How to fix nullpointer exception

Question

I keep on getting a null pointer exception when i invoke the add method the code seems fine but i dont know what is wrong.

public class CarRunner {

public static void main(String[] args) {
    TrafficLight light = new TrafficLight();
    RoadRunnable run1 = new RoadRunnable(1,light);
    String car="car";
    for (int i =0;i<100;i++)
    {
        car="car"+i;
        run1.add(car);
    }

    Thread traffic1 = new Thread(run1);
    traffic1.start();
}

}

public class RoadRunnable implements Runnable{

private LinkedList<String> queue;

private int number;

private TrafficLight light;

public RoadRunnable(int roadNumber, TrafficLight aLight) 

{ 
    number = roadNumber;

    light = aLight;

}

public void add(String car)

{

    queue.add(car);


}
 }

possible duplicate of [What is a Null Pointer Exception?](http://stackoverflow.com/questions/218384/what-is-a-null-pointer-exception) — Jonathon Reinhart, Jul 30 '13 at 04:08

score 2 · Answer 1 · answered Jul 30 '13 at 04:08

2

Your queue is not intantiated. In your constructor, you might want to do:

queue = new LinkedList<String>();

answered Jul 30 '13 at 04:08

vee

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@LucasLiu, You're welcome. Glad to be of help. – vee Jul 30 '13 at 04:17

score 0 · Answer 2 · answered Jul 30 '13 at 04:08

Null pointer exception happens when you try to access a null object method or params. In your code you have not initialized the queue:

private LinkedList<String> queue

and hence facing Null pointer exception while trying to add the elements in the queue using queue.add(car); in add method of your RoadRunnable class.

You should initialized your queue either at the declaration or in the constructor, something like:

public RoadRunnable(int roadNumber, TrafficLight aLight) 
{ 
    // queue initilization
    queue = new LinkedList<String>();
    number = roadNumber;
    light = aLight;
}

score 0 · Answer 3 · answered Jul 30 '13 at 04:10

0

Nothing in the RoadRunnable class ever creates the actual LinkedList for the queue variable to point to; therefore queue is null and you get the NPE when you try to use it.

answered Jul 30 '13 at 04:10

Ernest Friedman-Hill

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newuser · Answer 4 · 2013-07-30T04:20:29.333

0

You are not initialize the queue.

private LinkedList<String> queue = new LinkedList<String>();

instead of

private LinkedList<String> queue;

Reference for initializing LinkedList

edited Jul 30 '13 at 04:20

answered Jul 30 '13 at 04:15

newuser

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How to fix nullpointer exception

4 Answers4