How to store data read from a file into 2D array

Question

I'm currently working with my assignment which requires me to read input from a text file and store into a matrix. Basically I have part of my code here.

    int count = 0 ;
    double[][] matrix = null;

    while ((line = reader.readLine()) != null) 
    {   
        line2 = line.split(" ");

        double[] criteriaWeight = {Double.parseDouble(line2[0]),Double.parseDouble(line2[1]),Double.parseDouble(line2[2]),Double.parseDouble(line2[3])};

        for ( int i = 0 ; i < criteriaWeight.length ; i++ )
            matrix[count][i] = criteriaWeight[i];

        count++;
    }

Now, the logic of what I'm trying to do is that I read data from a text file and then convert it into a double and store into a 2D array ( matrix ). I managed to read the data from the file. That is error-free.

Now, my problem is at the matrix[count][i] = criteriaWeight[i]; where I get error

Exception in thread "main" java.lang.NullPointerException
at javaapplication2.JavaApplication2.readFile(JavaApplication2.java:42)
at javaapplication2.JavaApplication2.main(JavaApplication2.java:56)
Java Result: 1

Anyone can point to my mistakes here? Thank you very much.

Answer 1

NullPointerException

Thrown when an application attempts to use null in a case where an object is required. These include:

• Calling the instance method of a null object.
• Accessing or modifying the field of a null object.
• Taking the length of null as if it were an array.
• Accessing or modifying the slots of null as if it were an array.
• Throwing null as if it were a Throwable value.

So, In your code double[][] matrix = null;

You declared and initialized with null .

So when you write

  matrix[count][i]

That is still null. You need to initialize like

 double[][] matrix = new double[x][y];

If you are looking for a dynamic array ,consider using ArrayList

Answer 2

How about the following implementation

List<Integer>[] array;
array = new List<Integer>[10];
array[0] = new ArrayList<Integer>(); ....

It will work as dynamic two dimensional array