cannot access array from index i-1 through Null Pointer exception

Question

I'm working on a 2D array. The problem is really simple but I can't figure it out. When I try to access the value from index array[i-1][j] it is throwing a Null Pointer exception. While it seems completely legitimate to me to access a index like array[2-1][2] . Why it is not working? Is there any logic behind why I can't access array from one back index OR am I doing it wrong please explain.

Code to declare and initialize array:

this.Result = new Integer[len][6];

Code where Null pointer exception occurring:

for(int i=0;i<Result.length;i++) {
    if(i==0){
        Result[i][4]=0;
    }
    else if(Result[i][1]!=null) {
        Result[i][4]=(Result[i-1][2]+Result[i-1][3])+Result[i][2];
    }
}

Thanks in advance and please ignore grammar mistakes.

Can you share more code ? What is the declaration of your Result matrix ? — Ajinkya Patil, Apr 21 '16 at 12:41
Integer is a class so when you initialize it using class it's default values are null and when you access those null values you will get null pointer exception, use int instead, int will make values of all objects as 0. You can then use them or modify them. — Ajinkya Patil, Apr 21 '16 at 12:48

score 1 · Answer 1 · answered Apr 21 '16 at 12:42

1

Well, it looks like the fields in your array have not been initialized properly. While something like int[] intArray = new int[length]; initializes all values in the array to zero, Integer[] integerArray = new Integer[length]; will just fill the array with null values.

Are you sure you initialized all fields in your array properly?

answered Apr 21 '16 at 12:42

Tobias Brösamle

598
5
18

have `int[][] Result;` and `= new int[len][6]; – Joop Eggen Apr 21 '16 at 12:44

score 1 · Accepted Answer · answered Apr 21 '16 at 12:44

You are creating an array of the wrapper Integer type. This means that the default value for all elements is null.

On this line:

Result[i][4]=(Result[i-1][2]+Result[i-1][3])+Result[i][2];

you are accessing elements that may be uninitialised, which causes the NullPointerException.

I suggest you use the primitive int type for the array elements to avoid performing operations on nulls. Another option will be initialise all elements with the value 0 prior to trying to access and perform calculations on them.

score 0 · Answer 3 · answered Apr 21 '16 at 12:47

0

Exception Raised due to Integer is a Wrapper Class in java. So, this 2-D array would be filled with null not 0.

if you replace your code

this.Result = new Integer[len][6]; with this.Result = new int[len][6];

error will resolved.

answered Apr 21 '16 at 12:47

Vikrant Kashyap

6,398
3
32
52

cannot access array from index i-1 through Null Pointer exception

3 Answers3