Java 将填充有双精度字符的文本文件读取到二维数组中

免责声明：这是家庭作业，所以我不寻找确切的答案，因为我想自己做这件事。 我必须从一个文本文件中读取一个矩阵，该文件中填充了双精度字符，并放入一个2D数组中。 我目前正努力找出代码中的问题所在，因为我知道如何将内置Java扫描程序用于普通数组。下面的代码总是给我一个NullPointerException错误，这意味着我的矩阵2D数组是空的

public Matrix(String fileName){
    //reads first double and puts it into the [1][1] category
    //then moves on to the [1][2] and so on
    //once it gets to the end, it switches to [2][1]
    Scanner input = new Scanner(fileName);
    int rows = 0;
    int columns = 0;
    while(input.hasNextLine()){
        ++rows;
        Scanner colReader = new Scanner(input.nextLine());
        while(colReader.hasNextDouble()){
            ++columns;
        }
    }
    double[][]matrix = new double[rows][columns];
    input.close();

    input = new Scanner(fileName);
    for(int i = 0; i < rows; ++i){
        for(int j = 0; j < columns; ++j){
            if(input.hasNextDouble()){
                matrix[i][j] = input.nextDouble();
            }
        }
    }
}

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您的代码有几个错误-由于这是家庭作业，我将尽可能坚持您的原始设计：

public Matrix(String fileName){

    //First issue - you are opening a scanner to a file, and not its name.
    Scanner input = new Scanner(new File(fileName)); 
    int rows = 0;
    int columns = 0;

    while(input.hasNextLine()){
        ++rows;
        columns = 0; //Otherwise the columns will be a product of rows*columns
        Scanner colReader = new Scanner(input.nextLine());
        while(colReader.hasNextDouble()){
            //You must "read" the doubles to move on in the scanner.
            colReader.nextDouble();
            ++columns;
        }
        //Close the resource you opened!
        colReader.close();
    }
    double[][]matrix = new double[rows][columns];
    input.close();

    //Again, scanner of a file, not the filename. If you prefer, it's
    //even better to declare the file once in the beginning.
    input = new Scanner(new File(fileName));
    for(int i = 0; i < rows; ++i){
        for(int j = 0; j < columns; ++j){
            if(input.hasNextDouble()){
                matrix[i][j] = input.nextDouble();
            }
        }
    }
}

---

请注意注释-总体而言，您离功能代码不远，但理解错误很重要。

NullPointerException发生在哪里？查看堆栈跟踪，看看是否可以确定异常发生的位置。@Ravi这个问题非常笼统且过于宽泛。我不鼓励将此问题作为该问题的重复问题来结束。@DefenseTrator错误发生在我的toString函数中，这是我的嵌套for循环中第一个看到2d数组长度的函数。@azurefrog忘记将其更改回hasNextDouble，我只是在测试。@Michael在您发布的代码中没有对toString的调用。您发布的代码是否再现了错误？

public Matrix(String fileName){

    //First issue - you are opening a scanner to a file, and not its name.
    Scanner input = new Scanner(new File(fileName)); 
    int rows = 0;
    int columns = 0;

    while(input.hasNextLine()){
        ++rows;
        columns = 0; //Otherwise the columns will be a product of rows*columns
        Scanner colReader = new Scanner(input.nextLine());
        while(colReader.hasNextDouble()){
            //You must "read" the doubles to move on in the scanner.
            colReader.nextDouble();
            ++columns;
        }
        //Close the resource you opened!
        colReader.close();
    }
    double[][]matrix = new double[rows][columns];
    input.close();

    //Again, scanner of a file, not the filename. If you prefer, it's
    //even better to declare the file once in the beginning.
    input = new Scanner(new File(fileName));
    for(int i = 0; i < rows; ++i){
        for(int j = 0; j < columns; ++j){
            if(input.hasNextDouble()){
                matrix[i][j] = input.nextDouble();
            }
        }
    }
}