• Panel, which is only used for grouping components;
• Window, which is, as its name says, for creating and handling windows.

Java distinguishes two kinds of windows:

• dialog windows (e.g. dialogs for opening and saving files), implemented by the class Dialog,
• "ordinary windows," implemented by the class Frame.

Frame

The Applet

The Java Code

FrameApplet.java

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

public class FrameApplet extends Applet {
  Button openButton;
  InfoWindow window;

  public void init() {
    openButton = new Button("New Window");
    add(openButton);
  }

  public boolean action(Event evt, Object arg) {
    if (evt.target == openButton) {
      window = new InfoWindow("A Simple Window", "Example of a Window");
      window.show();
      return true;
    }
    else {
      return false;
    }
  }
}

The part in the Java code that may need explanation is for displaying the separate window. You create a new InfoWindow object, which will be an extension of a Frame, and invoke the show method in the following way:

window = new InfoWindow("A Simple Window", "Example of a Window");
window.show();

SimpleScratchpad.java

import java.awt.*;

public class InfoWindow extends Frame {
  Button closeButton;
  Label text;

  InfoWindow(String title, String message) {
    super(title);                   // create frame with specified title
    setLayout(new BorderLayout());  // set BorderLayout manager
    text = new Label(message);      // create label with specified text
    closeButton = new Button("Close Window"); // create close button
    add("Center", text);            // add label to frame
    add("South", closeButton);      // add button to frame
    pack();                         // resize frame to preferred size of its components
  }

  public boolean action(Event evt, Object arg) {
    if (evt.target == closeButton) {
      dispose();   // remove frame and give up resources of frame
      return true;
    }
    else {
      return false;
    }
  }

  public boolean handleEvent (Event evt) {
    if (evt.id == Event.WINDOW_DESTROY) {
      dispose();
    }
    return super.handleEvent(evt);
  }
}

The above code defines the InfoWindow class as a subclass of Frame. It contains a Button for closing the window and a label for displaying a short message. When the user clicks on the Close Window button, the window should close and release the resources that are used for the window. This is handled in the action method.

If you want the user to be able to close the window via the tools provided by your platform, e.g. via the Exit button in Windows 95, you need to provide a handler for the Window_DESTROY event. This is done in the handleEvent method.

You can go one step further with closing frames. Suppose that you have a Frame that is a root window for you application and that your application should stop when the window is destroyed, then this you should add the following lines of code to the above handleEvent method right after the invocation of dispose.

SecurityManager security = System.getSecurityManager();
if (security == null) {
  System.exit(0);
}

Panel

The Panel class is mainly used for holding components. The most important example of a Panel is an Applet. Components can be laid out within a container in two ways:

• via layout managers
• via absolute positioning

Hierarchy of Components