Recursive function changes original array?

Question

I've got a recursive functions which needs to create an array consists of special objects...

My custom object is populated from this class:

public class CategoryItem {

boolean hasSubCategories = false;
ArrayList<CategoryItem> subs;
ArrayList<Integer> positions;
String categoryName, categoryId;

// They have setter and getter methods

}

And here is my recursive function:

public ArrayList<CategoryItem> GetLists(ArrayList<Integer> positions, int to) {

    ArrayList<CategoryItem> items = new ArrayList<CategoryItem>();
    for(int i = 0; i < to; i++) {
        CategoryItem item = new CategoryItem();
        item.setHasSubCategories(RandomBool());
        item.setCategoryName("Category " + i);
        item.setCategoryId(RandomId());
        ArrayList<Integer> pos = positions;
        pos.add(i);
            Log.d(LOG, "positions: " + positions);
        Log.d(LOG, "pos: " + pos);
        item.setPositions(pos);
        if(item.isHasSubCategories()) {
            item.setSubs(GetLists(item.getPositions(), i));
        }
        items.add(item);
    }
    return items;

}

In this function, RandomBool() method returns true/false randomly... And RandomId() is also not important...

The problem is on "positions" array. I want to have every item has specific positions array, such as:

for the first step, every item needs to have: [0], [1], [2], [3] ...

for the next step, lets assume we picked positioned 3: [3,0], [3,1], [3,2]

But i found out that when i add an item to pos array, which i assigned it temporarly not to change the original one on recursive functions, it's been also added to positions array, the original one. So the result for the first step is like: [0,1,2,3] on every item.

And the log was like:

positions: []
pos: []
positions: [0]
pos: [0]
positions: [0, 1]
pos: [0, 1]
positions: [0, 1, 2]
pos: [0, 1, 2]
positions: [0, 1, 2, 0]
pos: [0, 1, 2, 0]
positions: [0, 1, 2, 0, 1]
pos: [0, 1, 2, 0, 1]

How to prevent this and make it work? Where is the problem? Any help is appreciated. Thanks...

Karthik T · Accepted Answer · 2013-01-16 07:48:48Z

1

which i assigned it temporarly not to change the original one on recursive functions

Are you from C/C++ background?

A = B

Does not make a copy in java. They both will point to the same object. It is sort of like all the variables are only C pointers.

You should use the copy constructor to make a copy of the list.

ArrayList<Integer> pos = new ArrayList<Integer>(positions);

edited Jan 16, 2013 at 7:48

answered Jan 16, 2013 at 7:43

Karthik T

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3 Comments

yahya Over a year ago

I also tried: ArrayList<Integer> pos = (ArrayList<Integer>) positions.clone(); but it didn't help either... So how to do it ?

yahya Over a year ago

Silly mistake :) Thank you so much... I'll accept your answer in a few minutes...

Karthik T Over a year ago

@yahya you are welcome, took me a while to grasp or indeed even recognise this fact.. If you are from C/C++ side, think of all variables in java as glorified pointers.

partlov · Accepted Answer · 2013-01-16 07:49:21Z

1

You can consider that ArrayList<Integer> pos = positions; is like you assigned pointer to an ArrayList (in C/C++ world), which means that you will modify original list inside your function. To Work on local list you will have to create new list and work with it:

ArrayLis<Integer> copiedList = new ArrayList<Integer>(ooriginalList);

answered Jan 16, 2013 at 7:49

partlov

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1 Comment

Karthik T Over a year ago

A pointer, and not a reference, also List wont work since his function expects ArrayList

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