Null Pointer Exception

[Adam Polak]

I have a null pointer exception when pressing on a button in the following applet, can someone help me out with this:

import java.awt.* ;
import java.applet.Applet ;
import java.awt.event.* ;
import java.util.StringTokenizer ;
 
public class Crypto extends Applet implements ActionListener
{
  private Button codeerKnop , decodeerKnop ;
  private TextField invoerVeld , gecodeerdVeld , gedecodeerdVeld ;
  public Cipher c1 ;
  public Reverse r1 ;
  public Caesar ces1 ;
  
  public void init()
  {
    setLayout( null ) ;
    resize( 300 , 250 ) ;
    
    codeerKnop = new Button( "Codeer" ) ;
    add( codeerKnop ) ;
    codeerKnop.addActionListener( this ) ;
    codeerKnop.setBounds( 50 , 50 , 180 , 20 ) ;
    
    decodeerKnop = new Button( "Decodeer" ) ;
    add( decodeerKnop ) ;
    decodeerKnop.addActionListener( this ) ;
    decodeerKnop.setBounds( 50 , 80 , 180 , 20 ) ;
    
    invoerVeld = new TextField() ;
    add( invoerVeld ) ;
    invoerVeld.setBounds( 50 , 110 , 180 , 20 ) ;
    
    gecodeerdVeld = new TextField() ;
    add( gecodeerdVeld ) ;
    gecodeerdVeld.setBounds( 50 , 140 , 180 , 20 ) ;
    gecodeerdVeld.setEditable( false ) ;
    
    gedecodeerdVeld = new TextField() ;
    add( gedecodeerdVeld ) ;
    gedecodeerdVeld.setBounds( 50 , 170 , 180 , 20 ) ;
    gedecodeerdVeld.setEditable( false ) ;
  }
  
  public void actionPerformed( ActionEvent event )
  {
    if ( event.getSource() == codeerKnop )
    {
      String codeerString = ( r1.encode( invoerVeld.getText() ) ) ;
      gecodeerdVeld.setText( codeerString ) ;
    }
    
    if ( event.getSource() == decodeerKnop )
    {
      String decodeerString = ( r1.decode( invoerVeld.getText() ) ) ;
      gedecodeerdVeld.setText( decodeerString ) ;
    }
  }
  
  public void paint( Graphics g )
  {
  }
}
 
abstract class Cipher
{
  public String encrypt( String s )
  {
    String result = "" ;
    StringTokenizer words = new StringTokenizer( s ) ;
    
    while ( words.hasMoreTokens() )
    {
      result = result + encode( words.nextToken() ) + " " ;
    }
    
    return result ;
  }
  
//public String decrypt(String s){
//}
  
  public abstract String encode( String word ) ;
  
  public abstract String decode( String word ) ;
}
 
class Reverse extends Cipher
{
  public String encode( String word )
  {
    StringTokenizer reverseString = new StringTokenizer( word , "" ) ;
    return word ;
  }
  
  public String decode( String word )
  {
    return word ;
  }
}
 
class Caesar extends Cipher
{
  public String encode( String word )
  {
    return word ;
  }
  
  public String decode( String word )
  {
    return word ;
  }
}

[ edited to preserve formatting using the [code] and [/code] UBB tags -ds ]
[ March 24, 2003: Message edited by: Dirk Schreckmann ]

[Steve Luke]

Originally posted by Adam Polak:
I have a null pointer exception when pressing on a button in the following applet, can someone help me out with this:

public void actionPerformed(ActionEvent event){

if (event.getSource()== codeerKnop){

String codeerString = ( r1.encode( invoerVeld.getText() ) );
gecodeerdVeld.setText(codeerString);

}

if (event.getSource()== decodeerKnop){

String decodeerString = ( r1.decode( invoerVeld.getText() ) );
gedecodeerdVeld.setText(decodeerString);

}

}

You are trying to refernce an object of a String that has not been created. Likely candidates are:
r1 when you do r1.encode or r1.decode.
The text returned from invoerVeld, when you do getTest() it can return a null.
Look at the error. It should give you a line number. Go to that line and see which object is not yet defined (I would guess r1). If it is r1, remeber you have to initialize r1 with r1=new Reverse() (or whatever it is) somewhere BEFORE you ever use it. You can check the text by enclosing all the actionPerformed statements into an:
if (invoerVeld != null) {
block.