why the concept of interfaces was implemented in JAVA

1. Interface!! Why?
A bit of history never hurt anyone.
The Java platform and language
began as an internal project at
Sun Microsystems in December
1990, providing an alternative to
the C++/C programming languages.
Engineer Patrick Naughton had
become increasingly frustrated with
the state of Sun's C++ and C APIs
(application programming interfaces)
and tools.[wikipedia]

2. This was one of his problems
Deadly Diamond of Death
 Class B and class C both inherit
from class A and both
override the print() method .
Both inherit the “i” instance
variable .
 Imagine that the “i” instance
variable is used by both B and
C ,with different values . What
happens if the D needs both
values of “i”?
 Problem with the multiple
inheritance. Which print()
method runs when you call
print() in class D.

3. Interface to the rescue!
A Java interface is pure abstract  Java gives us a solution . An
class. Interface. A Java interface solves
the problem by giving the
polymorphic benefits of the
multiple inheritance avoiding
Deadly Diamond of Death.
interface Pet  Java does that by making all the
methods abstract.That way the
subclass must implement the
methods ,so the JVM knows which
abstract void beFriendly(); two inherited version it needs to
call.
abstract void play();  Interface shows a “has-a”
relationship among classes.

4. The boring coding stuff
 To DEFINE an interface:
public interface Pet{
abstract void beFriendly();
abstract void play(); }
 To IMPLEMENT an interface:
public class dog extends Canine implements Pet{
void beFriendly(){….}
void play(){….}
}

5. It’s something like this

6. Reference materials
These are the places from where I used my
copy paste skills :
1. www.wikipedia.org
2. Head First Java - Kathy Sierra and Bert Bates .
(2nd edition, pages 244-247)

7. (I am outta here. Who’s next?)