此NullPointer异常的原因可能是什么？

Question

The following is a part of my code.

I get a NullPointerException in the line " buckets[i][h.hashBands(sum, bandRows)].add(j); ".

What could be the reason?

public static void generateBuckets()
{
    hash h = new hash();
    buckets = new ArrayList[bands][bandRows];

     for(int i=0; i<bands; i++)
     {
         for(int j=0; j<preprocessedList.size(); j++)
         {
             int[] sum = new int[bandRows];
             int a=0;
             for(int k=i; k<bands; k++)
             {
                 sum [a] = sigMatrix[k][j];
                 a++;
             }
             buckets[i][h.hashBands(sum, bandRows)].add(j);
         }
     }

The h.hashBands() function is as follows

public int hashBands(int[] in, int bucketSize) 
    {        
        BigInteger hashVal = BigInteger.ZERO;
        int k = in.length;
        BigInteger base = BigInteger.valueOf(3);
        BigInteger size = BigInteger.valueOf(bucketSize);

        for (int i = 0; i < in.length; i++)
            hashVal = (hashVal.add(BigInteger.valueOf(in[i]).multiply(base.pow(k-i-1))));

        hashVal = hashVal.mod(size);
        return hashVal.intValue();
    }

Answer 1

You are creating a 2D array of ArrayLists

buckets = new ArrayList[bands][bandRows];

but never filling it with ArrayLists so every element is null. When you call .add(), you get a NullPointerExcepion

Answer 2

You never initialize buckets[][] .
You will need to add a buckets = new BucketType[xlength][ylength]; somewhere before using it.

Answer 3

您尚未将buckets数组中的特定条目初始化为任何东西。

Answer 4

very hard to say for sure. either you haven't initialized buckets, or the elements in it, ... anyway, answering this for sure is pretty impossible. it would be easier if we saw a bit more of the code, and maybe the stacktrace.