嘗試寫入文件時出現NullPointerException

Question

我應該將以下合並排序算法跟蹤到文件，但是在測試它時，我在合並方法中始終收到NullPointerException異常。 我不知道為什么或如何解決它。 我跟蹤的其他排序算法都沒有這個問題。 這項任務又加入了另一個任務，因此我非常感謝您的幫助。

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]);
        }
    }
}

異常跟蹤

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)

這是

 writer.close();

Answer 1

看來您沒有設定

private static PrintWriter writer;

merge希望對此進行設置，但在排序上，您只能創建一個本地的。

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

嘗試將其更改為：

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

Answer 2

你的問題是

writer.close();

您需要使它脫離排序方法。 您的代碼在合並方法可以在文件中寫入條目之前關閉文件流

我建議您使用另一種方法來打開PrintWriter調用遞歸sort方法並關閉PrintWriter

Answer 3

您已經定義了PrintWriter的全局引用，僅使用它來創建書寫器並在finally塊中將其關閉。