Question

For example:

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

vs

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

What is the exact difference between these two?

When should we use the first one and when should we use the second?

Answer 1

The first form is the most desirable one because you hide the implementation (ArrayList) from the rest of your code and ensure your code only works with the abstraction (List). The advantage of this is that your code will be more generic and therefore easier to adapt, for example when you change from using an ArrayList to a LinkedList, Vector or own List implementation. It also means local changes are less likely to cause changes in other parts of your code (‘ripple-effect’), increasing your code’s maintainability.

You need the second form when you want to do things with your variable that are not offered by the List interface, for example ensureCapacity or trimToSize

EDIT: extra explanation of changing the implementation

Here is an example of declaring a variable as a Collection (an even more generic interface in java.util):

public class Example {

    private Collection<String> greetings = new ArrayList<String>();

    public void addGreeting(String greeting) {
        greetings.add(greeting);
    }
}

Now suppose you want to change the implementation in order to store unique greetings, and therefore switch from ArrayList to HashSet. Both are implementations of the Collection interface. This would be easy in this case because all the existing code treats the greetings field as a Collection:

public class Example {

    private Collection<String> greetings = new HashSet<String>();

    public void addGreeting(String greeting) {
        greetings.add(greeting);
    }
}

There is an exception. If there is code which casts the greetings field back to its implementation, this makes that code ‘implementation-aware’, violating the information-hiding you tried to achieve, for example:

ArrayList<String> greetingList = (ArrayList<String>) greetings;
greetingList.ensureCapacity(42);

Such code would cause a runtime error ‘java.lang.ClassCastException: java.util.HashSet incompatible with java.util.ArrayList’ if you change the implementation to HashSet, so this practice should be avoided if possible.