Java Applet NullPointerException

Question

I'm working on an Applet, which has a JButton that I want to use to enable another JButton. However, when I press the button, I get the error: Exception in thread "AWT-EventQueue-1" java.lang.NullPointerException. Why is this happening? It seems as though when I run the Applet, the global variables don't get instantiated (i.e. they are all "null"). In another program, everything works fine, and I can't find any difference between the two in terms of implementing this action.

Here is a bit of my code:

public class implementation2 extends Applet implements ActionListener {
  private static final long serialVersionUID = -4370650602318597069L;
  ...
  public JButton coreButton, testButton;
  ...
  public void init() {
    ...
    final JButton testButton = new JButton("Test);
    testButton.addActionListener(this);
    ...
    final JButton coreButton = new JButton("CORE");
    coreButton.addActionListener(this);
    coreButton.setEnabled(false);
    ...
  }
  ...
  public void actionPerformed(final ActionEvent event) {

  if(event.getActionCommand() == "Test") {
    coreButton.setEnabled(false);
  }
  ...

If anyone can point me in the direction towards fixing my code, that would be greatly appreciated! Thank you!

You should have a stack trace - look at that to pinpoint what's going on. You should also start using normal Java naming conventions (classes should be in PascalCase), and compare strings using equals() instead of ==. — Jon Skeet
– Jon Skeet, Commented Apr 8, 2011 at 11:14

Jon Skeet · Accepted Answer · 2011-04-08 11:15:53Z

1

This is the problem:

public JButton coreButton, testButton;

public void init() {
  final JButton testButton = new JButton("Test);

Here you've created a local variable which shadows the instance variable for testButton (and the same for coreButton). That means the instance variables are still null - so when you try to dereference them later, you get an exception. You don't want to declare new local variables in init - you just want to assign the values to the instance variables. Corrected code:

public class Implementation2 extends Applet implements ActionListener {
  private static final long serialVersionUID = -4370650602318597069L;
  ...
  public JButton coreButton, testButton;
  ...
  public void init() {
    ...
    testButton = new JButton("Test");
    testButton.addActionListener(this);
    ...
    coreButton = new JButton("CORE");
    coreButton.addActionListener(this);
    coreButton.setEnabled(false);
    ...
  }
  ...
  public void actionPerformed(final ActionEvent event) {
    if("Test".equals(event.getActionCommand())) {
      coreButton.setEnabled(false);
    }
    ...
  }
}

answered Apr 8, 2011 at 11:15

Jon Skeet

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1 Comment

TNC Over a year ago

Ah I see. I thought that it looked sketchy, but I'm largely reusing code (permissively, of course) so it can get messed up rather easily. Thank you very much!

Harry Joy · Accepted Answer · 2011-04-08 11:16:23Z

When you are declaring them global then why again declare them in init() in init() just write:

 public void init() {
    ...
    testButton = new JButton("Test");
    testButton.addActionListener(this);
    ...
    coreButton = new JButton("CORE");
    coreButton.addActionListener(this);
    coreButton.setEnabled(false);
    ...
  }

Possible bugs in your code:

public class implementation2 extends Applet implements ActionListener {
  private static final long serialVersionUID = -4370650602318597069L;
  ...
  public JButton coreButton, testButton;
  ...
  public void init() {
    ...
    final JButton testButton = new JButton("Test);  //---- Duplicate declaration which should not be done.
    //---- Forgot to write `"` to finish `Test` string
    testButton.addActionListener(this);
    ...
    final JButton coreButton = new JButton("CORE");  //---- Duplicate declaration which should not be done.
    coreButton.addActionListener(this);
    coreButton.setEnabled(false);
    ...
  }
  ...
  public void actionPerformed(final ActionEvent event) {

  if(event.getActionCommand() == "Test") { //--- use `.equals()` instead of `==`
    coreButton.setEnabled(false); //---- set it to `true` instead of `false`.
  }

Ingo · Accepted Answer · 2011-04-08 11:17:12Z

0

In init(), you make 2 local buttons that hide the outer ones, therefore, they are still null after init().

answered Apr 8, 2011 at 11:17

Ingo

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