Interface vs. Class for List and Set

[James DePrisco]

I have been using the following type code without thinking about it:

List<SubNode> sub = new ArrayList<SubNode>();

However, List is an interface, not a Class. I always thought an interface was a prepackaged group of functions you add in with "implements".

The above seems to say assign a new instance of ArrayList class to interface sub. Why wouldn't I do this instead:

ArrayList<SubNode> sub = new ArrayList<SubNode>(); ?

Thanks.

[Christophe Verré]

ArrayList implements List (check the class signature), or you could say that ArrayList IS-A List. So you can cast it to a List. A more explicit example of this use would look like this :

List<SubNode> sub = getSubNodes(parentNodes);

Here, you don't know (and you don't need to know) which concrete class you are using. The concrete class would be instanciated in getSubNodes():

public List<SubNode> getSubNodes(Node parent) {
	List<SubNode> sub = new ArrayList<SubNode>(); 
	...
	return sub;
}

The interesting point here is that you even if you'd change ArrayList to some other kind of List, and it would not impact on the code calling getSubNodes().

[Campbell Ritchie]

And welcome to the Ranch

You will often find that sort of coding described as "program to the interface, not the implemenation".