create junit test for runnable class

I have a runnable class as shown below,

public class WriteToOutputFile implements Runnable{
BlockingQueue<entry> queue;
File file;
boolean processentries;
WriteToOutputFile(){
this.queue = queue;
this.file = file;
this.processentries= tue;
}

@override 
public void run(){
//write entries in blocking queue to file
entry = queue.take();
if (entry== lastentry)break;
}

public void stop(){
processentries = false;
queue.put(lastentry);
}
}

I want to create Junit test cases for this runnable. As I am new to write Junit test cases , please let me know what is the best approach for this.