.swing.JSplitPane();
        jScrollPaneCanvas = new javax.swing.JScrollPane();
        jScrollPaneList = new javax.swing.JScrollPane();
        jListItem = new javax.swing.JList();
        setDefaultCloseOperation(javax.swing.WindowConstants.EXIT_ON_CLOSE);
        setTitle("zeeBookReader");
        jSplitPane1.setRightComponent(jScrollPaneCanvas);
        jListItem.setModel(new javax.swing.AbstractListModel() {
            String[] strings = { "Item 1", "Item 2", "Item 3", "Item 4", "Item 5" };
            public int getSize() { return strings.length; }
            public Object getElementAt(int i) { return strings; }
jScrollPaneList.setViewportView(jListItem);
jSplitPane1.setLeftComponent(jScrollPaneList);
javax.swing.GroupLayout layout = new javax.swing.GroupLayout(getContentPane());
getContentPane().setLayout(layout);
layout.setHorizontalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addComponent(jLayeredPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 400, Short.MAX_VALUE)
.addComponent(jSplitPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 400, Short.MAX_VALUE)
layout.setVerticalGroup(
layout.createParallelGroup(javax.swing.GroupLayout.Alignment.LEADING)
.addGroup(javax.swing.GroupLayout.Alignment.TRAILING, layout.createSequentialGroup()
.addComponent(jSplitPane1, javax.swing.GroupLayout.DEFAULT_SIZE, 264, Short.MAX_VALUE)
.addPreferredGap(javax.swing.LayoutStyle.ComponentPlacement.RELATED)
.addComponent(jLayeredPane1, javax.swing.GroupLayout.PREFERRED_SIZE, 30, javax.swing.GroupLayout.PREFERRED_SIZE))
pack();
}// </editor-fold>
* @param args the command line arguments
public static void main(String args[]) {
java.awt.EventQueue.invokeLater(new Runnable() {
public void run() {
new JFrameReader().setVisible(true);
// Variables declaration - do not modify
private javax.swing.JLayeredPane jLayeredPane1;
private javax.swing.JList jListItem;
private javax.swing.JScrollPane jScrollPaneCanvas;
private javax.swing.JScrollPane jScrollPaneList;
private javax.swing.JSplitPane jSplitPane1;
// End of variables declaration
=======================
By the way I am using Netbeans and all swing components are created using GUI editor.
Trying to build this app gives me following error:
===========================================
init:
deps-jar:
compile:
run:
Exception in thread "AWT-EventQueue-0" java.lang.NullPointerException
at javax.swing.JComponent.paintComponent(JComponent.java:766)
at org.zeenet.bookreader.JPanelCanvas.paintComponent(JPanelCanvas.java:54)
at org.zeenet.bookreader.JPanelCanvas.<init>(JPanelCanvas.java:30)
at org.zeenet.bookreader.JFrameReader.<init>(JFrameReader.java:22)
at org.zeenet.bookreader.JFrameReader$2.run(JFrameReader.java:80)
at java.awt.event.InvocationEvent.dispatch(InvocationEvent.java:209)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:597)
at java.awt.EventDispatchThread.pumpOneEventForFilters(EventDispatchThread.java:273)
at java.awt.EventDispatchThread.pumpEventsForFilter(EventDispatchThread.java:183)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThread.java:173)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:168)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:160)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:121)
BUILD STOPPED (total time: 10 seconds)
===============================================
Thanks

Answer 1

You should not call paintComponent explicitly (especially with a null Graphics object).
It is called internally when the component needs to repaint itself.
Read this: [Performing Custom Painting|http://java.sun.com/docs/books/tutorial/uiswing/painting/]