java addObserver generates NullPointerException with an observer that is not null

Question

I am trying to use the observer pattern in my GUI, and I get a NullPointerException even though the observer I am trying to add is definitely not null. here is the relevant extract of code:

    private class AuditButtonActionListener implements ActionListener {
    DirectoryAuditingProcessor directoryAuditingProcessor;
    DirectoryAuditingAgentObserver directoryAuditingAgentObserver;

    AuditButtonActionListener(){
        directoryAuditingAgentObserver = new DirectoryAuditingAgentObserver();
        directoryAuditingProcessor = new DirectoryAuditingProcessor();
        directoryAuditingProcessor.getDirectoryAuditingAgent().addObserver(directoryAuditingAgentObserver);
    }

    public DirectoryAuditingProcessor getDirectoryAuditingProcessor() {
        return directoryAuditingProcessor;
    }

    public void actionPerformed(ActionEvent ev){

        directoryAuditingProcessor.process(Paths.get(selectedSourcePathTextField.getText()),null);
        textAreaOutput.setText(directoryAuditingProcessor.getDirectoryAuditingAgent().
                                            getDirectoryVisitingAgent().getShaOneHashAndFileProperties().toString());
    }
}

class DirectoryAuditingAgentObserver implements Observer {
    @Override
    public void update(Observable o, Object message) {
        textAreaOutput.append((String)message);
    }
}

and the DirectoryAuditingAgent class does extends Observable. Thank you in advance for your help

In the future you will want to search on the exception before asking. Had you done this, you'd already know how important the stacktrace is, that it identifies the line that throws the exception, and that you should test the variables on that line to see which is null. — DontKnowMuchBut Getting Better, Feb 17 '18 at 14:38
And yes, something **is** null, but you (and we) don't know which variable is yet. — DontKnowMuchBut Getting Better, Feb 17 '18 at 14:38
My *guess* is that this: `getDirectoryAuditingAgent()` returns a null. But who the heck knows? Another suggestion -- do-chain method calls when debugging so that you know where the problem is. — DontKnowMuchBut Getting Better, Feb 17 '18 at 14:42
Thank you @DontKnowMuchButGettingBetter you are right, the problem came from getDirectoryAuditingAgent(). — abass.mahdavi, Feb 17 '18 at 14:52
Next suggestion/lesson: You state, ".... is not null". Never assume what is or isn't null, but *always* **test**. Glad you've identified the problem, next hopefully you'll be able to fix it. — DontKnowMuchBut Getting Better, Feb 17 '18 at 14:55

java addObserver generates NullPointerException with an observer that is not null

0 Answers0