Question

I copied this code exactly out of my parallel programming book. When I tried compiling it I go a nullpointerexception which seems to be happening at the code: t[i]=newThread(counters[i]); which according to the Eclipse is an obsolete method.

The only modification I made was adding a try{}catch{} to catch nullpointerexceptions to allow the program to actually run.

Does anyone know exactly what is going wrong/how to fix it.
Thanks in advance

CODE

import java.util.*;
import java.util.concurrent.*;


public class CountThrees implements Runnable
{
private static final int ARRAY_LENGTH=100000;
private static final int MAX_THREADS=10;
private static final int MAX_RANGE=100;
private static final Random random=new Random();
private static int count=0;
private static Object lock=new Object();
private static int[] array;
private static Thread[] t;

public static void main(String[] args)
{
    array=new int[ARRAY_LENGTH];


//initialize elements in the array
for(int i=0;i<array.length;i++)
{
    array[i]=random.nextInt(MAX_RANGE);
}

//create the threads
CountThrees[] counters=new CountThrees[MAX_THREADS];
int lengthPerThread=ARRAY_LENGTH/MAX_THREADS;

for(int i=0; i<counters.length; i++)
{
    counters[i]=new CountThrees(i*lengthPerThread,lengthPerThread);
}
//run the threads
for(int i=0;i<counters.length; i++)
{

    try
    {
    t[i]=new Thread(counters[i]); //NullPointerException Happens here
    t[i].start();
    }
    catch(NullPointerException d)
    {
        System.out.println("Null Pointer Exception Happened");
    }
}

for(int i=0;i<counters.length;i++)
{
    try
    {
    t[i].join();
    }
    catch(InterruptedException e)
    {}
    catch(NullPointerException f)
    {
        System.out.println("Null Pointer Exception Happened");
    }
}
//print the number of threes
System.out.println("Number of threes: " + count);
}

private int startIndex;
private int elements;
private int myCount=0;


public CountThrees(int start,int elem)
{
    startIndex=start;
    elements=elem;
}

//Overload of run method in the Thread class

public void run()
{

    for(int i=0;i<elements; i++)
    {
        if(array[startIndex+i]==3)
        {
            myCount++;
        }

    }


synchronized(lock)
{
    count+=myCount;
}
}
}

Answer 1

You never allocate the array t. The null pointer exception you’re getting is because your t array is null. You need to add this right at the beginning of your main method:

t = new Thread[MAX_THREADS];