Null pointer exception in button event handler.

[michael wrzos]

Hi, First time on this forum. Wonder if anyone could help me with a null pointer exception. Thanks, Michael

import javax.swing.*;
import java.awt.*;
import java.awt.event.*; 
 
public class PrimeNumbersSwing extends JFrame 
//implements ActionListener
	{
	private static final int WIDTH = 400;
	private static final int HEIGHT = 300;
	//private final static String newline = "\n";
 
	// Declare JFrame components:
    private  JLabel MaxNumberL; 

    private  JTextField MaxNumberTF;
    
    private JTextArea PrimeNumbersTA;

    private  JButton PrimeNumbersB , exitB , clearB;

    private PrimeNumbersButtonHandler pbHandler;
    private ClearButtonHandler  cbHandler;
    private ExitButtonHandler ebHandler;
    private JScrollPane scrollingResult;
    
   // Constructor
    public PrimeNumbersSwing()
    {
		// Set the title and Size:
        setTitle("Maximum Number to Test");
        setSize(WIDTH, HEIGHT);

        // Instantiate the JLabel components:
        MaxNumberL  = new JLabel("Enter the Maximum Number: ", SwingConstants.LEFT);
     
       // Instantiate the JTextFields:H
		MaxNumberTF  = new JTextField(10);

        // Make the JTextArea scrollable:
		PrimeNumbersTA = new JTextArea(10,1);
		scrollingResult = new JScrollPane(PrimeNumbersTA);


		// Instantiate and register the Calculate button for click events:
		PrimeNumbersB = new JButton("Prime Numbers");
        pbHandler = new PrimeNumbersButtonHandler();
        PrimeNumbersB.addActionListener(pbHandler);
        
     // Instantiate and register the Clear button for click events:
        clearB = new JButton("Clear");
        cbHandler = new ClearButtonHandler();
        clearB.addActionListener(cbHandler);

		// Instantiate and register the Exit button for click events:
		exitB = new JButton("Exit");
		ebHandler = new ExitButtonHandler();
        exitB.addActionListener(ebHandler);
        
        // Start to add the components to the JFrame:
        Container pane = getContentPane();
        pane.setLayout(new GridLayout(4, 2));

        pane.add(MaxNumberL);
        pane.add(MaxNumberTF);

        pane.add(scrollingResult);

        pane.add(PrimeNumbersB);
        pane.add(clearB);
        pane.add(exitB);

        // Show the JFrame and set code to respond to the user clicking on the X:
        setVisible(true);
        setDefaultCloseOperation(EXIT_ON_CLOSE);  
        
}//end constructor

    // Calculate Button Event Handler
	   private class PrimeNumbersButtonHandler implements ActionListener
	    {
           public void actionPerformed(ActionEvent e)
	       { 
	    	  int MaxNumber  = Integer.getInteger(MaxNumberTF.getText());
               String strOutput = "";                //initialize 
	    	   boolean isPrime;
	    	   
	    	 for(int i = 2; i < MaxNumber; i++)      //initialize, set limit, add to count
	    	 {
	    	     isPrime = true;
	    	                                   //check to see if the number is prime
	    	 for(int j = 2; j < i ; j++)
	    	 {
	    	    if(i % j == 0)
	    	   {
	    	     isPrime = false;
	    	     
	    	     break;
	    	   } // end if
	    	                                     
	    	 if(isPrime) 
	    	 {
	    		 
	    	    strOutput = strOutput + i + "\n";
	    	   PrimeNumbersTA.append(String.valueOf(strOutput) + "\n");  
	    	 }
	      }  // inner for
	      //Clear Button Event Handler
	     
	    }  // outer for
	    	 //PrimeNumbersTA.append(String.valueOf(strOutput) + "\n"); 
	} // end action performed
           
} // end class 

         private class ClearButtonHandler implements ActionListener
	      { 
	    	  public void actionPerformed(ActionEvent e)
	    	  {  
	             MaxNumberTF.setText("");
 		         PrimeNumbersTA.setText("");	
	 	      } 
	         // Exit Button Event Handler
	      }
         
	      private class ExitButtonHandler implements ActionListener
	      {
	    		      public void actionPerformed(ActionEvent e)
	    		      {
	  	    		    	  System.exit(0);
	                  }
	      }
	      
	   public static void main(String [] args )
       {
		// The only thing this main does is Instantiate the JFrame:
		// Note the name of the object is the name of the class from above.
        PrimeNumbersSwing PrimeNumbersProg = new PrimeNumbersSwing();
        }   
}   

[Mohamed Sanaulla]

Hi michael, Welcome to CodeRanch First things firsts- Please UseCodeTags to post your source code- You can do it by editing your post.That way the code is more readable. Also please TellTheDetails post the stacktrace for the exception. See that you are not using any of the uninitialized references in the event handler.

[Matthew Brown]

The nice thing about NullPointerExceptions is that they're usually pretty straightforward to track down...if you know what line it's happening on.

[Campbell Ritchie]

Mohamed Sanaulla wrote:Hi michael, Welcome to CodeRanch

Agree

. . . Please UseCodeTags to post your source code- . . .

Since he's new I have added the code tags.

Also please TellTheDetails post the stacktrace for the exception. See that you are not using any of the uninitialized references in the event handler.

No, it is a much more subtle error than that

[Campbell Ritchie]

Matthew Brown wrote: . . . usually pretty straightforward to track down... . . .

Who's the incurable optimist then?

There is another error waiting for us whenever we get rid of that Exception.

[Matthew Brown]

Campbell Ritchie wrote:No, it is a much more subtle error than that

Ooh, it is, isn't it?

Michael - you might want to check the Javadocs for the Integer class - and specifically the getInteger method.

[Matthew Brown]

Matthew Brown wrote:and specifically the getInteger method.

Incidentally - is it just me, or is that a ridiculous name for a method that does that?

[Campbell Ritchie]

. . . and some of the logic. It took about 21 seconds ( ) to work out prime numbers under 150, including 143. I didn't know 143 is a prime number. I always thought it was 11 × 13.
I would expect calculating prime numbers up to 150 to take maybe 1 millisecond.

[Campbell Ritchie]

Matthew Brown wrote: . . . is that a ridiculous name for a method that does that?

Close examination of the documentation for that method does not reveal a "since" clause. That suggests this method was introduced in the very earliest stages of development of Java™. There were quite a lot of mistakes made in the earlier stages (Joshau Bloch in Effective Java quotes "public class Stack extends Vector" as another example), and the name of that method was obviously one of them. The only way to correct it at this stage would be to add a @Depracted tag to that method and "@deprecated Use Integer#getIntegerFromPropertyName instead" to its documentation comments.

[michael wrzos]

Thanks for the help. Should have used Int.ParseInteger. Mike

[Campbell Ritchie]

Well done finding the problem. What was happening before was

You were passing the text, maybe "1234"

The System class looks for a system property called "1234"

It can't find anything by that name.

The getInteger method returns null

The JVM attempts to un-box that null, but that is not possible.

The Exception is thrown.

As we said, a fascinating error.

Have you got the finding of prime numbers worked out yet? It should take maybe 1 or 2 milliseconds to iterate through several thousand numbers with a Sieve of Eratosthenes. Something is going very wrong if it takes 21 seconds.

[michael wrzos]

I am still working on taking the output out of the loop. Right now it is outputing each prime several times, hence the slowdown.Thanks Mike

[Campbell Ritchie]

find out about the sieve of Eratosthenes.