Interfaces

Question 1

Interface

Answer 1

an interface is used as a contract to guarantee implementation of certain behaviors in a subclass.It’s an abstract entity - all methods are implicitly “public abstract” while instance variables, if there are any, are “public static final”.

Question 2

To implement an interface use

Answer 2

the implement keyword

Question 3

Interface methods and variables

Answer 3

• All methods are public abstract

- all variables are public static final

Question 4

Interfaces and Inheritance

Answer 4

• said to have “type” inheritance
• a class that implements an interface is polymorphically considered an instance of the interface
• An interface describes what behaviors a class should have but provides none of its own.

Question 5

Class and interfaces

Answer 5

-A class can implement multiple interfaces but can only extend one class

Question 6

Java 8 Interfaces

Answer 6

Public-facing interfaces (APIs) were difficult to update or change. So default methods were created.

Question 7

Default methods

Answer 7

-interface methods that are implemented in the interface
(Because an implementation is present in the interface, classes that implement that interface are not required to implement the method. To use a default method, you just add the default keyword before the method declaration in the interface.Then you can provide an implementation, like normal.) 
-implementing classes are not required to implement default methods, but they can override them
-the guarantee with an interface is that the implementing class will provide an implementation for those other methods, so they are safe to call.Interfaces are also allowed to have static methods with definitions.Like any other static method, you can invoke these without creating an instance of a class that implements the interface.And because they are static methods, they can only call other static methods, or access methods through local reference variable

Question 8

static methods

Answer 8

• can be called without creating an instance of the class

- can only call other static methods directly. Must use local reference variables to call non-static methods

Question 9

Multiple Inheritance

Answer 9

Use the super() keyword to refer to a specific interface method (in the case that two interfaces have default methods with the same name)

Question 10

Abstract Classes

Answer 10

• Can have instance variables that are not always public, static, final
• can have methods that are bot public

Question 11

What is the difference between interfaces vs abstract classes?*

Answer 11

• A class can extend only one class (abstract or not) but can implement multiple interfaces
• Interface variables are implicitly public static final. Abstract classes can declare any modifiers on instance variables.
• Interfaces can (as of Java 8) provide default methods, but all others are public abstract. Abstract classes can declare any modifiers on their methods.

Question 12

Why use an interface over an abstract class?

Answer 12

• In general, if you wanted to develop and enforce a certain hierarchy of functionality where state and behavior was tracked, then you would use an abstract class
• If you want a class to be free to implement functionality without constrained to a hierarchy, then an interface is a better approach.