Question

I have a 2D ArrayList. The ArrayList contains 10 ArrayLists. I tried the following code:

This is the main 2D ArrayList. Inside the main ArrayList there are10 ArrayLists:

 ArrayList<ArrayList<Items>> arrayList = new ArrayList<ArrayList<Items>>();

Here I tried to create a copy of one of the ArrayList (selectedRow is just a number which says which ArrayList I get)

ArrayList<Items> newList = new ArrayList<Items>(arrayList.get(selectedRow));

After that I create another ArrayList:

   ArrayList<Items> changeList = new ArrayList<Items>(it.returnTheNewArrayList(newList,randomItem));

Then in another class I created this method. The purpose of this method is to change an attribute of one of the objects.

public  ArrayList<Items> returnTheNewArrayList(ArrayList<Items> a,int item){

  int randomBin = r.nextInt(50);

    for(Items i:a){

        if(item==i.itemIds()){

            while(randomBin==i.bins()){
                randomBin = r.nextInt(50);

            }
            i.setBin(randomBin);

        }

    }
return a;

}

Finally, I set the new ArrayList in the 2D ArrayList

arrayList.set(whichList, changeList);

This is the procedure. I have find out, that when I run this procedure, both newList and changeList are the same. In both of these ArrayLists I’m saving the change I did in the method returnTheNewArrayList (I found out it with the debugging). But I want to change only one (the changeList).

What did I do wrong?

Answer 1

The lists contain references to objects. When you call i.setBin(...) that’s (presumably) making a change to the object itself.

Each list has an independent copy of the references – so you could remove an element from one list without affecting the other – but they’re only references.

Imagine you gave two people clipboards, with the same list of home addresses on. One person went and painted the front door of every house list on their clipboard red, then the second person visited all the same houses. The second person would see red doors, wouldn’t they? It’s the same thing here. The lists contain references, not objects.

If you want the lists to be completely independent, you’ll need to populate them with references to different objects.

EDIT: I’ve just noticed that you’ll also need to change your returnTheNewArrayList method, which actually isn’t even creating a new ArrayList in the first place!

public ArrayList<Items> returnTheNewArrayList(ArrayList<Items> a,int item) {
    // Stuff which doesn't change the value of a...

    return a;
}

Again, the value of a is just a reference to the list… so when you return the same reference back, you’re not returning a new ArrayList at all.

You really need to understand how references and objects work in Java – it’s absolutely crucial to working with the language.