java - 将充满 double 的文本文件读取到二维数组中

Question

这个问题已经有答案了:

What is a NullPointerException, and how do I fix it? (12 个回答)

已关闭 5 年前。

免责声明:这是家庭作业，所以我不想寻找确切的答案，因为我想自己做。我必须从充满 double 的文本文件中读取矩阵并将其放入二维数组中。我目前正在努力寻找代码中的问题所在，因为我知道如何使用内置的 Java Scanner 来处理普通数组。我下面的代码总是给我一个 NullPointerException 错误，这意味着我的 matrix 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();
            }
        }
    }
}

<小时/>

我的文本文件:

1.25 0.5 0.5 1.25
0.5 1.25 0.5 1.5
1.25 1.5 0.25 1.25

Answer 1

您的代码有几个错误 - 由于这是家庭作业，我会尽力坚持您的原始设计:

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 - 总的来说，您离功能代码不远了，但理解错误很重要。