1. 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
1

2. Recap
•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
2

3. Concepts of Interfaces
• What is an Interface ?
• English meaning
• Something that
connects two things
• Interface feature in Fig. 29.1. Interface between two programs
Java realises the •It is typically the programmer of
Object Oriented ”Other code” who has designed
the interface to be used in ”Your
Axiom code”
• Implementation •“Other code” defines Interface
should be separated •“Your code” implements the
Interface
from Interface
3

4. 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
4

5. 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)
5

6. 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
6

7. 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 7

8. Why Interface ? Reason #3
• To model multiple inheritance
• A class can implement multiple interfaces while
it can extend only one class
8

9. 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
9

10. 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
10

11. 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
11

12. 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();
12

13. 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
13

14. 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
14

15. 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
15

16. 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
16

17. 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
17

18. 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
18

19. Quiz
1.Which is NOT a part of an interface?
1. Methods
2. Constructors
3. Constants
4. None
19

20. 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

21. 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
21

22. 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