How to Use BorderLayout - Java Tutorial 5.0 英文版

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How to Use BorderLayout

Here's a snapshot of an application that uses a BorderLayout.

You can run BorderLayoutDemo using Java^TM Web Start. Its code is in BorderLayoutDemo.java.

As the preceding picture shows, a BorderLayout has five areas. These areas are specified by the BorderLayout constants PAGE_START, PAGE_END, LINE_START, LINE_END, and CENTER.

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Version note: Before 1.4, the preferred names for the various areas were different, ranging from points of the compass (for example, BorderLayout.NORTH for the top area) to wordier versions of the constants we use in our examples. The constants our examples use are preferred because they are standard and enable programs to adjust to languages that have different orientations.

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If you enlarge the window, the center area gets as much of the available space as possible. The other areas expand only as much as necessary to fill all available space. Often, a container uses only one or two of the areas of the BorderLayout — just the center, or center and bottom, for example.

The following code adds components to a frame's content pane. Because content panes use BorderLayout by default, the code doesn't need to set the layout manager. You can find the whole program in BorderLayoutDemo.java.

...//Container pane = aFrame.getContentPane()...
JButton button = new JButton("Button 1 (PAGE_START)");
pane.add(button, BorderLayout.PAGE_START);

//Make the center component big, since that's the
//typical usage of BorderLayout.
button = new JButton("Button 2 (CENTER)");
button.setPreferredSize(new Dimension(200, 100));
pane.add(button, BorderLayout.CENTER);

button = new JButton("Button 3 (LINE_START)");
pane.add(button, BorderLayout.LINE_START);

button = new JButton("Long-Named Button 4 (PAGE_END)");
pane.add(button, BorderLayout.PAGE_END);

button = new JButton("5 (LINE_END)");
pane.add(button, BorderLayout.LINE_END);

We strongly recommend that you specify the component's location (for example, BorderLayout.LINE_END) as one of the arguments to the add method. If you leave it out, the component will be added to the center, but your code will be much less clear. If you find that a component is missing from a container controlled by a BorderLayout, make sure that you specified the component's location and that you didn't put another component in the same location.

All our examples that use BorderLayout specify the component as the first argument to the add method. For example:

add(component, BorderLayout.CENTER)  //preferred

However, you might see code in other programs that specifies the component second. For example, here are alternate ways of writing the preceding code:

add(BorderLayout.CENTER, component)  //valid but old fashioned
    or
add("Center", component)             //valid but error prone

The BorderLayout API

BorderLayout defines a couple of constructors and some methods for adding space between components.

[PENDING: The following will be converted to be in the Tutorial's standard API table format.]

By default, a BorderLayout puts no gap between the components it manages. In the preceding applet, any apparent gaps are the result of the buttons reserving extra space around their apparent display area. You can specify gaps (in pixels) using the following constructor:

BorderLayout(int horizontalGap, int verticalGap)

You can also use the following methods to set the horizontal and vertical gaps, respectively:

void setHgap(int)
void setVgap(int)

Examples that Use BorderLayout

Here are a few of the many examples that use BorderLayout.

Example	Where Described	Notes
BorderLayoutDemo	This page	Puts a component in each of the five possible locations.
TabbedPaneDemo	How to Use Tabbed Panes	One of many examples that puts a single component in the center of a content pane, so that the component is as large as possible.
CheckBoxDemo	How to Use Check Boxes	Creates a JPanel that uses a BorderLayout. Puts components into the left (actually, LINE_START) and center locations.

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