Java: What is the difference between these methods of construction

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.

Answer 2

They are equal, but: the difference is that in the last example, you gain the benefit of being able to do more advanced initialization logic: error handling etc.