NullPointerException被抛出，但我不知道如何解决它。

Question

I have the following code. It is a constructer for a Matrix class. The main thread only calls the constructor, and then prints the matrix (I made a method for it).

public class Matrix{ 
    float [][] mainMatrix; 
    int rows; 
    int columns; 
    public Matrix(){
        System.out.printf("\nInput the number of rows \n") ;
    String rowR = new Scanner(System.in).next().replace(" ", "").replace("\n", "").toString();
    rows = Integer.parseInt(rowR);
    System.out.printf("\n Input the number of columns \n");
    String columnR = new Scanner(System.in).next().replace(" ", "").replace("\n", "").toString();
    columns = Integer.parseInt(columnR);
    System.out.printf("Input the rows, seperated by a \" \". Close the row with a \"]\"");
    System.out.println("Warning!!! Inproper input will cause the program to crash!"); 
    for(int r = 0; r < rows; r++){
        Scanner item = new Scanner(System.in);
        System.out.printf("\n["); 
        String[] raw = item.next().replace("]", "").replace("\n", "").split(" ");
        for(int c = 0; c < columns; c++){
            mainMatrix[r][c] = Float.parseFloat(raw[c]);  
        }
    }
    /*Bunch of methods*/ 
} 

For some reason, when the code is run, it returns a NullPointerException, and points to the line:

mainMatrix[r][c] = Float.parseFloat(raw[c]);

If it helps, the output looks like this:

 Input the number of columns 
2

Input the rows, seperated by a " ". Close the row with a "]"Warning!!! Inproper input will cause the program to crash!

[ 2 3] /*\n*/ 
[Ljava.lang.String;@29173efException in thread "main" java.lang.NullPointerException
    at mathProgs.linProg.Matrix.<init>(Matrix.java:51)
    at mathProgs.linProg.MatrixTest.main(MatrixTest.java:10)

the 2, 3, and ] are user inputs. Enter is pressed after the "]"

Answer 1

The reason is that you haven't initialized mainMatrix . You need something like:

mainMatrix = new int[rows][columns];

Otherwise the variable has its default value of null , so when you try to dereference it (assigning a value to an element of the array) you get the NullPointerException .

Note that unlike in some other languages, once you've created an array object it has a fixed size - you can't just add items to it later. For that, you'd need a List implementation such as ArrayList . Not a problem in this case, as you know how many rows and columns you've got to start with - but worth bearing in mind.

Answer 2

You haven't initialized your mainMatrix attribute so its default value will be null thus getting the NPE when using it. Initialize it when you have your row and column variables:

mainMatrix = new float[rows][columns];