Source

import java.applet.*;
import java.awt.*;

public class AnAppletWithPanels extends Applet {
	public void init() {
		setLayout(new BorderLayout());

		Panel p1 = new Panel();
		p1.setBackground(Color.blue);
		p1.add(new Button("1"));
		p1.add(new Button("2"));
		p1.add(new Button("3"));
		add(p1, BorderLayout.NORTH);

		Panel p2 = new Panel();
		p2.setBackground(Color.yellow);
		p2.add(new Button("4"));
		add(p2, BorderLayout.EAST);

		Panel p3 = new Panel();
		p3.setBackground(Color.green);
		p3.setLayout(new GridLayout(2, 0));		// 2 rows, variable columns
		p3.add(new Button("5"));
		p3.add(new Button("6"));
		p3.add(new Button("7"));
		p3.add(new Button("8"));
		p3.add(new Button("9"));
		add(p3, BorderLayout.CENTER);
	}
}

Description and Objective

• Illustrates how Panels may be used to achieve better control over layout
• Illustrates uses of the GridLayout class

Notes

• A panel is an interface component whose sole use is to act as a container for other components
• Panels provide the ability to group components
  • Placing a Panel in a region of a BorderLayout allows multiple components to be positioned in that region (by adding them to the Panel)
• Like Applet (n fact Applet is a form of Panel), the default layout for Panel is FlowLayout
  • This default can be overridden using Panel's setLayout method

• The GridLayout class provdes a layout consisting of a rectangular grid of regions
  • The number of rows/columns may be specified in the constructor
  • Adding components fills the grid from left-to-right, top-to-bottom, according to the number of rows/columns

This Applet's Behavior

• The applet's layout is set to BorderLayout
• Three panels are created and are placed in various regions of the applet
• Panels p1 and p2 add some buttons to themselves
  • Since no layout manager is explicitly specified, FlowLayout is used
• Panel p3 sets its layout manager to GridLayout with 2 rows and a variable number of columns and adds some buttons
• Each panel sets its background to a different color so the extent of the panel's real estate can be seen
• Note: In addition to allowing further subdivision of the applet's surface, Panels provide a way to avoid the 'stretching' effect of BorderLayout
  • It is the Panel thet is stretched not the contained buttons
    • THis can be seen in the North and East panels
  • The buttons in the center panel are stretched because GridLayout subdivides the ENTIRE surface of the panel into the grid
    • Each grid square is thus fairly large
    • Each button is then stretched to the size of its grid square

Things to Do

• The Panel class
  • What package is it in?
• The GridLayout class
  • What package is it in?
  • Describe the various constructors (i.e., how do they differ)