JAVA: Possible to add a runnable thread into a queue?

I recently began working with Threads and I am trying to complete a Java implementation of the Looper class in Android. Basically I am making a Java class that puts threads into a queue that will then be executed by the Looper class. I have the code completed for the most part but have an issue with the enqueuing of tasks.

In the Looper class I have the queue declared and my enqueue method:

List<Runnable> queue;   

public synchronized void enqueue(Runnable runnable) {
    queue.add(runnable);
    notify(); // signal a waiting thread
}

I then created another class called TaskManager to add Tasks into the queue. I receive the error when I call:

loop.enqueue(new Task());

Where Task() implements runnable and just adds two integers together in its run() method...this is just a test.

The error I receive is:

Exception in thread "Thread-0" java.lang.NullPointerException
at Looper.enqueue(Looper.java:20) (this is the queue.add(runnable))
at TaskMaker.run(TaskMaker.java:16) (this is the loop.enqueue(new Task())开发者_StackOverflow社区

I'm obviously doing something wrong and not implementing this right...how should I go about this? Is the way I am enqueuing the task right? Thanks for any help it is much appreciated!

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Are you initializing the queue variable? like:

List<Runnable> queue = new ArrayList<Runnable>();