What is more efficient: new ArrayList() vs clear()+add()?

Question

If I need to retrieve some collection of objects (assume its some Client), lets say from DB. So there are two possibilities:

1 in some class-receiver I would declare like:

...
 private final List<Client> clients = new ArrayList<Client>();
...
// the fillinng method:
fillClients(){
   dbHelper.getClients(clients);
}

So dbHelper's method will receive a ready to use collection it would NOT CREATE A NEW OBJECT (a list) but it will clear and fill the existing one, like:

public void getClients(List<Client>clients){
clients.clear();
...
while(cursor.moveToNext())
clients.add(new Client(...));
...
return;
}

And the other one pattern is:

...
 private List<Client> clients;
...
// the fillinng method:
fillClients(){
   clients = dbHelper.getClients();
}

So the corresponding method gonna CREATE A NEW OBJECT (a list):

public List<Client> getClients(){
 List<Client>clients = new ArrayList<Client> clients;
...
while(cursor.moveToNext())
clients.add(new Client(...));
...
return clients;
}

The question is related to a fact that a new object creating is a valuable operation.

Why not benchmark it and find out? One thing to consider: object allocation is _extremely_ cheap in Java. — Dmitri, Jun 06 '14 at 13:21

score 0 · Accepted Answer · answered Jun 06 '14 at 13:51

Actually I'll be careful reusing the same ArrayList. Because array behind the Arraylist would not be re-sized when we call clear().

Clear can be useful when you work with big number of elements, not creating a new object you are avoiding costs for array copy. In other cases just create a new object.

What is more efficient: new ArrayList() vs clear()+add()?

1 Answers1