Interfaces

Objectives

On completion of this period, you would be able to learn• Concepts of interfaces• Why interfaces are needed ?• Benefits of interfaces• Drawback of interfaces

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Recap

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•Inner classes

• Classes can defined as members a block of java code

• Member class

• Local class

• Anonymous class

• import statement

• Importing classes from other packages

Concepts of Interfaces

• What is an Interface ?• English meaning

• Something that connects two things

• Interface feature in Java realises the Object Oriented Axiom• Implementation

should be separated from Interface

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•It is typically the programmer of ”Other code” who has designed the interface to be used in ”Your code”•“Other code” defines Interface•“Your code” implements the Interface

Fig. 29.1. Interface between two programs

Concepts of Interfaces Contd..

• What is an Interface ?

• A collection of method declarations and constant fields

• It defines a standard and public way of specifying the behavior of classes

• Defines a contract – to implement all the methods of interface

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Concepts of Interfaces Contd..

• All methods of an interface are abstract methods

• Defines the signatures of a set of methods, without the body (implementation of the methods)

• A concrete class must implement the interface (all the abstract methods of the Interface)

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Why Interface ? Reason #1

• To reveal an object's programming interface (functionality of the object) without revealing its implementation• This is the concept of encapsulation• The implementation can change without

affecting the caller of the interface

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Why Interface ? Reason #2 • To have unrelated classes implement similar methods

(behaviors)

• One class is not a sub-class of another

• Example:

• Class Line and class MyInteger

• They are not related through inheritance

• But, both may want to implement comparison methods

• Check Is Greater (Object x, Object y)

• Check Is Less (Object x, Object y)

• Check Is Equal (Object x, Object y)

• Define Relation interface which has the three abstract methods above

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Why Interface ? Reason #3

• To model multiple inheritance • A class can implement multiple interfaces while

it can extend only one class

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Interfaces vs. Abstract Classes• All methods of an Interface are abstract methods

while some methods of an abstract class are abstract methods• Abstract methods of abstract class have

abstract modifier

• An interface can only define constants while abstract class can have fields

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Interfaces vs. Abstract Classes Contd..• Interfaces have no direct inherited relationship

with any particular class, they are defined independently• Interfaces themselves have inheritance

relationship among themselves

• To model multiple inheritance • A class can implement multiple interfaces while

it can extend only one class

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Interface As a Type

• When you define a new interface • you are defining a new reference type

• You can use interface names anywhere you can use any other type name

• If you define a reference variable whose type is an interface, any object you assign to it must be an instance of a class that implements the interface

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Example : Interface As a Type• Let's say Person class implements

PersonInterface interface• You can do

• Person p1 = new Person();• PersonInterface pi1 = p1;• PersonInterface pi2 = new Person();

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Interface vs. Class: Commonality

• Interfaces and classes are both types• This means that an interface can be used in

places where a class can be used• For example:

• // Recommended practice • PersonInterface pi = new Person(); • // Not recommended practice• Person pc = new Person();

• Interface and class can both have methods

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Interface vs. Class: Differences

• The methods of an interface are all abstract methods• They cannot have bodies

• You cannot create an instance from an interface• For example:

• PersonInterface pi = new PersonInterface(); //ERROR!

• An interface can only be implemented by classes or extended by other interfaces

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Benefits of Interfaces

• Provide multiple inheritance• Allow standard sets of methods to be used

across the class hierarchy• As interfaces are declared independently of

classes, they are unaffected by changes to specific classes

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Drawback of Interfaces

• Interfaces cannot grow• Cannot add more methods to the existing

interface• One has to re-implement all the classes which

implements the interface when it grows

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Summary

• In this class we have discussed– Basic concepts of interfaces– What is the need of interfaces– Comparison of interfaces with classes and

abstract classes– The benefits and drawbacks of interfaces

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Frequently Asked Questions

1. What is an interface ?

2. List the benefits of interface

3. Compare abstract classes and interfaces

4. Compare a class and interface

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Quiz

1.Which is NOT a part of an interface?1. Methods

2. Constructors

3. Constants

4. None

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Quiz Contd..

2 .What is the commonality between interfaces

and abstract classes?

1. Both have methods

2. Both have abstract methods

3. Both have variables

4. No commonality

Quiz Contd..

3.Which of the statement is true about interface?1. It contains only methods

2. It contains both methods and constants

3. It contains both abstract methods and constants

4. None

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Quiz Contd..

4. What is the commonality between interfaces and classes?

1. Both have methods

2. Both define new types

3. Both have variables

4. No commonality