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Thread: Help with Error Debugging

  1. #1
    Junior Member
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    Default Help with Error Debugging

    I am trying to implement the producer consumer problem by creating my own semaphores. When compiling and running the program I am getting the error :

    Exception in thread "Producer" java.lang.NullPointerException
    at Producer.run(Producer.java:23)
    Exception in thread "Consumer" java.lang.NullPointerException
    at Consumer.run(Consumer.java:24)

    I know that this has something to do with the methods valueConsumed.P() and valueProduced.P(). Did I not implement these methods right in the Producer, Consumer, and Semaphore Classes?

    Producer
    public class Producer extends Thread{
    	private Buffer sharedLocation;
    	private Semaphore valueProduced;
    	private Semaphore valueConsumed;
     
    	public Producer (Buffer shared) {
    		super("Producer");
    		sharedLocation = shared;
    	}
     
    	public void run() {
    		for (int count = 1; count <= 4; count++){
    			try {
    				Thread.sleep((int) (Math.random() * 3001));
    				valueConsumed.P();
    				//check to see if the value has been consumed P( valueConsumed );
    				sharedLocation.set(count);
    				//let consumer thread know it can consume the value
    				valueProduced.V();
    			}
    			catch (InterruptedException exception) {
    				exception.printStackTrace();
    			}
    		}
    		System.err.println(getName() + " done producing.");
    	}
    }

    Consumer
    public class Consumer extends Thread{
    	private Buffer sharedLocation;
    	private Semaphore valueProduced;
    	private Semaphore valueConsumed;
     
    	public Consumer (Buffer shared) {
    		super("Consumer");
    		sharedLocation = shared;
     
    	}
    	public void run() {
    		int sum = 0;
    		for (int count=1; count<=4; count++) {
    			try{
    				Thread.sleep((int) (Math.random() * 3001));
    				valueProduced.P();
    				sum += sharedLocation.get();
    				valueConsumed.V();
    			}
    			catch (InterruptedException exception) {
    				exception.printStackTrace();
    			}
    		}
    		System.err.println(getName() + " done consuming. and sum= " + sum);
    	}
    }

    Semaphore
    public class Semaphore{
    	private int value;
    	public Semaphore(int initval)
    	{
    		value=initval;
    	}
    	public void P(){
    		if( value>0)
    			value=value-1;
    		else
    			try {
    				Thread.sleep((int) (Math.random() * 3001));
    			}
    		catch (InterruptedException exception){
    			exception.printStackTrace();
    		}
    	}
    	public void V(){
    		value=value+1;
    	}
    }


  2. #2
    Super Moderator helloworld922's Avatar
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    Default Re: Help with Error Debugging

    The problem isn't that you didn't implement them correctly, but that you forgot to instantiate them in your Producer class.

  3. #3
    Member Darryl.Burke's Avatar
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    Default Re: Help with Error Debugging


  4. #4
    Super Moderator helloworld922's Avatar
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    Default Re: Help with Error Debugging

    meh asking the same question on different forums isn't the same as asking the same question on the same forum.

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