interface as a method parameter in Java

Question

I had an interview days ago and was thrown a question like this.

Q: Reverse a linked list. Following code is given:

public class ReverseList {      interface NodeList {         int getItem();         NodeList nextNode();     }     void reverse(NodeList node) {      }     public static void main(String[] args) {      } } 

I was confused because I did not know an interface object could be used as a method parameter. The interviewer explained a little bit but I am still not sure about this. Could somebody enlighten me?

Answer 1

This is in fact one of the most common and useful ways to use an interface. The interface defines a contract, and your code can work with any class that implements the interface, without having to know the concrete class - it can even work with classes that didn't exist yet when the code was written.

There are many examples in the Java standard API, especially in the collections framework. For example, Collections.sort() can sort anything that implements the List interface (not just ArrayList or LinkedList, though implementing your own List is uncommon) and whose contents implement the Comparable interface (not just String or the numerical wrapper classes - and having your own class implement Comparable for that purpose is quite common).

Answer 2

It's not the interface "object" being passed to the method, still just a regular object. It's just a way of saying "this parameter will accept any object that supports this interface". It's equivalent to accepting some object of a base class type, even if you're passing in a subclass.