[SOLVED] [newbie] java.lang.NullPointerException

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

:confused:

Code:

---

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.

You can:

Code:

---

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.

Thanks, I'll add tags from now on.

The code breaks where I have commented //null ?

Quote:

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Originally Posted by jon80

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.

Quote:

---

Originally Posted by Tolls

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.

---

Code:

---

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

---

OK, you're still using the shorthand.

What

Code:

---

for (int i : arrayOne)
{}

---

Expands into is something along the lines of:

Code:

---

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:

Code:

---

int i = arrayOne[a];

---

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

Code:

---

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

---

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