我在加粗的代码行中收到 NULL 指针异常？ 我究竟做错了什么？

Question

public class kmeansgeneral {
    public static void main(final String args[]) throws Exception {

        int words = 0;
        int chars = 0;
        int lines = 0;

        String s, sp1;
        StringTokenizer st;
        final ArrayList<Double> x = new ArrayList<Double>();

        final FileReader fr = new FileReader("G:/t1.txt");
        final BufferedReader buf = new BufferedReader(fr);

        // Counting number of words and lines
        while ((s = buf.readLine()) != null) {
            lines++;
            st = new StringTokenizer(s, " ,;:.");

            while (st.hasMoreTokens()) {

                words++;
                s = st.nextToken();
                chars += s.length();

                final Double y = new Double(s);
                x.add(y);

            }

        }

        System.out.println("Word Count : " + words / lines);
        System.out.println("Line Count : " + lines);
        // Counting and printing number of words and lines ENDS

        Double ct[] = new Double[0];
        ct = x.toArray(ct);

        // Input array, values to be read in successively, float
        // double[][] indat = new double[lines][lines*words];
        final double[][] indat = new double[10][10];
        int inval = 0;

        final BufferedReader buf1 = new BufferedReader(new InputStreamReader(System.in));
        System.out.print("Enter K-Value : ");
        sp1 = buf1.readLine();

        final Integer ky = new Integer(sp1);
        final int k = ky;

        System.out.println("K ==  " + k);

        // Now read in input array values, successively

        for (int i = 0; i < lines; i++) {
            for (int j = 0; j < words / lines; j++) {

                indat[i][j] = ct[inval];
                inval++;

                System.out.print(indat[i][j]);
                System.out.print("\t");
            }
            System.out.println();
        }


        // Initial Clusters
        System.out.println("   ");
        System.out.println(k + " seed  points ");

        final double[][] Clusters = new double[k][lines * words];
        // double[][] calcnt = new double[lines][words];
        final double[][] calcnt = new double[1000][1000];
        final double[][] array = new double[k][lines * words];

        System.out.println("Clusters==>");
        // int pos=0;
        for (int i = 0; i < k; i++) {
            for (int j = 0; j < words / lines; j++) {

                Clusters[i][j] = indat[i][j];
                // pos= j;

                System.out.print(Clusters[i][j]);
                System.out.print("\t");
            }
            System.out.println();

        }
        System.out.println("PRINTING VECTOR ELEMENTS:");
        final Vector FinalClusters[][] = new Vector[100][100];

        for (int i = 0; i < k; i++) {
            for (int j = 0; j < words / lines; j++) {
                final String tempString = String.valueOf(Clusters[i][j]);
                FinalClusters[i][j].add(tempString);
            }

        }

        // Inital Cluster Array
        System.out.println("Initial Cluster Array");
        int b = 0;
        final double[] arr = new double[2000];

        for (int i = 0; i < k; i++) {
            for (int j = 0; j < words / lines; j++) {

                arr[b] = Clusters[i][j]; // = indat[i][j];
                System.out.print(arr[b]);
                System.out.print("\t");
                b++;
            }

        }
        System.out.println();


        System.out.println("Centroids");

        for (int i = 0; i < k; i++) {
            for (int j = 0; j < words / lines; j++) {

                calcnt[i][j] = (Clusters[i][j] + indat[k][j]) / 2;

                System.out.print(calcnt[i][j]);
                System.out.print("\t");

            }

        }

        // Claculate Distances

        // System.out.println("MAGIC # 3");

        final double[] dist = new double[k];

        for (int i = 0; i < k; i++) {

            double dis = 0;
            for (int j = 0; j < words / lines; j++) {

                dis = dis + (Math.pow(Clusters[i][j] - indat[k][j], 2));

            }
            dist[i] = (Math.sqrt(dis));
            System.out.println("From Cluster K = " + i + "\t" + "Distance" + dist[i]);
        }

        System.out.println("To Find Minimum Distance ");

        double min = dist[0];

        int y = 0;

        for (int m = 0; m < k; m++) {

            if (dist[m] < min) {
                min = dist[m];
                y = m;
            }
        }
        System.out.print("Min Value =" + min + "\t" + "For Cluster :" + y);

        System.out.println();
        System.out.print("Added Cluster =");
        final double[] temp = new double[lines * (words / lines)];

        for (int j = 0; j < words / lines; j++) {

            temp[j] = indat[k][j];

            System.out.print(temp[j]);
            System.out.print("\t");

        }
        System.out.println();

        final Vector[] vector = new Vector[k];
        for (int i = 0; i < k; i++) {
            vector[i] = new Vector<Object>();
        }

        for (int i = 0; i < words / lines; i++) {
            vector[y].add(String.valueOf(temp[i]));
        }

        System.out.println(Arrays.toString(vector));


    }
}

Answer 1

首先，您应该重新编辑您的帖子，以更清楚地说明问题所在。

然后删除所有不相关的代码。

然后看看你的堆栈跟踪，它应该说哪一行是错误的，那一行你应该发布。

我猜它是带有 ** 的，否则你的代码将无法编译

所以我假设它的这一行：

      FinalClusters[i][j].add(tempString);//is it this line ?
      // where did you initialise FinalClusters[i][j] ??
      // maybe you first need FinalClusters[i][j] = new Vector(); 

如果是，那么您的错误是您确实初始化了数组，但不是数组中的每个单独元素。

旁注：你真的需要那个向量数组吗？

Answer 2

我在以下输入文件上运行了您的代码：

t1.txt：

123
234
345
34456 

Output：

Word Count : 1
Line Count : 4
Enter K-Value : 3
K ==  3
123.0   
234.0   
345.0   
34456.0 

3 seed  points 
Clusters==>
123.0   
234.0   
345.0   
PRINTING VECTOR ELEMENTS:
Exception in thread "main" java.lang.NullPointerException
    at test.kmeansgeneral.main(kmeansgeneral.java:106)

就是这段代码：

System.out.println("PRINTING VECTOR ELEMENTS:");
final Vector FinalClusters[][] = new Vector[100][100];

for (int i = 0; i < k; i++) {
    for (int j = 0; j < words / lines; j++) {
        final String tempString = String.valueOf(Clusters[i][j]);
        // NPE OCCURS IN THE LINE BELOW
        FinalClusters[i][j].add(tempString);
    }
}

问题是数组“FinalClusters”包含 null 值，而不是空向量。 所以这是一个修复，在发生错误的行上方添加以下行：

if (FinalClusters[i][j] == null) {
    FinalClusters[i][j] = new Vector();
}

那么output就是：

Word Count : 1
Line Count : 4
Enter K-Value : 3
K ==  3
123.0   
234.0   
345.0   
34456.0 

3 seed  points 
Clusters==>
123.0   
234.0   
345.0   
PRINTING VECTOR ELEMENTS:
Initial Cluster Array
123.0   234.0   345.0   
Centroids
17289.5 17345.0 17400.5 From Cluster K = 0  Distance34333.0
From Cluster K = 1  Distance34222.0
From Cluster K = 2  Distance34111.0
To Find Minimum Distance 
Min Value =34111.0  For Cluster :2
Added Cluster =34456.0  
[[], [], [34456.0]]

我不知道这应该做什么，所以无法判断这是否正确。 至少不再抛出异常。

Answer 3

让我猜猜：您可能在 eclipse 或某些无法为 jvm 提供控制台的编辑器中调试此代码时遇到此错误。

如果是这种情况，那么您可能会在sp1 = buf1.readLine();处遇到空指针。