Question

I’m getting weird problems in my android app and I think it may be linked to the behavior of the way the ArrayList works. Check the following code and please tell me if I’m correct or doing something wrong:

ArrayList<ArrayList<String>> arr = new ArrayList<ArrayList<String>>();
ArrayList<String> tmp = new ArrayList<String>();
tmp.add("test");
arr.add(tmp);
tmp.clear();

After the last line the contents of arr[0] is emptied. So does that mean that when adding one ArrayList to another it does it by reference?

So if I have the following method:

void addArray(ArrayList<String> arr) {
 group.add(arr); // group is ArrayList<ArrayList<String>>;
};

I must change it to:

void addArray(ArrayList<String> arr) {
 ArrayList<String> tmp = new ArrayList<String>();
 tmp.addAll(arr);
 group.add(tmp); // group is ArrayList<ArrayList<String>>;
};

to make sure that if I clear the incoming array somewhere else that nothing happens to the group array?

Answer 1

In Java there is no passing by value, every object is passed by reference. So in your case arr[0] and tmp are the same object and clearing it will result in arr[0] being cleared. Hope this helps.

EDIT

As a quick answer to the second part of your question: you don’t need to use the tmp ArrayList inside the addArray method. The argument of the addArray method is a reference to the object, passed by value. So changing it won’t have any effect outside of the addArray method. Hope it’s clear.