Solving Common Painting Problems - Java Tutorial 5.0 英文版

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Solving Common Painting Problems

Problem: I don't know where to put my painting code.

• Painting code belongs in the paintComponent method of any component descended from JComponent. See How Swing Components Are Displayed for details.

Problem: The stuff I paint doesn't show up.

• Check whether your component is showing up at all. Solving Common Component Problems should help you with this.
• Check whether repaint is invoked on your component whenever its appearance needs to be updated.

Problem: The background of my applet shows up, but the foreground stuff doesn't show up.

• Did you make the mistake of performing painting directly in a JApplet subclass? If so, then your contents will be covered by the content pane that is automatically created for every JApplet instance. Instead, create another class that performs the painting and then add that class to the JApplet's content pane. See How Swing Components Are Displayed for more information on how painting in Swing works.

Problem: My component's foreground shows up, but its background is invisible. The result is that one or more components directly behind my component are unexpectedly visible.

• Make sure your component is opaque. JPanels, for example, are opaque by default in many but not all look and feels. To make components such as JLabels and GTK+ JPanels opaque, you must invoke setOpaque(true) on them.
• If your custom component extends JPanel or a more specialized JComponent descendant, then you can paint the background by invoking super.paintComponent before painting the contents of your component.
• You can paint the background yourself using this code at the top of a custom component's paintComponent method:

  g.setColor(getBackground());
  g.fillRect(0, 0, getWidth(), getHeight());
  g.setColor(getForeground());
  

Problem: I used setBackground to set my component's background color, but it seemed to have no effect.

• Most likely, your component isn't painting its background, either because it's not opaque or your custom painting code doesn't paint the background. If you set the background color for a JLabel, for example, you must also invoke setOpaque(true) on the label to make the label's background be painted. For more help, see the preceding problem.

Problem: I'm using the exact same code as a tutorial example, but it doesn't work. Why?

• Is the code executed in the exact same method as the tutorial example? For example, if the tutorial example has the code in the example's paintComponent method, then this method might be the only place where the code is guaranteed to work.

Problem: How do I paint thick lines? patterns?

• The Java^TM 2D API provides extensive support for implementing line widths and styles, as well as patterns for use in filling and stroking shapes. See the 2D Graphics trail for more information on using the Java 2D API.

Problem: The edges of a particular component look odd.

• Because components often update their borders to reflect component state, you generally should avoid invoking setBorder except on JPanels and custom subclasses of JComponent.
• Is the component painted by a look and feel such as GTK+ or Windows XP that uses UI-painted borders instead of Border objects? If so, don't invoke setBorder on the component.
• Does the component have custom painting code? If so, does the painting code take the component's insets into account?

Problem: Visual artifacts appear in my GUI.

• If you set the background color of a component, be sure the color has no transparency if the component is supposed to be opaque.
• Use the setOpaque method to set component opacity if necessary. For example, the content pane must be opaque, but components with transparent backgrounds must not be opaque.
• Make sure your custom component fills its painting area completely if it's opaque.

Problem: The performance of my custom painting code is poor.

• If you can paint part of your component, use the getClip or getClipBounds method of Graphics to determine which area you need to paint. The less you paint, the faster it will be.
• If only part of your component needs to be updated, make paint requests using a version of repaint that specifies the painting region. An example of doing this is in the updateSize method in SelectionDemo.java.
• For help on choosing efficient painting techniques, look for the string "performance" in the Java 2D API home page.

Problem: The same transforms applied to seemingly identical Graphics objects sometimes have slightly different effects.

• Because the Swing painting code sets the transform (using the Graphics method translate) before invoking paintComponent, any transforms that you apply have a cumulative effect. This doesn't matter when doing a simple translation, but a more complex AffineTransform, for example, might have unexpected results.

If you don't see your problem in this list, see Solving Common Component Problems and Solving Common Layout Problems.

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