Frequently Asked Questions About Java

Answer: You are probably not calling repaint on the correct component. You must call repaint on the component that you want redrawn. For example, if you want a JPanel redrawn, you should call repaint on that JPanel. If you instead try to call repaint on the JFrame containing that JPanel, or on the content pane containing that JPanel, nothing will happen because neither of them will ask their own components to redraw themselves.

Answer: The way I determine the difference is to have the application define a boolean flag which it sets to true when it calls repaint. paintComponent tests this flag and then re-sets it to false. If someone has a better way of doing this, I'd be happy to hear about it.

Answer: Java does strange things with the clip region. Before Java calls paintComponent, it tiles the clip region with a previous image of the window. As far as I can tell, you cannot prevent Java from doing this. Therefore you might as well call super.paintComponent to blank the clipping region, then redraw into that clipping region. In order to control the clipping region that you are given, give a damaged region to the repaint method when you call it. If you don't, Java assumes the clipping region should be the entire component. Java uses the RepaintManager class to collect damaged regions. Each time that your application calls repaint, an instance of the RepaintManager gets called and it folds the dirty region specified by repaint into the existing dirty region for the component. It appears that the RepaintManager can only maintain one region per component. Unfortunately, this means that it is difficult to implement a display manager that only redraws changed objects, because the union of the changed objects' bounding boxes will typically include some undamaged objects. One way to avoid this problem is call paintImmediately rather than repaint but the Java documentation does not suggest doing this. In your sokoban game, the changed objects will always be contiguous so you can get away with drawing only the changed objects. One other trick that you might try but which won't work is to give repaint a tiny damaged region and then try to enlarge the clipping region in paintComponent. Java won't let you enlarge the clipping region, you can only narrow it.

Answer: The documentation for a JScrollPane says that it gets its sizing information from the getPreferredScrollableViewportSize method. This method returns a Dimension so we can return a Dimension object with the desired width. Unfortunately, we may still want the default height. According to the documentation, the default height accommodates 8 rows. In order to calculate the number of pixels needed to display 8 rows, the default implementation of getPreferredScrollableViewportSizemethod uses 16 pixels per row. Hence the default height is 128 pixels and you can get your desired scroll pane using the following code:

JList myList = new JList() {
  public Dimension getPreferredScrollableViewportSize() {
    return new Dimension(desiredWidth, 128);
  }
}

Answer: An instance of an inner class can access instance variables of its enclosing class which means that it needs to have an instance of its enclosing class as a "scope". However, if you try to create an instance of an inner class from another class, Java does not have such an outer object to provide as a scope. Hence only the class containing an inner class can create instances of its inner classes.

Answer: clearRect does not seem to be a reliable way to clear a display area. If you are writing to an off-screen image, which is what you are doing by default since JFrames are double-buffered by default, then clearRect is not guaranteed to be correct, but it appears to fail in other situations as well. The best way to clear an area is to retrieve the background color from the JComponent, set the color to this background color, and then call fillRect. For example:

void paintComponent(Graphics g) {
  ...
  g.setColor(getBackground());
  // this command clears the entire component. To clear a smaller area you
  // would provide your own coordinates and size.
  g.fillRect(0, 0, getWidth(), getHeight());
  ...
}

Answer: Call revalidate on the JComponent or JPanel's whose size has changed if you want to have it re-laid out. If you want to also adjust the window size, then call pack on the component's JFrame. revalidate will recursively traverse up the component hierarchy, invalidating the intervening container objects, until it reaches a container object that knows that its size will not be affected by the change. Most frequently this is the JRootPane itself, although sometimes it can stop earlier, such as at a JTextField. The layout manager will then be invoked on the topmost invalidated container object, and all intervening container objects will also have their layout managers invoked.

Note that revalidation will not cause the window size to change. The layout manager may re-allocate space among a container's components, but it will not change the size of a window. If you want the window to adjust its size, then you need to call the window's pack method.