exception java.lang.InterruptedException is never thrown in body of corresponding try statement

[Jon Camilleri]

Any idea why I am getting a compilation error, and, how I could go about it?

/**
   @version 1.32 2004-07-27
   @author Cay Horstmann
*/

import java.awt.*;
import java.awt.event.*;
import java.awt.geom.*;
import java.util.*;
import javax.swing.*;

/**
   Shows an animated bouncing ball.
*/
public class BounceThread
{
   public static void main(String[] args)
   {
      JFrame frame = new BounceFrame();
      frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
      frame.setVisible(true);
   }
}

/**
   A runnable that animates a bouncing ball.
*/
class BallRunnable implements Runnable
{
   /**
      Constructs the runnable.
      @aBall the ball to bounce
      @aPanel the component in which the ball bounces
   */
   public BallRunnable(Ball aBall, Component aComponent)
   {
      ball = aBall;
      component = aComponent;
   }

   public void run()
   {
      try
      {
         for (int i = 1; i <= STEPS; i++)
         {
            ball.move(component.getBounds());
            component.repaint();
            //Thread.sleep(DELAY);
         }
      }
      catch (InterruptedException e)
      {
		  e.printStackTrace();
      }
   }

   private Ball ball;
   private Component component;
   public static final int STEPS = 1000;
   public static final int DELAY = 5;
}

/**
   A ball that moves and bounces off the edges of a
   rectangle
*/
class Ball
{
   /**
      Moves the ball to the next position, reversing direction
      if it hits one of the edges
   */
   public void move(Rectangle2D bounds)
   {
      x += dx;
      y += dy;
      if (x < bounds.getMinX())
      {
         x = bounds.getMinX();
         dx = -dx;
      }
      if (x + XSIZE >= bounds.getMaxX())
      {
         x = bounds.getMaxX() - XSIZE;
         dx = -dx;
      }
      if (y < bounds.getMinY())
      {
         y = bounds.getMinY();
         dy = -dy;
      }
      if (y + YSIZE >= bounds.getMaxY())
      {
         y = bounds.getMaxY() - YSIZE;
         dy = -dy;
      }
   }

   /**
      Gets the shape of the ball at its current position.
   */
   public Ellipse2D getShape()
   {
      return new Ellipse2D.Double(x, y, XSIZE, YSIZE);
   }

   private static final int XSIZE = 15;
   private static final int YSIZE = 15;
   private double x = 0;
   private double y = 0;
   private double dx = 1;
   private double dy = 1;
}

/**
   The panel that draws the balls.
*/
class BallPanel extends JPanel
{
   /**
      Add a ball to the panel.
      @param b the ball to add
   */
   public void add(Ball b)
   {
      balls.add(b);
   }

   public void paintComponent(Graphics g)
   {
      super.paintComponent(g);
      Graphics2D g2 = (Graphics2D) g;
      for (Ball b : balls)
      {
         g2.fill(b.getShape());
      }
   }

   private ArrayList<Ball> balls = new ArrayList<Ball>();
}

/**
   The frame with panel and buttons.
*/
class BounceFrame extends JFrame
{
   /**
      Constructs the frame with the panel for showing the
      bouncing ball and Start and Close buttons
   */
   public BounceFrame()
   {
      setSize(DEFAULT_WIDTH, DEFAULT_HEIGHT);
      setTitle("BounceThread");

      panel = new BallPanel();
      add(panel, BorderLayout.CENTER);
      JPanel buttonPanel = new JPanel();
      addButton(buttonPanel, "Start",
         new ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               addBall();
            }
         });

      addButton(buttonPanel, "Close",
         new ActionListener()
         {
            public void actionPerformed(ActionEvent event)
            {
               System.exit(0);
            }
         });
      add(buttonPanel, BorderLayout.SOUTH);
   }

   /**
      Adds a button to a container.
      @param c the container
      @param title the button title
      @param listener the action listener for the button
   */
   public void addButton(Container c, String title, ActionListener listener)
   {
      JButton button = new JButton(title);
      c.add(button);
      button.addActionListener(listener);
   }

   /**
      Adds a bouncing ball to the canvas and starts a thread
      to make it bounce
   */
   public void addBall()
   {
      Ball b = new Ball();
      panel.add(b);
      Runnable r = new BallRunnable(b, panel);
      Thread t = new Thread(r);
      t.start();
   }

   private BallPanel panel;
   public static final int DEFAULT_WIDTH = 450;
   public static final int DEFAULT_HEIGHT = 350;
   public static final int STEPS = 1000;
   public static final int DELAY = 3;
}

Source Core Java Vol.2 (7th Ed.) written for JDK 1.5.

Error java:52: exception java.lang.InterruptedException is never thrown in body of corresponding try statement catch (InterruptedException e)

[Jesper de Jong]

Read the error message from the compiler carefully.

What do you think it is trying to tell you?

[John Jai]

Were you able to figure out the source of error in the below code?

try  
      {  
         for (int i = 1; i <= STEPS; i++)  
         {  
            ball.move(component.getBounds());  
            component.repaint();  
            //Thread.sleep(DELAY);  
         }  
      }  
      catch (InterruptedException e)  
      {  
          e.printStackTrace();  
      }  

[Jon Camilleri]

John Jai wrote:Were you able to figure out the source of error in the below code?

try  
      {  
         for (int i = 1; i <= STEPS; i++)  
         {  
            ball.move(component.getBounds());  
            component.repaint();  
            //Thread.sleep(DELAY);  
         }  
      }  
      catch (InterruptedException e)  
      {  
          e.printStackTrace();  
      }  

Well is it safe to remove the catch? In any case the 'ball' does not seem to move anywhere with this code, when I remove the catch clause.

[Jesper de Jong]

Jon Camilleri wrote:Well is it safe to remove the catch? In any case the 'ball' does not seem to move anywhere with this code, when I remove the catch clause.

So, let's look at the error message from the compiler:

exception java.lang.InterruptedException is never thrown in body of corresponding try statement catch (InterruptedException e)

Read it carefully and try to understand it. What does it say? It says that in the try-block there isn't any code that can ever throw InterruptedException. In other words, catching InterruptedException is unnecessary here, because it will never be thrown from the try-block.

So, what do you think, is it safe to remove the catch?

[Jon Camilleri]

Jesper de Jong wrote:

Jon Camilleri wrote:Well is it safe to remove the catch? In any case the 'ball' does not seem to move anywhere with this code, when I remove the catch clause.

So, let's look at the error message from the compiler:

exception java.lang.InterruptedException is never thrown in body of corresponding try statement catch (InterruptedException e)

Read it carefully and try to understand it. What does it say? It says that in the try-block there isn't any code that can ever throw InterruptedException. In other words, catching InterruptedException is unnecessary here, because it will never be thrown from the try-block.

So, what do you think, is it safe to remove the catch?

Horstmann reads that the catch is recommended to be there, whilst the compiler thinks that it is not.

[Jesper de Jong]

Since the code in the try-block will never throw InterruptedException, catching this exception is unnecessary, you should remove the catch for that exception. If the code is exactly copied from the book (and not modified in any way) then that is a mistake in the book.

The compiler is always right. (Unless there is a bug in the compiler, but it will be very, very rare to encounter a bug in the compiler).

[John Jai]

just double check that you have not mistakenly commented the below statement which is the cause for the error.

//Thread.sleep(DELAY);