Null pointer in a 2D array

Question

I have been attempting to find the null pointer issue for days. The code compiles fine but when it is run the message java.lang.NullPointerException at RandomArray.getRow(RandomArray.java:28) [this being the line indicated below <--]

Am I stupid?

    
  //An instance variable to hold an array of integers
  int[][] intHolder;
  
  public RandomArray(int rows, int cols, int range) { 
    //Create a 2D array with values between 0-range
    int[][] intHolder = new int[rows][cols];
 
    for(int i = 0; i < rows; i++){
     for(int j = 0; j < cols; j++){
      intHolder[i][j] = (int)((range-1)*Math.random());
      System.out.println(intHolder[i][j]);  //PRINT FOR TESTING
      }
    }
  }//constructor

  public int[] getRow(int r){
    //Return a copy (clone) of row r of the array
    int[] arrayReturn = new int[intHolder[r].length];  //I was having problems here too, no idea why this works
    
    for(int i = 0; i < intHolder[r].length; i++){
      arrayReturn[i] = intHolder[r][i];    //The problem is here <--------------
      System.out.println(arrayReturn[i]);
    }
    return arrayReturn;
  }//getRow method
  
  public int[] getCol(int c){
    //Return a copy (clone) of column c of the array
    int[] arrayReturn = new int[intHolder[c].length];
    
    for(int i = 0; i < intHolder[c].length; i++){
      arrayReturn[i] = intHolder[c][i];
      System.out.println(arrayReturn[i]);
    }
    return arrayReturn;
  }//getCol method

}//RandomArray class```

Unrelated to your issue, but the entire getRow() method can be replaced with a call to Arrays#copyOf(). — Charlie Armstrong
– Charlie Armstrong, Commented Feb 26, 2021 at 22:04

camickr · Accepted Answer · 2021-02-26 21:56:26Z

3

You have two variables with the same name:

int[][] intHolder;

You define an "instance" variable, but it is null.

public RandomArray(int rows, int cols, int range) { 
    int[][] intHolder = new int[rows][cols];

The above code defines a "local" variable in the constructor and can't be accessed by any other method.

To initialize your "instance" variable so it can be used in any method you use:

//int[][] intHolder = new int[rows][cols];
intHolder = new int[rows][cols];

answered Feb 26, 2021 at 21:56

camickr

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1 Comment

ThePyroGod Over a year ago

A good reminder to be diligent with bugfixing! Thank you so much

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