尝试写入文件时出现NullPointerException

Question

I'm supposed to trace the following merge sort algorithm to a file but I keep getting a NullPointerException in the merge method when I test it. I don't know why or how to fix it. None of the other sort algorithms I traced had this issue. This assignment builds into another so I'd really appreciate some help here.

import java.io.*;

public class MergeSortWithTrace
{
private static PrintWriter writer;
/** Sort the array using the merge sort algorithm
        pre: table contains Comparable objects
        post: table is sorted
        @param table The array to be sorted
*/
public static <T extends Comparable<T>> void sort(T[] table)
{
    try
    {
        PrintWriter writer = new PrintWriter("Merge_Sort_Trace.txt");
        // a table with one element is sorted already
        if (table.length > 1)
        {
            // Split the table into halves.
            int halfSize = table.length / 2;
            T[] leftTable = (T[]) new Comparable[halfSize];
            T[] rightTable = 
                            (T[]) new Comparable[table.length - halfSize];
            System.arraycopy(table, 0, leftTable, 0, halfSize);
            System.arraycopy(table, halfSize, rightTable, 0, table.length - halfSize);
            writer.print("\nSplit the array in two...");
            writer.print("\nThe left array: ");
            for (int i = 0; i < leftTable.length; i++)
            {
                writer.print("     " + leftTable[i]);
            }
            writer.println("\nThe right array: ");
            for (int i = 0; i < rightTable.length; i++)
            {
                writer.print("     " + rightTable[i]);
            }
            // Sort the halves
            sort(leftTable);
            sort(rightTable);

            // merge the halves
            merge(table, leftTable, rightTable);
        }
        writer.close();
    }
    catch (FileNotFoundException fnfe)
    {
        writer.println(fnfe.getMessage());
    }
}

/** merge two sequences.
        pre: leftSequence and rightSequence are sorted
        post: outputSequence is the merged result and is sorted.
        @param outputSequence The destination
        @param leftSequence The left input
        @param rightSequence The right input
 */
 private static <T extends Comparable<T>> void merge(T[] outputSequence,
                                                                                                         T[] leftSequence,
                                                                                                         T[] rightSequence)
 {
        int i = 0; // Index into the left input sequence
        int j = 0; // Index into the right input sequence
        int k = 0; // Index into the output sequence
        // While there is data in both input sequences
        while (i < leftSequence.length && j < rightSequence.length)
        {
             // Find the smaller and insert it into the output sequence
             if (leftSequence[i].compareTo(rightSequence[j]) < 0)
             {
                    outputSequence[k++] = leftSequence[i++];
             }
             else
             {
                 outputSequence[k++] = rightSequence[j++];
             }
        }
        // assert: one of the sequences has more items to copy
        // copy remainings input from left sequence into the output.
        while (i < leftSequence.length)
        {
            outputSequence[k++] = leftSequence[i++];
        }
        // Copy remaining input from right sequence into the output.
        while (j < rightSequence.length)
        {
            outputSequence[k++] = rightSequence[j++];
        }
        writer.println("\nThe sorted sequence: ");
        for (int z = 0; z < outputSequence.length; z++)
        {
            writer.print("     " + outputSequence[z]);
        }
    }
}

Exception trace

Exception in thread "main" java.lang.NullPointerException
    at MergeSortWithTrace.merge(MergeSortWithTrace.java:81)
    at MergeSortWithTrace.sort(MergeSortWithTrace.java:39)
    at MergeSortWithTrace.sort(MergeSortWithTrace.java:35)
    at MergeSortWithTrace.main(MergeSortWithTrace.java:89)

which is

 writer.close();

Answer 1

It looks like you aren't setting

private static PrintWriter writer;

merge expects this to be set, but in sort, you only create a local one.

PrintWriter writer = new PrintWriter("Merge_Sort_Trace.txt");

trying changing it to:

writer = new PrintWriter("Merge_Sort_Trace.txt");

Answer 2

Your problem is

writer.close();

You need to take it out of sort method. Your code closes file stream before merge method could write entries in the file

I suggest you have another method which opens PrintWriter calls recursive sort method and closes PrintWriter

Answer 3

您已经定义了PrintWriter的全局引用，仅使用它来创建书写器并在finally块中将其关闭。