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Solving Common Component Problems - Java Tutorial 5.0 英文版

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Trail: Creating a GUI with JFC/Swing
Lesson: Using Swing Components

Solving Common Component Problems

This section discusses problems that you might encounter while using components. If you don't find your problem in this section, consult the following sections:

Problem: I'm having trouble implementing a model (or some other code that's similar to something already in Java 2 Platform, Standard Edition).

  • Look at the J2SE source code. It's distributed with the J2SE SDK, and it's a great resource for finding code examples of implementing models, firing events, and the like.
Problem: Whenever the text in my text field updates, the text field's size changes.
  • You should specify the preferred width of the text field by specifying the number of columns it should have room to display. To do this, you can use either an int argument to the JTextField constructor or the setColumns method.
Problem: Certain areas of the content pane look weird when they're repainted.
  • If you set the content pane, make sure it's opaque. You can do this by invoking setOpaque(true) on your content pane. Note that although JPanels are opaque in most look and feels, that's not true in the GTK+ look and feel, which was introduced in 1.4.2. See Adding Components to the Content Pane for details.
  • If one or more of your components performs custom painting, make sure you implemented it correctly. See Solving Common Painting Problems (in the Creating a GUI with JFC/Swing trail) for help.
  • You might have a thread safety problem. See the next entry.

Problem: My program is exhibiting weird symptoms that sometimes seem to be related to timing.

Problem: My modal dialog gets lost behind other windows.

  • If the dialog has a null parent component, try setting it to a valid frame or component when you create it.
  • See bug # 4255200 (outside of the tutorial) for a possible workaround.

Problem: The scroll bar policies don't seem to be working as advertised.

  • Some Swing releases contain bugs in the implementations for the VERTICAL_SCROLLBAR_AS_NEEDED and the HORIZONTAL_SCROLLBAR_AS_NEEDED policies. If feasible for your project, use the most recent release of Swing.
  • If the scroll pane's client can change size dynamically, the program should set the client's preferred size and then call revalidate on the client.
  • Make sure you specified the policy you intended for the orientation you intended.
Problem: My scroll pane has no scroll bars.
  • If you want a scroll bar to appear all the time, specify either VERTICAL_SCROLLBAR_ALWAYS or HORIZONTAL_SCROLLBAR_ALWAYS for the scroll bar policy as appropriate.
  • If you want the scroll bars to appear as needed, and you want to force the scroll bars to be needed when the scroll pane is created, you have two choices: either set the preferred size of scroll pane or its container, or implement a scroll-savvy class and return a value smaller than the component's standard preferred size from the getPreferredScrollableViewportSize method. Refer to Sizing a Scroll Pane for information.
Problem: The divider in my split pane don't move! Problem: The setDividerLocation method of JSplitPane doesn't work.
  • The setDividerLocation(double) method has no effect if the split pane has no size (typically true if it isn't onscreen yet). You can either use setDividerLocation(int) or specify the preferred sizes of the split pane's contained components and the split pane's resize weight instead. Refer to Positioning the Divider and Restricting Its Range for information.
Problem: The borders on nested split panes look too wide. Problem: The buttons in my tool bar are too big.
  • Try reducing the margin for the buttons. For example: 
    button.setMargin(new Insets(0,0,0,0));
Problem: The components in my layered pane aren't layered correctly. In fact, the layers seem to be inversed — the lower the depth the higher the component.
  • This can happen if you use an int instead of an Integer when adding components to a layered pane. To see what happens, make the following change to LayeredPaneDemo:

    Change this...to this...
    layeredPane.add(label, new Integer(i)); layeredPane.add(label, i);

Problem: The method call colorChooser.setPreviewPanel(null) does not remove the color chooser's preview panel as expected.

  • A null argument specifies the default preview panel. To remove the preview panel, specify a standard panel with no size, like this: colorChooser.setPreviewPanel(new JPanel());
Problem: I can't make HTML tags work in my labels or buttons or... (See Using HTML in Swing Components for an example.)
  • Make sure your program is running in a release that supports HTML text in the desired component. The following table shows which releases support HTML in which components.

    Java 2 Release Corresponding JFC 1.1 Release Status of HTML support
    J2SE v 1.2, v 1.2.1 JFC 1.1 (with Swing 1.1) HTML supported in styled text components only.
    J2SE v 1.2.2 JFC 1.1 (with Swing 1.1.1) HTML support added for JButton, JLabel, JMenuItem, JMenu, JCheckBoxMenuItem, JRadioButtonMenuItem, JTabbedPane, and JToolTip.

    Because table cells and tree nodes use labels to render strings, tables and trees automatically support HTML, as well.

    Java 2 v 1.3 none HTML support added for JToggleButton, JCheckBox, and JRadioButton.

  • If you can't guarantee that your program will be executed only with a release that supports HTML text in the desired component, don't use that feature!


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