Using Layout Managers - Java Tutorial 5.0 英文版

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Trail: Creating a GUI with JFC/Swing
Lesson: Laying Out Components Within a Container

Using Layout Managers

A layout manager is an object that implements the LayoutManager interface* and determines the size and position of the components within a container. Although components can provide size and alignment hints, a container's layout manager has the final say on the size and position of the components within the container.
This section discusses some of the common tasks related to using layout managers:

Setting the Layout Manager
Adding Components to a Container
Providing Size and Alignment Hints
Putting Space Between Components
Setting the Container's Orientation
It ends with Tips on Choosing a Layout Manager.

Setting the Layout Manager

As a rule, the only containers whose layout managers you need to worry about are JPanels and content panes. Each JPanel object is initialized to use a FlowLayout, unless you specify differently when creating the JPanel. Content panes use BorderLayout by default. If you don't like the default layout manager that a panel or content pane uses, you're free to change it to a different one.
You can set a panel's layout manager using the JPanel constructor. For example:
JPanel panel = new JPanel(new BorderLayout());
After a container has been created, you can set its layout manager using the setLayout method. For example:
Container contentPane = frame.getContentPane();
contentPane.setLayout(new FlowLayout());
Although we recommend that you use layout managers, you can perform layout without them. By setting a container's layout property to null, you make the container use no layout manager. With this strategy, called absolute positioning, you must specify the size and position of every component within that container. One drawback of absolute positioning is that it doesn't adjust well when the top-level container is resized. It also doesn't adjust well to differences between users and systems, such as different font sizes and locales .

Adding Components to a Container

When you add components to a panel or content pane, the arguments you specify to the add method depend on the layout manager that the panel or content pane is using. For example, BorderLayout requires that you specify the area to which the component should be added, using code like this:
pane.add(aComponent, BorderLayout.PAGE_START);
The how-to section for each layout manager has details on what, if any, arguments you need to specify to the add method. Some layout managers, such as GridBagLayout and SpringLayout, require elaborate setup procedures. Many layout managers, however, simply place components based on the order they were added to their container.
Swing containers other than JPanel and content panes generally provide API that you should use instead of the add method. For example, instead of adding a component directly to a scroll pane (or, actually, to its viewport), you either specify the component in the JScrollPane constructor or use setViewportView. Because of specialized API like this, you don't need to know which layout manager (if any) many Swing containers use. (For the curious: scroll panes happen to use a layout manager named ScrollPaneLayout.)
For information about how to add components to a specific container, see the how-to page for the container. You can find the component how-to pages using A Visual Index to the Swing Components .

Providing Size and Alignment Hints

Sometimes you need to customize the size hints that a component provides to its container's layout manager, so that the component will be laid out well. You can do this by specifying one or more of the minimum, preferred, and maximum sizes of the component. You can invoke the component's methods for setting size hints — setMinimumSize, setPreferredSize, and setMaximumSize. Or you can create a subclass of the component that overrides the appropriate getter methods — getMinimumSize, getPreferredSize, and getMaximumSize. Here is an example of making a component's maximum size unlimited:
component.setMaximumSize(new Dimension(Integer.MAX_VALUE,
                                       Integer.MAX_VALUE));
Many layout managers don't pay attention to a component's requested maximum size. However, BoxLayout and SpringLayout do.
Besides providing size hints, you can also provide alignment hints. For example, you can specify that the top edges of two components should be aligned. You set alignment hints either by invoking the component's setAlignmentX and setAlignmentY methods, or by overriding the component's getAlignmentX and getAlignmentY methods. Although most layout managers ignore alignment hints, BoxLayout honors them. You can find examples of setting the alignment in How to Use BoxLayout.

Putting Space Between Components

Three factors influence the amount of space between visible components in a container:

The layout manager
Some layout managers automatically put space between components; others don't. Some let you specify the amount of space between components. See the how-to page for each layout manager for information about spacing support.
Invisible components
You can create lightweight components that perform no painting, but that can take up space in the GUI. Often, you use invisible components in containers controlled by BoxLayout. See How to Use BoxLayout for examples of using invisible components.
Empty borders
No matter what the layout manager, you can affect the apparent amount of space between components by adding empty borders to components. The best candidates for empty borders are components that typically have no default border, such as panels and labels. Some other components might not work well with borders in some look-and-feel implementations, because of the way their painting code is implemented. For information about borders, see How to Use Borders .

Setting the Container's Orientation

This website is written in English, with text that runs from left to right, and then top to bottom. However, many other languages have different orientations. The componentOrientation property provides a way of indicating that a particular component should use something different from the default left-to-right, top-to-bottom orientation. In a component such as a radio button, the orientation might be used as a hint that the look and feel should switch the locations of the icon and text in the button. In a container, the orientation is used as a hint to the layout manager.
To set a container's orientation, you can use either the Component-defined method setComponentOrientation or, to set the orientation on the container's children as well, applyComponentOrientation. The argument to either method can be a constant such as ComponentOrientation.RIGHT_TO_LEFT, or it can be a call to the ComponentOrientation method getOrientation(Locale). For example, the following code causes all JComponents to be initialized with an Arabic-language locale, and then sets the orientation of the content pane and all components inside it accordingly:
JComponent.setDefaultLocale(new Locale("ar"));
JFrame frame = new JFrame();
...
Container contentPane = frame.getContentPane();
contentPane.applyComponentOrientation(
    ComponentOrientation.getOrientation(
        contentPane.getLocale()));
Here are two pictures showing how FlowLayout lays out components in containers that are exactly the same, except for their orientation.

Default orientation (left-to-right)

Right-to-left orientation
The standard layout managers that support component orientation are FlowLayout, BorderLayout, BoxLayout, GridBagLayout, and GridLayout.

Tips on Choosing a Layout Manager

Layout managers have different strengths and weaknesses. This section discusses some common layout scenarios and which layout managers might work for each scenario. If none of the layout managers we discuss is right for your situation, feel free to use other layout managers that you write or find. Also keep in mind that flexible layout managers such as GridBagLayout and SpringLayout can fulfill many layout needs.

Scenario: You need to display a component in as much space as it can get.
If it's the only component in its container, use GridLayout or BorderLayout. Otherwise, BorderLayout or GridBagLayout might be a good match.
If you use BorderLayout, you'll need to put the space-hungry component in the center. With GridBagLayout, you'll need to set the constraints for the component so that fill=GridBagConstraints.BOTH. Another possibility is to use BoxLayout, making the space-hungry component specify very large preferred and maximum sizes.

Scenario: You need to display a few components in a compact row at their natural size.
Consider using a JPanel to group the components and using either the JPanel's default FlowLayout manager or the BoxLayout manager. SpringLayout is also good for this.

Scenario: You need to display a few components of the same size in rows and columns.
GridLayout is perfect for this.

Scenario: You need to display a few components in a row or column, possibly with varying amounts of space between them, custom alignment, or custom component sizes.
BoxLayout is perfect for this.

Scenario: You need to display aligned columns, as in a form-like interface where a column of labels is used to describe text fields in an adjacent column.
SpringLayout is a natural choice for this. The SpringUtilities class used by several Tutorial examples defines a makeCompactGrid method that lets you easily align multiple rows and columns of components.

Scenario: You have a complex layout with many components.
Consider either using a very flexible layout manager such as GridBagLayout or SpringLayout, or grouping the components into one or more JPanels to simplify layout. If you take the latter approach, each JPanel might use a different layout manager.

* Way back in JDK 1.1 a second interface, LayoutManager2, was introduced. LayoutManager2 extends LayoutManager, providing support for maximum size and alignment. Many layout managers don't use those features, however.

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