[SOLVED] [newbie] java.lang.NullPointerException

[jon80]

I'm getting a Null.PointerException when I try to iterate through the array.

import java.util.*;
public class TestFour {

	public static void main(String[] args) {
		
	try {
				
		Integer[] arrayOne = new Integer[100];
		
		/*
		 Generate random numbers and iterate through 
		 the array assigning a random number 
		 within arrayOne[] 
		 */
		Random randomNumber = new Random(1000);
		for (int i : arrayOne ) //breaks here...
		{
			//arrayOne[i] = randomNumber.nextInt();
			arrayOne[i] = 1;
		}
		
		//iterate through array and display contents
		for (int i : arrayOne )
		{
			System.out.println(i);
		}
	}
	catch (AssertionError ex)
	{
System.out.println(ex.toString() + ": variable has a " + ex.getMessage() + " value.");
	}

	}

}

NOTE: It would be nice to have html tags (e.g. <code> </code> that distinguish code snippets from text.

[Tolls]

You can:

import java.util.Random;

public class Test4 {
	public static void main(String[] args) {

		try {

			Integer[] arrayOne = new Integer[100];

			/*
			 * Generate random numbers and iterate through the array assigning a
			 * random number within arrayOne[]
			 */
			Random randomNumber = new Random(1000);
			for (int i : arrayOne) // null?
			{
				// arrayOne[i] = randomNumber.nextInt();
				arrayOne[i] = 1;
			}

			// iterate through array and display contents
			for (int i : arrayOne) {
				System.out.println(i);
			}
		} catch (AssertionError ex) {
			System.out.println(ex.toString() + ": variable has a "
					+ ex.getMessage() + " value.");
		}

	}

}

The hash button sticks code tags around.

You ought to say where the NPE is being thrown from.

ETA: The line you've marked isn't doing what you think. Try writing it as a full for loop instead of using the shorthand and you might see the problem.

[jon80]

Thanks, I'll add tags from now on.

The code breaks where I have commented //null ?

[Tolls]

Thanks, I'll add tags from now on.

The code breaks where I have commented //null ?

As I say, in the edit (you probably posted before that), try expanding the for loop, rather than using the shorthand.

The int i you're using is actually the value of whatever part of the array you've iterated to, and not the index into the array.

[jon80]

As I say, in the edit (you probably posted before that), try expanding the for loop, rather than using the shorthand.

The int i you're using is actually the value of whatever part of the array you've iterated to, and not the index into the array.

...
Random randomNumber = new Random(1000);
		int idx = 0;
		for (int i : arrayOne ) //still breaks here
		{
			arrayOne[idx] = randomNumber.nextInt();
			idx++;
		}
...

[Tolls]

OK, you're still using the shorthand.

What

for (int i : arrayOne)
{}

Expands into is something along the lines of:

for (int a = 0; a < arrayOne.length; a++)
{
	int i = arrayOne[a];
}

(this isn't actually correct, since in reality it uses Iterators I think, but for the purposes of this explanation it'll do).

As you can see, you're 'i' isn't the index, which is "hidden" from you (it's 'a').

Now, the line:

int i = arrayOne[a];

is itself shorthand, since arrayOne[a] is an Integer and not an int...it expands to:

int i = arrayOne[a].intValue();

And this is where you're null pointer exception comes from.