October 30th, 2013
Graphical Applications and Frame Windows
A graphical application shows information inside a frame: a window with a title bar.
To show a frame, construct a JFrame object, set its size, and make it visible
import javax.swing.JFrame;
public class EmptyFrameViewer
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
frame.setSize(300, 400);
frame.setTitle("An Empty Frame");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
}
}
The JFrame class is a part of the javax.swing package. Swing is the nickname for the graphical user interface library in Java. The “x” in javax denotes the fact that Swing started out as a Java extension before it was added to the standard library.
In order to display a drawing in a frame, declare a class that extends the JComponent class.
Place drawing instructions inside the paintComponent method. That method is called whenever the component needs to be repainted.
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JComponent;
/*
A component that draws two rectangles.
*/
public class RectangleComponent extends JComponent
{
public void paintComponent(Graphics g)
{
// Recover Graphics2D
Graphics2D g2 = (Graphics2D) g;
// Construct a rectangle and draw it
Rectangle box = new Rectangle(5, 10, 20, 30);
g2.draw(box);
// Move rectangle 15 units to the right and 25 units down
box.translate(15, 25);
// Draw moved rectangle
g2.draw(box);
}
}
When the component is shown for the first time, the paintComponent method is called automatically.
The method is also called when the window is re-sized, or when it is shown again after it was hidden.
The Graphics class is primitive. The designers of Java created the Graphics2D class, which extends the Graphics class. Whenever the Swing toolkit calls the paintComponent method, it actually passes a parameter of type Graphics2D
import javax.swing.JFrame;
public class RectangleViewer
{
public static void main(String[] args)
{
JFrame frame = new JFrame();
frame.setSize(300, 400);
frame.setTitle("Two rectangles");
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
RectangleComponent component = new RectangleComponent();
frame.add(component);
frame.setVisible(true);
}
}
Applets have two advantages. They don’t need separate component and viewer classes; you only implement a single class. And, more importantly, applets run inside a web browser, allowing you to place your creations on a web page for all the world to admire.
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import javax.swing.JApplet;
/*
An applet that draws two rectangles.
*/
public class RectangleApplet extends JApplet
{
public void paint(Graphics g)
{
// Prepare for extended graphics
Graphics2D g2 = (Graphics2D) g;
// Construct a rectangle and draw it
Rectangle box = new Rectangle(5, 10, 20, 30);
g2.draw(box);
// Move rectangle 15 units to the right and 25 units down
box.translate(15, 25);
// Draw moved rectangle
g2.draw(box);
}
}
Here is the simplest possible file to display the rectangle applet:
If you know HTML, you can proudly explain your creation, by adding text and more HTML tags:
Two rectangles Here is my first applet:
To run the applet, you have two choices. You can use the applet viewer, a program that is included with the Java Software Development Kit from Sun Microsystems. You simply start the applet viewer, giving it the name of the HTML file that contains your applets:
appletviewer RectangleApplet.html
The applet viewer only shows the applet, not the HTML text (see the left figure).
You can also show the applet inside any Java-enabled web browser, such as Firefox or Safari. (If you use Internet Explorer, you probably need to configure it. By default, Microsoft supplies either an outdated version of Java or no Java at all. Go to the java.com web site and install the Java plugin.) The second figure shows the applet running in a browser. As you can see, both the text and the applet are displayed.
Drawing Text:
You often want to put text inside a drawing, for example, to label some of the parts. Use the drawString method of the Graphics2D class to draw a string anywhere in a window. You must specify the string and the x- and y-coordinates of the basepoint of the first character in the string. For example,
g2.drawString("Message", 50, 100);
Colors
Color magenta = new Color(255, 0, 255);
import java.awt.Color;
import java.awt.Graphics;
import java.awt.Graphics2D;
import java.awt.Rectangle;
import java.awt.geom.Ellipse2D;
import java.awt.geom.Line2D;
import javax.swing.JComponent;
/*
A component that draws an alien face
*/
public class FaceComponent extends JComponent
{
public void paintComponent(Graphics g)
{
// Recover Graphics2D
Graphics2D g2 = (Graphics2D) g;
// Draw the head
Ellipse2D.Double head = new Ellipse2D.Double(5, 10, 100, 150);
g2.draw(head);
// Draw the eyes
g2.setColor(Color.GREEN);
Rectangle eye = new Rectangle(25, 70, 15, 15);
g2.fill(eye);
eye.translate(50, 0);
g2.fill(eye);
// Draw the mouth
Line2D.Double mouth = new Line2D.Double(30, 110, 80, 110);
g2.setColor(Color.RED);
g2.draw(mouth);
// Draw the greeting
g2.setColor(Color.BLUE);
g2.drawString("Hello, World!", 5, 175);
}
}
Exercise 1: In this exercise, you will explore a simple way of visualizing a Rectangle object. The setBounds method of the JFrame class moves a frame window to a given rectangle. Complete the following program to visually show the translate method of the Rectangle class:
import java.awt.Rectangle;
import javax.swing.JFrame;
import javax.swing.JOptionPane;
public class TranslateDemo
{
public static void main(String[] args)
{
// Construct a frame and show it
JFrame frame = new JFrame();
frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
frame.setVisible(true);
// Your work goes here:
// Construct a rectangle and set the frame bounds
JOptionPane.showMessageDialog(frame, "Click OK to continue");
// Your work goes here:
// Move the rectangle and set the frame bounds again
}
}