Null pointer exception when trying to access triangular array in method

Question

I'm having an issue with trying to access my triangular array separately using a method, each time i try to use tarray[i][j] I get a null pointer exception unless it's done within the class creation, for example I have a get method and have used return tarray[0][0] and it just throws me the error even though it prints out fine within the creation.

I know I'm probably doing something stupid but I just can't figure it out,

public class Triangular<A> implements Cloneable
{

    private int inRa;
    private A [][] tarray;

    /**
     * Constructor for objects of class Triangular
     * @param indexRange - indices between 0 and indexRange-1 will be legal to index
     *                     this a triangular array
     * @throws IllegalArgumentException - if indexRange is negative                    
     */
    public  Triangular(int indexRange) throws IllegalArgumentException
    { 
        inRa=indexRange;
        int n = inRa; 
        int fill = 1;
        Object [][] tarray = new Object [inRa + 1][];
          for (int i = 0; i <= inRa; i++){
           tarray[i] = new Object [n];

          }

          for (int i = 0; i < tarray.length; i++){

          for (int j = 0; j + i < tarray[i].length; j++){
          tarray[i][j + i] = fill;
          fill ++;
        }
        }

        for (int i = 0; i < tarray.length; i++) {
        for (int j = 0; j + i < tarray[i].length; j++){
        System.out.print(tarray[i][j + i] + " ");  
       }
       System.out.println();

       }

    }
}

thank you for your help!

Answer 1

You don't initialize anything to the tarray field in the constructor, you initialize a local variable of the same name; this one:

Object [][] tarray = new Object [inRa + 1][]; // doesn't access the tarray field

You have to assign something to the tarray field, however, to fix the NPE.

BTW: It's better not to use local variables that have the same name as a field.