Weird error using a scrollpane

Question

Hi there,
I'm trying to make a map distance calculator for my assignment. I have a custom Jpanel where the map ( a GIF or JPEG) gets drawn in paintComponent using :
          g2.drawImage(MAP, 0, 0, this); //MAP is an Image
Now the custom Jpanel (PaintPanel) is viewed from a scrollpane, since diffrenet maps have different size.
Up until now, i had my program set up so a default pic that i specified gets loaded at start-up, then if i opened a new one from the File Menu it gets loaded into the custom JPanel and setViewportView is called on the scrollpane to update the view. Everything was working like a charm up until the point where i wanted to put a blank panel as the startup state (ie you have to chose File > open to load a map).
So in my constructor i changed the line where i set the viewportView to :
          setViewportView(blank);
where blank is a panel with grey background.
this is the code that runs in the actionperfromed when i click on the open button:
if (fc.showOpenDialog(this) == JFileChooser.APPROVE_OPTION)
{     File f = fc.getSelectedFile();
     String filename = f.getAbsolutePath();
     image = Toolkit.getDefaultToolkit().createImage(filename);
     getPicSize(image); //sets two variables w,h, to the image's width and height
     inkPanel.setImage(image); //changes the map to "image", then calls paintComponent
     inkPanel.setPreferredSize(new Dimension(w,h));
          inkPanel.revalidate();
          inkPanel.repaint();
For the record, actionperformed calls setImage on the PaintPanel, setImage calls paintComponent on the paintPanel and paintComponent calls super.paintComponent (those are the 3 lines related to MyFrame in the error log below)
so now, the program starts-up with the grey panel. However when i try to open a new file nothing happens (ie scrollpane still shows the blank panel), and i get the following error:
java.lang.Exception: Stack trace
at java.lang.Thread.dumpStack(Thread.java:1064)
at javax.swing.SwingGraphics.createSwingGraphics(SwingGraphics.java:147)
at javax.swing.JComponent.paintComponent(JComponent.java:539)
at MyFrame$PaintPanel.paintComponent(a3.java:683)
at MyFrame$PaintPanel.setImage(a3.java:677)
at MyFrame.actionPerformed(a3.java:401)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:17
86)
at javax.swing.AbstractButton$ForwardActionEvents.actionPerformed(Abstra
ctButton.java:1839)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel
.java:420)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258
at javax.swing.AbstractButton.doClick(AbstractButton.java:289)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1
113)
at javax.swing.plaf.basic.BasicMenuItemUI$MouseInputHandler.mouseRelease
d(BasicMenuItemUI.java:943)
at java.awt.Component.processMouseEvent(Component.java:5100)
at java.awt.Component.processEvent(Component.java:4897)
at java.awt.Container.processEvent(Container.java:1569)
at java.awt.Component.dispatchEventImpl(Component.java:3615)
at java.awt.Container.dispatchEventImpl(Container.java:1627)
at java.awt.Component.dispatchEvent(Component.java:3477)
at java.awt.LightweightDispatcher.retargetMouseEvent(Container.java:3483
at java.awt.LightweightDispatcher.processMouseEvent(Container.java:3198)
at java.awt.LightweightDispatcher.dispatchEvent(Container.java:3128)
at java.awt.Container.dispatchEventImpl(Container.java:1613)
at java.awt.Window.dispatchEventImpl(Window.java:1606)
at java.awt.Component.dispatchEvent(Component.java:3477)
at java.awt.EventQueue.dispatchEvent(EventQueue.java:456)
at java.awt.EventDispatchThread.pumpOneEventForHierarchy(EventDispatchTh
read.java:201)
at java.awt.EventDispatchThread.pumpEventsForHierarchy(EventDispatchThre
ad.java:151)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:145)
at java.awt.EventDispatchThread.pumpEvents(EventDispatchThread.java:137)
at java.awt.EventDispatchThread.run(EventDispatchThread.java:100)
java.lang.NullPointerException
at javax.swing.JComponent.paintComponent(JComponent.java:544)
at DemoMouseInkFrame$PaintPanel.paintComponent(a3.java:683)
at DemoMouseInkFrame$PaintPanel.setImage(a3.java:677)
at DemoMouseInkFrame.actionPerformed(a3.java:401)
at javax.swing.AbstractButton.fireActionPerformed(AbstractButton.java:17
86)
at javax.swing.AbstractButton$ForwardActionEvents.actionPerformed(Abstra
ctButton.java:1839)
at javax.swing.DefaultButtonModel.fireActionPerformed(DefaultButtonModel
.java:420)
at javax.swing.DefaultButtonModel.setPressed(DefaultButtonModel.java:258
at javax.swing.AbstractButton.doClick(AbstractButton.java:289)
at javax.swing.plaf.basic.BasicMenuItemUI.doClick(BasicMenuItemUI.java:1
113)
at javax.swing.plaf.basic.BasicMenuItemUI$MouseInputHandler.mouseRelease
d(BasicMenuItemUI.java:943)
...it continues but i think this is enough
Any thoughts? as i have no idea what seems to be wrong
Sorry for being so lengthy but i wanted to make sure to explain the situation in detail.
Thank you

Answer 1

Hello,
Have you considered adding your image to a JLabel instead? That way, you won't have to bother at all about overriding paint methods or even calling drawImage yourself.
File f = fc.getSelectedFile();
String filename = f.getAbsolutePath();
image = Toolkit.getDefaultToolkit().createImage(filename);
myScrollPane.setViewportView(new JLabel(new ImageIcon(image));