Java MalformedURLException 被捕获

Question

该程序假设调用以下 URL http://www.google.com当选择退出时，所有其他按钮都工作正常，但是当我单击“退出”时没有任何反应。
我还得到:

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

请帮忙

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

关于警告:“无法访问的 catch 子句 catch( IOException iox )” 该 try block 中没有任何内容抛出 IOException。 URL 构造函数抛出 MalformedURLException 并且您正在捕获它。 IOException 的 catch block 不是必需的，并且永远不会执行(即它无法访问)。

顺便说一句，MalformedURLException 扩展了 IOException。