NullPointerException in Java while trying to print an array

Question

The thing is that I'm trying to print the matrix previously created in the constructor, but seems that it's empty.

Here's the code of the constructor:

public Matrix(int row_col){
    int [][] randomMatrix = new int[row_col][row_col];
    Random rand = new Random();
    if (row_col > 0 && row_col < ROW_LIMIT && row_col < COL_LIMIT)
    for (int i = 0; i < randomMatrix.length; i++) 
     for (int j = 0; j < randomMatrix[0].length; j++)
         randomMatrix[i][j] = rand.nextInt(51);
}

And the code of the method print:

public void print(){
    int row = randomMatrix.length;
    int col = randomMatrix[0].length;
    for(int i=0 ; i < row ; i++)
     for(int j=0 ; j < col ; j++)
        System.out.print(randomMatrix[i][j]);
}

Greetings!

JB Nizet · Accepted Answer · 2012-12-15 16:13:41Z

4

Replace

int [][] randomMatrix = new int[row_col][row_col];

by

this.randomMatrix = new int[row_col][row_col];

The constructor initializes and fills a local variable instead of initializing and filling the instance field used by the print() method.

answered Dec 15, 2012 at 16:13

JB Nizet

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2 Comments

Z i i t o x Over a year ago

Thanks for the answer, now seems to work, but in a strange way. I set the dimension to 2x2, but when I call to the print method, it prints such a huge chain of numbers... do I have something wrong in the constructor?

jlordo Over a year ago

@Ziitox what else do you expect? you never call System.out.println() or have a "\n" in your output when using your print() method.

Miquel · Accepted Answer · 2012-12-15 16:14:06Z

1

It looks like randomMatrix is defined directly in the scope of the constructor, and is not being stored in a field of the class.

If you already have a randomMatrix as a field, remove the int[][] in the first line of the constructor method, so you refer to the field instead of declaring a new variable.

answered Dec 15, 2012 at 16:14

Miquel

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Comments

Abubakkar · Accepted Answer · 2012-12-15 16:22:34Z

0

It is beacuse you have declared and initialized your array randomMatrix inside your constructor and as soon as the contructor's code is executed your randomMatrix array goes out of scope of print method.

And so when you try to access it in print method there is no such randomMatrix object, so you get NullPointerException

edited Dec 15, 2012 at 16:22

answered Dec 15, 2012 at 16:14

Abubakkar

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