Question

In most of the Java code that uses collections,i see something like this.

List<String> list = new ArrayList<String>(); 

But i always wondered why it is preferred over

ArrayList<String> = new ArrayList<String>();

I researched a bit but wasnt able to understand a lot.

Answer 1

Because when using the object by its interface you can later change the implementation. For example:

public ArrayList<String> doSomething(..) {
    ...
    return arrayList;
}

But after a while it appears ArrayList is inefficient for what it is mostly used by the callers of this method, so you decide to change it to LinkedList.

• if your method returns just List you are done
• if your method returns ArrayList you should go to all places that the method is invoked and change that. And what if someone used some arraylist-specific methods – you have to think how to refactor that. And that’s the good scenario. If your code is a library and other branches/companies are using it you may be stuck with the inefficient code.

See also:

• What does it mean to "program to an interface"?
• "Program to an interface". What does it mean?