Question

the program suppose to invoke the following URL http://www.google.com When exit is selected,
all the other buttons are working fine but when I click on “exit” nothing happens.
I also get:

warning: unreachable catch clause
    catch( IOException iox ) {
    ^
thrown type MalformedURLException has already been caught

Please help

import java.awt.*;
import java.lang.*;
import java.applet.*;
import java.util.Vector;
import java.io.IOException;
import java.net.*;
import java.net.MalformedURLException;
import java.applet.Applet.*;

public class Stage extends Canvas implements Runnable
{
public class Stage2 extends Applet
{

        public Stage2() {};


    }
Stage2 stage2= new Stage2();


Graphics offGraphics = null;
Image    offImage;



Thread   conductor;

Ball     balls[];
int      numBalls;
int      numBallsAllocated;

int      width;
int      height;

int      sleepy = 5;


// ----- constructor
public Stage( int width,int height )  {
this.width  = width;
this.height = height;
    setBackground( Color.black );
    numBalls = 0;
    numBallsAllocated = 10;
    balls = new Ball[numBallsAllocated];
    conductor = null;
} // end of Stage constructor



//----- methods for setting and maintaining the size of the canvas

public Dimension preferredSize() {
return( new Dimension( width,height ));
} // end of preferredSize()

public Dimension minimumSize() {
return( new Dimension( width,height ));
} // end of minimumSize()



//----- methods for the Bounce applet object to call

public void start() {
    if ( conductor == null ) {
        conductor = new Thread(this, "Stage");
        conductor.start();
    }
else {
        for ( int i = 0; i < numBalls; i++ ) {
            balls[i].start();
        }
        conductor.resume();
    }
} // end of start()

public void stop() {
    for( int i = 0; i < numBalls; i++ ) {
        balls[i].stop();
    }
    conductor.suspend();
} // end of stop()

public void addBall() {
Color color = chooseColor( numBalls );
Ball ball = new Ball( "Ball "+(numBalls+1),color,this,sleepy );
System.out.println( "here "+ball.toString() );
// enlarge ball array if necessary.
if ( numBalls == numBallsAllocated ) {
    Ball newBalls[];
    numBallsAllocated *= 2;
    newBalls = new Ball[numBallsAllocated];
    System.arraycopy( balls,0,newBalls,0,numBalls );
    balls = newBalls;

}
balls[numBalls] = ball;
numBalls++;
ball.start();

} // end of addBall()



//----- methods for conductor thread to run

public void run() {
    while ( true ) {
        repaint();
        try {
            Thread.sleep( sleepy );
        }
    catch ( InterruptedException ix ) {
            break;
        }
    }
} // end of run()

public void faster() {
if ( sleepy > 0 ) {
    sleepy--;
}
for ( int i=0; i<numBalls; i++ ) {
    balls[i].setSleepy( sleepy );
}
System.out.println( "faster... " + sleepy );
} // end of faster()

public void slower() {
sleepy++;
for ( int i=0; i<numBalls; i++ ) {
    balls[i].setSleepy( sleepy );
}
System.out.println( "slower... " + sleepy );
} // end of slower()

public void exit()
{
    try {
      URL url = new URL( "http://www.google.com" );
      stage2.getAppletContext().showDocument( url );
    }
    catch( MalformedURLException murlx ) {
    }
    catch( IOException iox ) {
    }
} // end of exit()

// we have overridden update() instead of paint() since the
// background does not need to be cleared when doing double
// buffering.
public synchronized void update( Graphics g ) {
if ( offGraphics == null ) {
    offImage = createImage( width,height );
    offGraphics = offImage.getGraphics();
}
offGraphics.setColor( getBackground() );
offGraphics.fillRect( 0,0,width,height );
for (int i = 0; i < numBalls; i++) {
    balls[i].paint( offGraphics );
}
g.drawImage( offImage, 0, 0, this );
} // end of update()



//----- private methods.

private Color chooseColor( int i ) {
    switch (i % 5) {
        case 0: return Color.white;
        case 1: return Color.red;
        case 2: return Color.blue;
        case 3: return Color.green;
        case 4: return Color.yellow;
    }
    // Not reached
    return Color.white;
} // end of chooseColor()



 } // end of Stage class

Answer 1

Others have adequately explained the cause of the compilation error.

---

I’ve some comments to make first, then some suggestions on how to diagnose the remaining problem:

1) It seems that you were running an application that had compilation errors. Some IDE’s will let you do this, but it is a somewhat dangerous thing to do. It is much safer to turn off this option and only run code that compiles.

IDE’s (specifically Eclipse) deals with methods that don’t compile by generating method code that throws an unchecked exception saying there was a compilation error. If you call such a method from the main thread, you’ll get a stack trace. If you call it from a child thread, you may not see a stack trace … depending on whether the thread has an “uncaught exception handler”. I suspect that that was happening here!

Morals:

• Fix your compilation errors before you run your code.
• Install a default uncaught exception handler so that you get a stacktrace for any Thread that dies due to an unchecked exception. (This is a bit advanced for where you are currently at, but try and remember it for later on.)

2) Your catch block for MalformedURLException is squashing exceptions. That is, it is catching that exception, saying nothing about it, and then proceeding as if nothing bad had happened. In this case, you need to know if that exception was throw because it means that you’ve got a bug in your program; i.e. the hard-wired URL is incorrect.

Morals:

• Don’t squash unexpected exceptions. Any unexpected exception should at least be logged. (An unchecked exception can also be allowed to propagate. If you have a checked exception that you don’t want to handle at that point, you can either declare it in method signature, or wrap it in an unchecked exception, though the former approach is usually better.)
• If you decide that an exception is expected and doesn’t need to be reported, put a comment in the catch block to explain what is going on.

---

Here’s what I think you should do to move forward:

1) Fix the compilation errors. (You’ve done this I gather)

2) Add some code to the catch clause to (at a minimum) send a stacktrace to the console.

If that doesn’t work then:

3a) Run the code in a debugger

or

3b) Add some traceprints, and temporarily add catch (Throwable ex) {ex.printStackTrace();} to see if there is some other unchecked exception being thrown.

There are a number of possible causes of your observed “nothing happens”, and you need to figure out which of the possible causes is the actual cause.