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edumorlom
4,073 PointsWhy are Lists an interface?
I thought interfaces were just a contract that you had to implement their methods. Like if I implement Animal interface, I am agreeing to write their methods.
For example:
public class Mammal implements Animal {}
This implements the Animal interface.
List<String> listOfAnimals;
Why are lists referred to as interfaces?
Thanks!
3 Answers
Jennifer Nordell
Treehouse TeacherHi there! As I understand it an Interface gives an object a way to interact with the "world" outside through a series of methods. Here's some documentation from Oracle about interfaces and here's some documentation specifically on the List interface. I feel like the big distinction here is that an interface is a type of object that only implements methods and no properties (at least not publicly exposed).
But someone please tell me if I've got this incorrect!
Hope this helps!
Craig Dennis
Treehouse TeacherList is an interface that provides methods like add contains and delete. There are several concrete implementations that implement the List interface such as ArrayList, LinkedList etc.
Here is some more on the List interface.
What you are talking about here is a parameterized type or generic. List<Animal> can be read as a list of animals. And only Animals can be added, removed, etc.
List<Animal> animals = new ArrayList<Animal>();
See how we still choose the concrete implementation?
That help?
edumorlom
4,073 PointsI dont understand why Lists are considered interfaces. Arent they Classes? They have methods. It confuses me.
Darth R3id4k
Courses Plus Student 10,125 PointsClasses - you can make object of class, you can inherit class, but only one. If you inherit class you can choose some of its methods you need (not all if you don't need). Interface - it is full abstract class - it means all its methods are abstract (have only signature). Only talk what to do but don't how to do. You are obligated to provide your own implementation for all methods from interface. But you cannot inherit interfaces - you can only implement them. You cannot create object from interface. But you can implement many interfaces.
I know that some thing called "composition" let to omit issue concerning one class inheritance.
P.S. Thanks to many developers we don't have to make our own implementation (but we can if you are so smart) of Collection interfaces. We can use already prepared good stuff like ArrayList<>() or map etc. There are many of them because there isn't one simple way to resolve many issues in specified short among of time. We have to consider their + and - and wisely choose one of them to achieve deliberate effect.