[hw] how to fix this "java.lang.NullPointerException"

[Cheetoz]

public class Guess 
{
    double gOld[], gNew;
    String result = "";
    int i;
    
   double a = Math.random();
    
    Guess(double guess)
    {
        [COLOR="DarkOrange"]gOld[i]=guess;[/COLOR]
            
        while(Math.abs(gNew - gOld[i])>0.000000005)
        {   
            result = result + gNew + "\n";
            gNew = (Math.pow(gOld[i],2)-a)/2*gOld[i];
            i++;
            gOld[i] = gNew;
        }
    }

    public String toString()
    {
        return result;
    }
    

}

I am getting the error at orange part. I've read that the error means something could not initialized. But I don't see the error here. "i" is default at zero, so it should take the value "guess" and store it at index 0 of the array.

 

[Python]

Looks like to me you never init int i so gOld is throwing a null pointer error.

 

[sickwookie]

int i is fine it is gOld that is never initialized.

Add:
gOld = new double[size];

 

[Python]

yea, i just played with it in Eclipse. You need to give your array a size. I didn't even notice that.

 

[tim_m]

even if i defaults to 0 if not specified, it is always good practice do explicitly define your variables.

 

[S3th13]

But its not the problem of defining i, sickwookie was correct in saying that he never instantiates gOld.

 

[tim_m]

yes, having seen that the answer was provided, i merely offered a suggestion for good practice.