Question

For example, I have an ArrayList:

ArrayList<Animal> zoo = new ArrayList<Animal>();
// some code to add animals to zoo

I want to create a copy of this old zoo to new zoo, and I can use remove/add… to process this new zoo.(and of course, it will not affect to old zoo). So, I have a way that: create new zoo that clone each animals in old zoo (it means create animal objects).
So, it too SLOW.

I have an another way: create a reference of each animals of old zoo and copy to new zoo. So, new zoo just hold reference to animals of zoo. So, each time I remove/add,… It just delete reference to animal objects. No affect old zoo, and NO CREATE a new object.

But, I don’t know how to do it in Java. Please teach me.

Thanks 🙂

Answer 1

This is very simple in Java:

ArrayList<Animal> copy = new ArrayList<Animal>(zoo);

The constructor ArrayList<T>(Collection<? extends E> c) performs a shallow copy of the list that you supply as parameter, which is precisely the semantics that you are looking for.