Solving Common Layout Problems

[转载]Solving Common Layout Problems

Note: This lesson covers writing layout code by hand, which can be challenging. If you are not interested in learning all the details of layout management, you might prefer to use the GroupLayout layout manager combined with a builder tool to lay out your GUI. One such builder tool is the NetBeans IDE. Otherwise, if you want to code by hand and do not want to use GroupLayout, then GridBagLayout is recommended as the next most flexible and powerful layout manager.

Problem: How do I specify a component's exact size?

• A handful of the more modern layout managers provide ways to override the size set by the component. Check whether the layout manager you are using allows you to specify component sizes.

   

• Make sure that you really need to set the component's exact size. Each Swing component has a different preferred size, depending on the font it uses and the look and feel. For this reason, it often does not make sense to specify a Swing component's exact size.

   

• If the component is not controlled by a layout manager, you can set its size by invoking the setSize or setBounds method on it. Otherwise, you need to provide size hints and then make sure you are using a layout manager that respects the size hints.

   

• If you extend a Swing component class, you can give size hints by overriding the component's getMinimumSize, getPreferredSize, and getMaximumSize methods. What is nice about this approach is that each getXxxxSize method can get the component's default size hints by invoking super.getXxxxSize(). Then it can adjust the size, if necessary, before returning it. This is particularly handy for text components, where you might want to fix the width, but have the height determined from the content. However, sometimes problems can be encountered with GridBagLayout and text fields, wherein if the size of the container is smaller than the preferred size, the minimum size gets used, which can cause text fields to shrink quite substantially.

   

• Another way to give size hints is to invoke the component's setMinimumSize, setPreferredSize, and setMaximumSize methods.

   

• If you specify new size hints for a component that is already visible, you then need to invoke the revalidate method on it, to make sure that its containment hierarchy is laid out again. Then invoke the repaint method.

Note:  No matter how you specify your component's size, be sure that your component's container uses a layout manager that respects the requested size of the component. The FlowLayout and GridBagLayout managers use the component's preferred size (the latter depending on the constraints that you set), but BorderLayout and GridLayout usually do not. The BoxLayout manager generally uses a component's preferred size (although components can be larger), and is one of the few layout managers that respects the component's maximum size.

Problem: My component does not appear after I have added it to the container.

• You need to invoke revalidate and repaint after adding a component before it will show up in your container.

Problem: My custom component is being sized too small.

• Does the component implement the getPreferredSize and getMinimumSize methods? If so, do they return the right values?
• Are you using a layout manager that can use as much space as is available? See Tips on Choosing a Layout Manager for some tips on choosing a layout manager and specifying that it use the maximum available space for a particular component.