解决常见的组件问题

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

• 使用其他 Swing 特性解决常见问题
• 解决常见布局问题
• 解决常见的事件处理问题
• 解决常见的绘画问题

Problem:I am having trouble implementing a model (or some other code that is similar to something already in Java SE Platform, Standard Edition).

• Look at the Java SE source code. It is distributed with the JDK, and it is 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 are repainted.

• If you set the content pane, make sure it is 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 is not true in the GTK+ look and feel. 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 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.

• Make sure your code is thread-safe. See Concurrency in Swing for details.

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.
• This bug was fixed in the 6.0 release. For more information, see 4255200.

Problem:The scroll bar policies do not 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 does not move!

• You need to set the minimum size of at least one of the components in the split pane. Refer to Positioning the Divider and Restricting Its Range for information.

Problem:The setDividerLocation method of JSplitPane does not work.

• The setDividerLocation(double) method has no effect if the split pane has no size (typically true if it is not 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.

• If you nest split panes, the borders accumulate — the border of the inner split panes display next to the border of the outer split pane causing borders that look extra wide. The problem is particularly noticeable when nesting many split panes. The workaround is to set the border to null on any split pane that is placed within another split pane. For information, see bug # 4131528 in the Java Bug Database.

Problem:The buttons in my tool bar are too big.

• Try reducing the margin for the buttons. 例如：

  button.setMargin(new Insets(0,0,0,0));
  

Problem: The components in my layered pane are not 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, in the LayeredPaneDemo class, change
  layeredPane.add(label, new Integer(i));
  to
  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());