An interface defines a new type of data by specifying a set of method that any implementing class must provide. Unlike a class, an interface can not include method implementations—these are left to the implementing classes.

import java.util.List;
import java.util.ArrayList;
import java.util.LinkedList;

public class InterfaceExample {
    public static void main(String[] args) {
        List<String> arrayList = new ArrayList<String>();
        List<String> linkedList = new LinkedList<String>();

        arrayList.add("Apple");
        linkedList.add("Banana");

        System.out.println(arrayList.get(0));  // Apple
        System.out.println(linkedList.get(0)); // Banana
    }
}

In this case both ArrayList and LinkedList are both implementations of the interface List, they both implement the same methods and conform to the same specification.