Interfaces

Question 1

Limits of Inheritance Polymorphism

Answer 1

 Java is a single inheritance language
⚫ Classes can extend precisely one other class. No more.
⚫ So what if we want to build a class capable capable of more than one role?
⚫ As an abstract example:
 Man and a Woman are both arguably subclasses of Person
 A Lecturer is a Person, but will also likely be a Man or Woman
 Try implementing that in an inheritance hierarchy without duplication!

⚫ Some other languages are multiple inheritance, and can have an unlimited
number of parent classes (e.g. C++).
⚫ By polymorphism instances can be treated as a type of any parent class.
⚫ This provides additional flexibility, but brings complexity.
⚫ How to ratify multiple definitions and implementations of functionality from multiple parent classes? And how does this pass on to subsequent subclasses?

Question 2

Interfaces: Motivation

Answer 2

 Separating Polymorphism from Inheritance
⚫ Inheritance guarantees that a subclass of a class has all the external capabilities (visible methods and instance variables) of a superclass.
⚫ This means polymorphism can occur without compromising type safety.
⚫ Need inheritance be the only way to guarantee a class has specific
capabilities?

Question 3

Interfaces: Definition 1

Answer 3

 Interfaces
⚫ An Interface is an abstract specification of zero or more methods.
⚫ Interfaces cannot contain any instance variables.
⚫ Interfaces cannot contain any method implementation.
⚫ They can be considered similar to an abstract class, but without any
functionality.
⚫ A class can elect to implement any number of interfaces.
⚫ By contract, that class must then implement the methods specified in the
interface. This is enforced at compile time.
⚫ We can therefore guarantee that any class implementing an interface has
all the methods defined in that interface…

Question 4

Interfaces: Definition 2

Answer 4

 Interfaces and Polymorphism
⚫ Polymorphism in Java is also applied to interfaces.
⚫ As we can definitively say a class has the capabilities defined in an interface,
we can apply polymorphism without compromising type safety.
⚫ In other words, a class can be treated as an ‘instance’ of any interface it
chooses to implement.
⚫ As classes can implement many interfaces, this provides a clean mechanism that addresses the limitations of single inheritance languages, whilst sidestepping the complexity of multiple inheritance.
⚫ A complete definition of Polymorphism in Java :
⚫ A class can be treated as a type of its class, any of its super classes, or any of the interfaces it implements.

Question 5

Interface Example

Answer 5

 The Java API contains a multitude of interfaces…
⚫ Consider Swing’s ActionListener interface as a familiar example.
⚫ This specifies an interface with a single method:

Question 6

Interfaces and Inheritance

Answer 6

 Interfaces also have an inheritance hierarchy
⚫ Interfaces can extend other interfaces.
⚫ Methods defined in a super-interface are included in all sub-interfaces
⚫ Polymorphism is transient, as with object inheritance. An abstract example:

 However:
⚫ The interface hierarchy is totally distinct from the class hierarchy.
⚫ An interface cannot extend a class.
⚫ A class cannot extend an interface.

Question 7

AbstractButton: addActionListener

Answer 7

 Consider what is happening here.
⚫ This method (in AbstractButton) takes a single parameter – an object reference to an instance that implements the ActionListener interface.
⚫ It can then store these (in a heterogeneous collection), until needed.
⚫ It can then invoke the actionPerformed method on those instances when the
time is right (in this case, the button is clicked).
⚫ AbstractButton had no knowledge of our class (ChessBoard) when it was written, yet can invoke methods on it in a type safe manner!

Question 8

Roll your own interfaces

Answer 8

 It is very straightforward to create your own interfaces
⚫ Java interfaces can be used anytime you want to promote polymorphism, but
don’t want to be restricted by inheritance.
⚫ Promotes extensibility and elegance.
⚫ Allows you to define what other programmers must do in order to interface
with your code. Then your code can support working with objects that haven’t
even been written yet.
 Remember:
⚫ Interfaces define only methods. No instance variables allowed
⚫ Interfaces are merely a specification. No implementation of the methods.
⚫ If you need to provide default behaviour, use inheritance*
.

Question 9

Example custom interface: Creation

Answer 9

 Interfaces are defined much like classes
⚫ Interfaces are named, in capitalized camel case.
⚫ Filename should match interface name.

Question 10

Example custom interface: Usage

Answer 10

 Classes use implements keyword to implement your interface.
⚫ Can then be treated as a type of your new interface.