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# Interfaces

* [Interfaces](/workspace/languages/java/intermediate/oop/interfaces.md#interfaces)
  * [What?](/workspace/languages/java/intermediate/oop/interfaces.md#what)
  * [Why use?](/workspace/languages/java/intermediate/oop/interfaces.md#why-use)
  * [Override method](/workspace/languages/java/intermediate/oop/interfaces.md#override-method)
  * [Inheritence](/workspace/languages/java/intermediate/oop/interfaces.md#inheritence)
  * [Default Methods (JAVA8 +)](/workspace/languages/java/intermediate/oop/interfaces.md#default-methods-java8-)
  * [other](/workspace/languages/java/intermediate/oop/interfaces.md#other)
  * [Links](/workspace/languages/java/intermediate/oop/interfaces.md#links)

## What?

* is an abstract type that is used to specify a behavior that classes must implement.
  * Is just a contract. The implementing class ensures, that it will have these methods that can be used on it. It is basically a contract or a promise the class has to make.
  * A class can implement multiple interfaces
* A Java interface is a bit like a class, except a Java interface can only contain method signatures and fields. An Java interface cannot contain an implementation of the methods, only the signature (name, parameters and exceptions) of the method.
* Interfaces cannot be instantiated (does not container constructor), but rather are implemented. A class that implements an interface must implement all of the non-default methods described in the interface, or be an abstract class.
* Object references in Java may be specified to be of an interface type; in each case, they must either be null, or be bound to an object that implements the interface.
* Interfaces can extend multiple interfaces
* <https://www.quora.com/If-asked-on-a-coding-interview-What-is-the-primary-difference-between-an-interface-and-abstract-class-what-is-the-correct-answer>

## Why use?

* Interfaces are the best way to maintain well decoupled constructs.
* You can use interfaces in Java as a way to achieve polymorphism.

## Override method

* annotation
  * Use of @Override above method declaration, to say it is using the method from the interface
* Why?
  * <https://stackoverflow.com/questions/94361/when-do-you-use-javas-override-annotation-and-why>
  * <https://javarevisited.blogspot.com/2012/11/why-use-override-annotation-in-java.html>

## Inheritence

* inherit from interfaces
  * classes
  * interfaces
* multiple inheritence

## Default Methods (JAVA8 +)

* Issues
  * Multiple inheritence

## other

* methods
* Reference types
  * polymorphism
  * cannot use concrete implementation class which has its own method with this ref type

## Links


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