Java Programing

The reason is that in changeColor( ) we increment colorIndex before testing its value. That means that for some brief period of time while Java is running through our code, colorIndex can have a value that is past the end of our array. If our currentColor( ) method happened to run at that same moment, we would see a runtime “array out of bounds” error. There are, of course, ways in which we could fudge around the problem in this case, but this simple example is representative of more general synchronization issues we need to address. In the next section, you’ll see that Java makes dealing with these problems easy through language-level synchronization support. .4 HelloJava4: Netscape’s Revenge We have explored quite a few features of Java with the first three versions of the HelloJava application. But until now, our application has been rather passive; it has waited patiently for events to come its way and responded to the whims of the user. Now our application is going to take some initiative HelloJava4 will blink! Here is the code for our latest version: //file: HelloJava4.javaimport java.awt.*; import java.awt.event.*; import javax.swing.*; public class HelloJava4extends JComponentimplements MouseMotionListener, ActionListener, Runnable { // Coordinates for the message int messageX = 125, messageY = 95; String theMessage; JButton theButton; int colorIndex; // Current index into someColors. static Color[] someColors = { Color.black, Color.red, Color.green, Color.blue, Color.magenta }; boolean blinkState; public HelloJava4(String message) { theMessage = message; theButton = new JButton(”Change Color”); setLayout(new FlowLayout( )); add(theButton); theButton.addActionListener(this); addMouseMotionListener(this); Thread t = new Thread(this); t.start( ); } public void paintComponent(Graphics g) { g.setColor(blinkState ? getBackground() : currentColor( )); g.drawString(theMessage, messageX, messageY); } public void mouseDragged(MouseEvent e) { messageX = e.getX( ); messageY = e.getY( ); repaint( ); } public void mouseMoved(MouseEvent e) {} - 48

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