Question

我完全从我的并行编程书中复制了这段代码。当我尝试编译它时，我会在代码中发生nullpointerexception： t [i] = newThread（counters [i]）; 根据Eclipse是一个过时的方法。

我做的唯一修改是添加一个try {} catch {}来捕获nullpointerexceptions以允许程序实际运行。

有没有人确切知道出了什么问题/如何修复它。提前致谢

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

你永远不会分配数组t。你得到的空指针异常是因为你的t数组是null。您需要在main方法的开头添加此权限：

t = new Thread[MAX_THREADS];

Answer 2

我浏览了它，但我认为问题是你在尝试将数据分配给t之前没有为t分配内存。 Java中的数组是指针。你要做的是在C：

中这样做

static thread* t;
//Need to initialize t here using t = new t[MAX_THREADS]
t[i] = ...

将抛出NullPointerException。

Java Nullpointerexception和过时的方法

2 个答案: