Java Lecture 5

[Previous Lecture] [Lecture Index] [Next Lecture]

Java Graphical User Interfaces

Java's GUI packages:

java.awt - Classes dealing with visual appearance (buttons, lines, placement, etc.)
java.awt.event - Classes for events: `messages' between user interface components
java.awt.datatransfer - Classes dealing with cut and paste operations
java.awt.image - Classes dealing with image manipulation

Two types of programs use java.awt:

applications: normal program (has main(), etc.) with a GUI.
applets: special: special program, typically runs within a browser.
Uses java.applet package.

Both types use event driven programming model.

HelloApplet.java

import java.applet.Applet;
import java.awt.Graphics;

public class HelloApplet extends Applet {

  public void paint(Graphics g) {
    g.drawString("Hello, World!", 50, 25);
  }

}

HelloApplet.html


<html>
  <head>
    <title>HelloApplet Applet tester</title>
  </head>
  <body>
    A test applet:
    <applet code=HelloApplet width=200 height=200>
      If you see this, you do not have a
      browser that knows about java...
    </applet>
  </body>
</html>

Introduction to Applets

applets `called' from .html file
two ways to run an applet:
- appletviewer: cut down browser - only runs applets
- web browser: e.g., netscape
both look for/use <applet ...>...</applet> in a .html file
fields in <applet> tag:
- code: which class to call (.class suffix not needed). Must be derived from java.applet.Applet class
- codebase: where to look for class, image, sound files (default is where .html file came from)
- archive: name of a .zip or .jar file to extract classes from
- width : defines width of applet's screen area
- height : defines height of applet's screen area
- others: align alt name object hspace vspace
inside <applet>...</applet>:
```
<param name=.. value=...>
```
everything else is ignored if applet is recognized; otherwise it is displayed.

LineDraw1.java

/*
 * <applet code=LineDraw1 width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class LineDraw1
        extends java.applet.Applet
        implements MouseListener
{
  private int nPoints = 0;
  private int x1, y1;
  private int x2, y2;

  public void init() {
    addMouseListener(this);
  }

  public void paint(Graphics g) {
    if (nPoints == 1)
        g.fillRect(x1 - 1, y1 - 1, 3, 3);
    else if (nPoints == 2)
        g.drawLine(x1, y1, x2, y2);
  }

  public void mouseClicked(MouseEvent me) {
    if (nPoints != 1) {
        nPoints = 1;
        x1 = me.getX();
        y1 = me.getY();
    } else {
        nPoints++;
        x2 = me.getX();
        y2 = me.getY();
    }
    repaint();
  }
  public void mouseEntered(MouseEvent me) { }
  public void mouseExited(MouseEvent me) { }
  public void mousePressed(MouseEvent me) { }
  public void mouseReleased(MouseEvent me) { }
}

LineDraw2.java

/*
 * <applet code=LineDraw2 width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class LineDraw2
        extends java.applet.Applet
{
  private int nPoints = 0;
  private int x1, y1;
  private int x2, y2;

  public void paint(Graphics g) {
    if (nPoints == 1)
        g.fillRect(x1 - 1, y1 - 1, 3, 3);
    else if (nPoints == 2)
        g.drawLine(x1, y1, x2, y2);
  }

  public void mouseClicked(MouseEvent me) {
    if (nPoints != 1) {
        nPoints = 1;
        x1 = me.getX();
        y1 = me.getY();
    } else {
        nPoints++;
        x2 = me.getX();
        y2 = me.getY();
    }
    repaint();
  }
}

class HelperClass extends MouseAdapter
{
  HelperClass(LineDraw2 parent) {
      this.parent = parent;
      parent.addMouseListener(this);
  }

  public void mouseClicked(MouseEvent me) {
    parent.mouseClicked(me);
  }

  private LineDraw2 parent;
}

LineDraw3.java

/*
 * <applet code=LineDraw3 width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class LineDraw3
        extends java.applet.Applet
{
  private int nPoints = 0;
  private int x1, y1;
  private int x2, y2;

  public void init() {
    addMouseListener(
        new MouseAdapter() {
          public void mouseClicked(MouseEvent me) {
            if (nPoints != 1) {
                nPoints = 1;
                x1 = me.getX();
                y1 = me.getY();
            } else {
                nPoints++;
                x2 = me.getX();
                y2 = me.getY();
            }
            repaint();
          }
        }
      );
  }

  public void paint(Graphics g) {
    if (nPoints == 1)
        g.fillRect(x1 - 1, y1 - 1, 3, 3);
    else if (nPoints == 2)
        g.drawLine(x1, y1, x2, y2);
  }
}

Intro to AWT Events

General Event Design Pattern

XXXEvent: a class containing information about XXX events
XXXListener: an interface used by classes wanting to receive XXX event
XXXAdapter: a class implementing XXXListener
- makes it easy to define only desired method
- typically used in anonymous inner classes
components that generate XXX events have the methods:
- addXXXListener(XXXListener)
- removeXXXListener(XXXListener)

Commonly Used Listeners/Adapters:

ActionListener: generated by components (buttons, menues, etc.)
MouseListener: generated by mouse clicks, presses, releases, enters and exits
MouseMotionListener: generated by mouse moves and drags
KeyListener: generated by key presses and releases
ComponentListener: generated by component resizes, moves, hides, and shows

AWT Elements

basic drawing: java.awt.Graphics class
composing screens: java.awt
Screens created using:
- Component - individual items that make up interface:
- Container - collections of components:
- layout managers - organize components in containers:
presenting images: java.awt.image class

ButtonTest.java

/*
 * <applet code=ButtonTest width=200 height=200>
 * </applet>
 */
import java.awt.*;
import java.awt.event.*;

public class ButtonTest
        extends java.applet.Applet
        implements ActionListener
{
  Button aButton;
  Button bButton;

  public void init() {
    aButton = new Button("First button");
    bButton = new Button("B");

    add(aButton);
    add(bButton);

    aButton.addActionListener(this);
    bButton.addActionListener(this);
  }

  public void actionPerformed(ActionEvent ae) {
      if (ae.getSource() == aButton)
          System.out.println("First pressed");
      else if (ae.getSource() == bButton)
          System.out.println("B pressed");
  }
}

[Previous Lecture] [Lecture Index] [Next Lecture]