Question

              Input file- 
              5 
              1 2 3 4 5
              2 3 4 5 6
              3 2 1 5 8

我的工作是我应该读取此输入文件my.txt并交换其第一列和最后一列并将其输出到另一个文件comp.txt但我得到一个空白文件comp.txt

import java.io.BufferedReader;
import java.io.FileReader;
import java.io.PrintStream;
import java.io.*;

// to swap first and last column of 2D array
public class Swap
{
    private static BufferedReader in = null;
    private static int row = 0;
    private static int column = 0;
    private static int[][] matrix = null;

    public static void main(String[] args) throws Exception
    {
        try
        {
            // String filepath = args[0];
            int lineNum = 0;
            int row = 0;
            in = new BufferedReader(new FileReader("my.txt"));
            String line = null;
            while ((line = in.readLine()) != null)
            {
                lineNum++;
                if (lineNum == 1)
                {
                    column = Integer.parseInt(line);
                }
                else
                {
                    String[] tokens = line.split(",");
                    for (int j = 0; j < tokens.length; j++)
                    {
                        if (j == 0) matrix[row][0] = Integer.parseInt(tokens[column]);
                        else matrix[row][j] = Integer.parseInt(tokens[j]);
                    }
                    row++;
                }
            }
        }
        catch (Exception ex)
        {
            System.out.println("The code throws an exception");
            System.out.println(ex.getMessage());
        }
        finally
        {
            if (in != null) in.close();
        }

        try
        {
            PrintStream output = new PrintStream(new File("comp.txt"));
            for (int i = 0; i < row; i++)
            {
                for (int j = 0; j < column; j++)
                {
                    output.println(matrix[i][j] + " ");
                }
            }
            output.close();
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
    }
}

以下是我在控制台获得的输出 -

           The code throws an exception
           null

除此之外，我得到一个空白的comp.txt文件

Answer 1

1）改变

System.out.println("The code throws an exception");  
System.out.println(ex.getMessage());

获取例外原因的详细说明。

System.out.println("The code throws an exception");  
ex.printStackTrace();

2）来自代码＆amp;异常消息似乎有一个NullPointerException，因为你的数组matrix被初始化为null而不是匹配大小的数组。

异常的解决方案是[假设你的核心逻辑很好]，在为matrix数组赋值之前，用相关大小进行初始化，即最大行数和最大值。列

Answer 2

用

替换了你的catch块之后

    catch (Exception ex)
    {
        System.out.println("The code throws an exception");
        System.out.println(ex.getMessage());
        ex.printStackTrace();
    }

我可以说错误是

java.lang.NullPointerException
    at test.example.code.Swap.main(Swap.java:37)
The code throws an exception
null

该行

                    if (j == 0) matrix[row][0] = Integer.parseInt(tokens[column]);

因为您正在尝试访问tokens的第5列，但它实际上是一个5长度的数组，Java中的索引和大多数基于C的语言通常都是zero-indiced有效指数从0到4。

另外，我不完全确定你要对这条线做什么，所以我不会修复它，但那是错误。

编辑：

现在这不是一件合法的事情，但我解决了它，就像我自己做的那样，之后我会解释它。如果你不能阅读代码，因为你的导师＆＃34;会反对它，然后只读这篇文章的最后几行并自己解决。

// to swap first and last column of 2D array
public class Swap
{
    private static BufferedReader in = null;
    private static int column = 0;
    private static List<int[]> matrix = null;

    public static void main(String[] args) throws Exception
    {
        try
        {
            // String filepath = args[0];
            in = new BufferedReader(new FileReader("my.txt"));
            String line = null;

            matrix = new ArrayList<int[]>(); //array length is variable length
                                             //so using a 2D array without knowing the row size is not right
            boolean isFirstLine = true;

            while ((line = in.readLine()) != null)
            {
                if (isFirstLine)
                {
                    column = Integer.parseInt(line); //first line determines column length
                    isFirstLine = false;
                }
                else
                {
                    String[] tokens = line.split(" "); // there are no commas in input file so split on spaces instead
                    matrix.add(new int[column]);
                    for (int i = 0; i < tokens.length; i++)
                    {
                        matrix.get(matrix.size() - 1)[i] = Integer.parseInt(tokens[i]);
                        //store the lines of the currently read line in the latest added array of the list
                    }
                }
            }
        }
        catch (Exception ex)
        {
            System.out.println("The code throws an exception");
            ex.printStackTrace(); //changed exception write out for better info
        }
        finally
        {
            if (in != null)
                in.close();
        }

        // swapping the elements of first and last in each int array of list
        for(int[] intLines : matrix)
        {
            int temp = intLines[0];
            intLines[0] = intLines[intLines.length-1];
            intLines[intLines.length-1] = temp;
        }

        BufferedWriter output = null;
        try
        {
            output = new BufferedWriter(new OutputStreamWriter(new FileOutputStream(new File(
                    "comp.txt")))); //i just prefer bufferedwriters don't mind me
            output.write("" + column); //write the first line that is the array length into file
            output.newLine();
            for (int[] intLines : matrix) //write out each line into file
            {
                for (int i = 0; i < intLines.length; i++)
                {
                    output.write("" + intLines[i]);
                    if (i < (intLines.length - 1))
                    {
                        output.write(" ");
                    }
                }
                output.newLine();
            }
        }
        catch (FileNotFoundException e)
        {
            e.printStackTrace();
        }
        finally
        {
            if (output != null)
            {
                try
                {
                    output.close();
                }
                catch (IOException e)
                {
                }
            }
        }
    }
}

所以基本上我将2D数组更改为数组列表，读入文件，修复了你用逗号而不是空格分割，并实际进行了交换。你不必采取这种方法，特别是如果这违反了规则。如果你不这样做，甚至不读代码。

在java中的文本文件中交换2D数组的第一列和最后一列

2 个答案: