CODEWITHSUNDEEP
java
mistahenry
mistahenry

Reputation: 8724

Difference in arrayList size

If I have an ArrayList of Objects in Java, is the underlying implementation really just an array of object references? I'm creating this method:

//method returns all combinations of Objects
public ArrayList<ArrayList<Object>> getAllCombinations(ArrayList<Object> allObjects){

}

As I started thinking about the problem, I wondered if I could just return an ArrayList<ArrayList<Integer>> where the ints were the position in allObjects (essentially thinking this would use drastically less memory). But then I started thinking that each value of the array implementation just points to the memory location of the objects in the and that the array wasn't creating new objects for each ArrayList I was creating (which would definitely use more memory). So in reality, it would use about the same memory if I just implement the method I mentioned above, right?

I think I need to understand Java's memory allocation a little bit better...

Upvotes: 0

Views: 65

Answers (2)

assylias
assylias

Reputation: 328568

Yes, an array or an arraylist only contains references to objects, not the objects themselves. So each item in your array uses 4 bytes of memory (on a 32 bit machine), regardless of the size of the underlying objects.

So keep it simple and return whatever makes sense.

Upvotes: 1

gzm0
gzm0

Reputation: 14842

You got that right: The elements of your ArrayList will only point to the objects (except if you copy them explicitly). So both implementations will use about the same memory as you say.

This is by the way something important to consider, if you want to alter the state of your objects as they will also seem altered in the old array where they originated from.

Upvotes: 1

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