This week's book giveaway is in the OCAJP 8 forum. We're giving away four copies of OCA Java SE 8 Programmer I Study Guide and have Edward Finegan & Robert Liguori on-line! See this thread for details.
A class must be declared abstract when it has an abstract method. Its common to declare an abstract class when the class represents a base class in a hierarchy. Any extender that's not abstract must implement its abstract methods.
Interfaces are public contracts. They list the signatures of public methods that implementers must implement.
You can't instantiate interfaces (only anonymous inner interfaces) or abstract classes.
Abstract class vs an interface;
An interface is like a 100% Abstract class where ALL the methods are implicitly public abstract and it is up to the class implementing the interface to provide an implementation for all these methods.
On the other hand, the Abstract class might have some OR none of his methods abstract: if you declare just one method to be abstract then you must declare the class to be abstract, however you can still declare a class to be abstract without having any of his methods declared as abstract method. Moreover an abstract class CANNOT be instantiated (ex. you cannot create objects of the abstract class) , it can only be extended so that implementations could be given to the abstract methods if he has any.