Answer Posted / chitij mehrotra

1 A class may inplement several interfaces. A class may
extend only one abstract class.
2 An interface can have only abstract methods. An abstract
class can have both abstract methods and concrete methods.
At one abstract method in the abstract class.
3 Interfaces cannot have constructors. Abstract class can
have constructors.
4 Only static final variables are allowed to be decelared.
Both static and instance variables are allowed.
5 All decelared methods are implicitly public. Can have any
kind of methods.
6 We should initialize variables in an interface. Not
necessary to initialize variables in an abstract class.
7 In the case of interface you can make any class in the
hierarchy. In the case of abstract class you know the class
hierarchy in advance.
8 An interface implementation may be added to any existing
third party class. A third party class must be rewritten to
extend only from the abstract class.
9 Interfaces can be implemented by classes that are not
related to one another Abstract classes are used only when
there is a “is-a” type of relationship between the classes.

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