What happens if i remove an object, which has another object in it that is pointed by an ArrayList

Question

In Java, I have a Class called Couple which has a String and an int as istance variables. And i have an ArrayList which holds istances of Class Couple. A method foo, adds new couples to the ArrayList cop_list. But also, add every message to another ArrayList, called msg_list.

foo(String msg,int id)
{
   Couple cop = new Couple(msg, id);
   //ArrayList<Couple>
   cop_list.add(cop);
   //ArrayList<String>
   msg_list.add(msg);
   ...
}

After that, When i call a delete(id) method, it should search and remove a couple by its id, but it should also remove the message from msg_list. So, my question is, when i delete a Couple object from the cop_list, what happens to the message in msg_list? msg_list still points to it until i explicitly remove it? String object msg is still on the heap?

delete(int id)
{
   //search and find the couple, save its msg in a variable
   msg = cop.getMsg();
   cop_list.remove(cop);

   //at this point, can/should i remove msg from msg_list?
   //what happens if i call:
   msglist.remove(msg);
}

Simply put: an object continues to live until its reference count drops to zero. One reference doesn't disappear due to another reference to the same object being nullified. — laune, Jun 01 '15 at 16:23
Yes, it will be not collected by GC until there are no references (it is simplified explanation). When you do msg = cop.getMsg you increase reference count to that object. — edmarisov, Jun 01 '15 at 16:25
@laune A bit oversimplified - circular references can be collected even though the circular nature means the refcount is > 0. — Thorn G, Jun 01 '15 at 16:34

score 1 · Answer 1 · answered Jun 01 '15 at 16:26

You will have to call msglist.remove(msg) to clear message after removing cop from the list.
The msglist still have reference to String from original cop object.

When you call cop_list.remove(cop) the reference to String in msglist still stays. So you will have to remove it explicitly.

score 0 · Accepted Answer · answered Jun 01 '15 at 16:23

So, my question is, when i delete a Couple object from the cop_list, what happens to the message in msg_list? msg_list still points to it until i explicitly remove it? String object msg is still on the heap?

Yes, you still have a reference to the String in your msg_list ArrayList, so that message is still in memory. The object won't be eligible for garbage collection while any references point to it.

What happens if i remove an object, which has another object in it that is pointed by an ArrayList

2 Answers2