Exception in thread "Thread-4" java.lang.NullPointerException in call the constructor

Question

I am building a midi piano in java. In this case, I want to play pitches ,record pitches and playback them.

Then I used java thread to add pitch to queue and add delay to queue. I used thread object to these process and call it in constructor. After the call this constructor it gives NullPointException.

Can you help me found out why?

public PianoPlayer() throws MidiUnavailableException{
     queue = new LinkedBlockingQueue<NoteEvent>();  
     delayQueue = new LinkedBlockingQueue<NoteEvent>();
     machine = new PianoMachine(this);
     processQueue.start();
     processDelayQueue.start();
}

Thread processDelayQueue = new Thread() {
    public void run(){
        while(true){
            if(queue.isEmpty()){
            }
            else{
                try {
                    NoteEvent e=queue.take();
                    midi.Midi.wait(100);
                    queue.put(e);            
                } catch (InterruptedException ex) {
                    Logger.getLogger(PianoPlayer.class.getName()).log(Level.SEVERE, null, ex);
                }
            }
        }
   }
};

Where do you initialize processDelayQueue? – matt Oct 28 '18 at 11:28 — matt, Oct 28 '18 at 11:28
Actually I should have said processQueue. – matt Oct 28 '18 at 12:24 — matt, Oct 28 '18 at 12:24

score 0 · Answer 1 · answered Oct 28 '18 at 12:33

Check if processQueue and processDelayQueue are not null. NullPointerException is usually caused when you try to call a method or get a value from a variable in an object that is null.

Such as

MyClass c = null;
// NullPointerException
String name = c.getName();

Possible Fix

MyClass c = null;
if (c == null) {
    // initialize
    c = new MyClass();
}
// This will work
String r = c.getName();

You need to always have a null check because it is very common to get a null variable.

Exception in thread "Thread-4" java.lang.NullPointerException in call the constructor

1 Answers1