Question

What’s the difference between these two methods of initializing the observers ArrayList. Or any other type for that matter. Is one faster than the other? Or am I missing some other benefit here.

class Publisher implements Observerable
{
     private ArrayList observers = new ArrayList();
}

class Publisher implements Observerable
{
    private ArrayList observers; 

    public Publisher()
    {
        observers = new ArrayList();
    }
}

Answer 1

They’re equivalent. In fact, if you compile the two you’ll see that they generate exactly the same byte code:

Publisher();
  Code:
   0:   aload_0
   1:   invokespecial   #1; //Method java/lang/Object."<init>":()V
   4:   aload_0
   5:   new     #2; //class java/util/ArrayList
   8:   dup
   9:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
   12:  putfield        #4; //Field observers:Ljava/util/ArrayList;
   15:  return    
}

Given that they’re the same code, there clearly can’t be any speed difference 🙂

Note that in C# they’re not quite equivalent – in C# the initializer for observers would run before the base constructor call; in Java they really are the same.

Which you use is a matter of taste. If you have several different constructors which all initialize a variable in the same way it would make sense to use the first form. On the other hand, it’s generally a good idea if you have several constructors to try to make most of them call one “core” constructor which does the actual work.