can anyone help me,a complete newbie?

[keye906]

I have just started to learn java.

these days, I am always bothered by NullpointerException.

I try to pass a object to a method of its own class, but whenever

I want to read the data of that object or try to modify the content

of the object, I always get a NullpointerException. I don't know why this

happens. can some kind veteran give me a hand? here is an code encounter

this problem:

I first defined a class called matrix which mainly consist of a 2D array, "enter" is a method of this class that receive a matrix object and return a matrix object.

in main method, I first instantiate an object of the matrix class and then
pass it to the "enter " method.

matrix h=new matrix(i,j);
h=h.enter(h);

public matrix enter(matrix a) // "enter method"
{
try{
for(p=0;p<a.row;p++)
{

StreamTokenizer stok=new StreamTokenizer(br);
q=0;
while(stok.nextToken() !=';' )
{
a.matrix[p][q]=stok.nval; // the exception occurs at this line.
q++;
}
}

for(p=0;p<a.col;p++)
{
for(q=0;q<a.col;q++)
{
System.out.print(a.matrix[p][q]+ " ");
}
System.out.println();
}
} catch (IOException e) {throw e;}
return a;
}

[r035198x]

NullPointerException a.k.a NullReferenceException - Java insights

[Eranga]

I always get a NullpointerException. I don't know why this happens.

Because at runtime, one of variable not assign with the correct/valid value.

[keye906]

Because at runtime, one of variable not assign with the correct/valid value.

do you mean a object was not properly instantiated, the problem is about this method?

[r035198x]

Do yourself a favour and read the link posted in the first reply.

[cselic]

a.matrix[p][q]=stok.nval; // the exception occurs at this line.

try to initialize object a.

For example:

Matrix[][] a = new Matrix[p][q];

[Tolls]

a.matrix[p][q]=stok.nval;

Either a is null, a.matrix is null, a.matix[p] is null or stok is null.

Now, since a.row() a couple of lines earlier didn't throw an NPE, and stock.hasNext() didn't either, these are not null.

So that leaves the matrix attribute in your matrix class (and classes should start with a capital, so that would be Matrix), or part of its array.

[Eranga]

do you mean a object was not properly instantiated, the problem is about this method?

Yes. Initialized with a null value.

[Eranga]

a.matrix[p][q]=stok.nval;

Either a is null, a.matrix is null, a.matix[p] is null or stok is null.

Check this out.

[keye906]

thanks everyone. I figure it out.

[Eranga]

If you've solve the problem, please mark the thread solved.

[Eranga]

And also please use code tags when you post code segments in the forum next time. Unformated codes are really hard to read. If you don't know how to do it, check on my forum signature. You can find the relevant link.

[keye906]

Thank you, I will pay attention to my format next time

[Eranga]

That's much better.