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Java AWT

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7.8 Combining Layouts

If you can't create the display you want with any of the standard layout managers, or you are unable to figure out GridBagLayout, you may want to try combining several different layouts. This technique can often help you build the display you want. Figure 7.12 shows a display that uses three panels and three different layouts.

Here's the source code to generate the display in Figure 7.12:

import java.awt.*;
public class multi extends java.applet.Applet {
    public void init() {
        Panel s = new Panel();
        Panel e = new Panel();
        setLayout (new BorderLayout ());
        add ("North", new Label ("Enter text", Label.CENTER));
        add ("Center", new TextArea ());
        e.setLayout (new GridLayout (0,1));
        e.add (new Button ("Reformat"));
        e.add (new Button ("Spell Check"));
        e.add (new Button ("Options"));
        add ("East", e);
        s.setLayout (new FlowLayout ());
        s.add (new Button ("Save"));
        s.add (new Button ("Cancel"));
        s.add (new Button ("Help"));
        add ("South", s);

Figure 7.12: Multipanel screen using several layouts

[Graphic: Figure 7-12]

The display in Figure 7.12 is created by adding four sections to a single BorderLayout. The north region contains a panel with a single Label in it. The panel uses its default LayoutManager, which is a FlowLayout. Why bother with this panel? Why not just add a label at the north position in the BorderLayout? Our strategy gives the label the position and size we want: the label is centered and displayed at its preferred size. If we had added the label directly to the BorderLayout, it would have been left justified and resized to fill the region.

The TextArea has no special requirements, so we added it directly to the center of the BorderLayout.

The three buttons on the right of the screen were arranged in a panel with a GridLayout; then this panel was placed in the east region of the BorderLayout.

To create the buttons at the bottom of the screen, we used another Panel with a FlowLayout. It centers the three buttons and displays them at their preferred size, with a gap between them.

With a little work, we could have created this display using a single Panel with a GridBagLayout. The result would have been more efficient; placing panels within panels has performance implications. Each container in the display has its own peer object, which uses up system resources. Furthermore, in the 1.0 version of AWT, nesting containers complicates event handling. However, using a GridBagLayout would have required much more work: figuring out the right GridBagConstraints for each component would be time consuming and result in code that is harder to understand. Sometimes, it's best to settle for the easy solution: a hybrid layout composed of several simple panels, rather than a single very complex panel.

In Java 1.1, you can make this program even more efficient in its resource usage by using a lightweight component instead of panels. This is particularly easy because the panels in the multipanel screen exist strictly to help with layout and not for partitioning event handling. Therefore, you can define a LightweightPanel that extends Container, with no methods. Use this class instead of Panel. The LightweightPanel allows you to lay out areas without creating unnecessary peers. Here's all the code for the LightweightPanel:

// Java 1.1 only
import java.awt.*;
public class LightweightPanel extends Container {

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