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I'm trying to write a simple game where an enemy chases the player on a grid. I'm using the simple algorithm for pathfinding from the Wikipedia page on pathfinding. This involves creating two lists with each list item containing 3 integers. Here's test code I'm trying out to build and display such a list.

When I run the following code, it prints out the same numbers for each array in the ArrayList. Why does it do this?

public class ListTest {

public static void main(String[] args) {
    ArrayList<Integer[]> list = new ArrayList<Integer[]>(); 
    Integer[] point = new Integer[3];
    for (int i = 0; i < 10; i++) {
        for (int j = 0; j < 3; j++) {
            point[j] = (int)(Math.random() * 10);
        }            

        //Doesn't this line add filled Integer[] point to the 
        //end of ArrayList list?
        list.add(point);      

        //Added this line to confirm that Integer[] point is actually 
        //being filled with 3 random ints.
        System.out.println(point[0] + "," + point[1] + "," + point[2]);
    }
    System.out.println();

    //My current understanding is that this section should step through 
    //ArrayList list and retrieve each Integer[] point added above. It runs, but only 
    //the values of the last Integer[] point from above are displayed 10 times.
    Iterator it = list.iterator();
    while (it.hasNext()) {
        point = (Integer[])it.next();  
        for (int i = 0; i < 3; i++) {
            System.out.print(point[i] + ",");
        } 
            System.out.println(); 
        } 
    }
}
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3
  • 1
    What happens when you compile this code? Commented Dec 2, 2012 at 0:31
  • It will run (I'm writing in Netbeans IDE), but it prints the same random numbers, say 9,9,2 ten times. Commented Dec 2, 2012 at 0:36
  • Did your original code using int[]s do the same thing? Commented Dec 3, 2012 at 0:02

4 Answers 4

Reset to default
14

First of all, several of the other answers are misleading and/or incorrect. Note that an array is an object. So you can use them as elements in a list, no matter whether the arrays themselves contain primitive types or object references.

Next, declaring a variable as List<int[]> list is preferred over declaring it as ArrayList<int[]>. This allows you to easily change the List to a LinkedList or some other implementation without breaking the rest of your code because it is guaranteed to use only methods available in the List interface. For more information, you should research "programming to the interface."

Now to answer your real question, which was only added as a comment. Let's look at a few lines of your code:

Integer[] point = new Integer[3];

This line creates an array of Integers, obviously.

for (int i = 0; i < 10; i++) {
    for (int j = 0; j < 3; j++) {
        point[j] = (int)(Math.random() * 10);
    }            

    //Doesn't this line add filled Integer[] point to the 
    //end of ArrayList list?
    list.add(point);
    //...
}

Here you assign values to the elements of the array and then add a reference to the array to your List. Each time the loop iterates, you assign new values to the same array and add another reference to the same array to the List. This means that the List has 10 references to the same array which has been repeatedly written over.

Iterator it = list.iterator();
while (it.hasNext()) {
    point = (Integer[])it.next();
    for (int i = 0; i < 3; i++) {
        System.out.print(point[i] + ",");
    }
    System.out.println();
}

Now this loop prints out the same array 10 times. The values in the array are the last ones set at the end of the previous loop.

To fix the problem, you simply need to be sure to create 10 different arrays.

One last issue: If you declare it as Iterator<Integer[]> it (or Iterator<int[]> it), you do not need to cast the return value of it.next(). In fact this is preferred because it is type-safe.

Finally, I want to ask what the ints in each array represent? You might want to revisit your program design and create a class that holds these three ints, either as an array or as three member variables.

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2 Comments

Wow, thanks so much for that thorough response. I made the changes you suggested, and it works perfectly. Furthermore, you really made it click in my mind. The integer arrays represent x and y coordinate pairs with an integer counter that represents steps out on the grid from the starting point. I think I see what you mean about creating a class Point to do the comparisons and such. I'm going to work on that right away. Cheers!
@vitriolik In that case, I suggest that Point just include x and y. The counter should most likely be stored separately.
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I would highly recommend to enclose the integer array of 3 numbers into a meaningful class, that would hold, display and control an array of 3 integers.

Then in your main, you can have an growing ArrayList of objects of that class.

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1 Comment

I suggest going a step further by creating a class that clearly describes the meaning of these three ints. For example, if they are (x, y, z) coordinates, create a class named Point with three member variables named x, y, and z.
2

You have an extra ) here:

element = (int[])it.next()); //with the extra parenthesis the code will not compile

should be:

element = (int[])it.next();
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1 Comment

@Code-Guru I have made the necessary changes in my answer.
1

Besides the problem in the other answer, you cal it.next() two times, that cause the iterator move forward two times, obviously that's not what you want. The code like this:

element = (int[])it.next()); 
String el = (String)element;

But actually, I don't see you used el. Although it's legal, it seems meaningless.

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1 Comment

Thanks for that. I should have been clear that I was including both lines just to illustrate what I am probably misunderstanding about how objects can be retrieved from the list.

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