Abstract classes
Abstract class is class which contain one or more abstract methods, which is implemented by sub classes. An abstract class can contain no abstract methods but also containe mehtod with body. Abstract classes are useful in a situation when some general methods should be implemented and specialization behavior should be implemented by subclasses.Abstract class can contain private as well as protected members.
When to use Abstract Class-If we have a base class where all the classes will perform the same function, then we can define that in our Abstract class. If you plan on updating this base class throughout the life of your program, it is best to allow that base class to be an abstract class. Because you can make a change to it and all of the inheriting classes will now have this new functionality.
Interface
Interface is extremely useful when you don’t want a big hierarchical type framework. As interfaces are implicitly abstract, they cannot be directly instantiated except when instantiated by a class which implements the said interface. The class must implement all of the methods described in the interface, or be an abstract class. An Interface can only have public members. A class implementing an interface must implement all of the methods defined in the interface, while a class extending an abstract class need not implement any of the methods defined in the abstract class.Maintainability–if you want to add a new feature (method) in its contract, then you must implement those method in all of the classes which implement that interface. However, in the case of an abstract class, the method can be simply implemented in the abstract class and the same can be called by its subclass.
When To use interface – Interfaces are useful when you do not want classes to inherit from unrelated classes just to get the required functionality.It is used where there of chances adding new method in future. Interfaces are more used to set standards. interface gave merely a specification,nothing implemented for any standalone project which can be changed at will its more design flexible and it can be utilized to model multiple inheritance.
Ref – http://www.interview-questions-java.com/java-questions/java-abstract-class-and-interface-interview-questions
