接口VS抽象类

msdn版:

Here are some recommendations to help you to decide whether to use an interface or an abstract class to provide polymorphism for your components.

• If you anticipate creating multiple versions of your component, create an abstract class. Abstract classes provide a simple and easy way to version your components. By updating the base class, all inheriting classes are automatically updated with the change. Interfaces, on the other hand, cannot be changed once created. If a new version of an interface is required, you must create a whole new interface.
• If the functionality you are creating will be usefulacross a wide range of disparate objects, use an interface. Abstract classes should be used primarily for objects that are closely related, whereas interfaces are best suited for providing common functionality to unrelated classes.
• If you are designing small, concise bits of functionality, use interfaces. If you aredesigning large functional units, use an abstract class.
• If you want to provide common, implemented functionality among all implementations of your component, use an abstract class. Abstract classes allow you to partially implement your class, whereas interfaces contain no implementation for any members.

JAVA版:

• Main difference is methods of a Java interface are implicitly abstract and cannot have implementations. A Java abstract class can have instance methods that implements a default behavior
• Variables declared in a Java interface is by default final. An  abstract class may contain non-final variables.
  
• Members of a Java interface are public by default. A Java abstract class can have the usual flavors of class members like private, protected, etc..
  
• Java interface should be implemented using keyword “implements”; A Java abstract class should be extended using keyword “extends”.
  
• An interface can extend other Java interface only, an abstract class can extend another Java class and implement multiple Java interfaces.
  
• A Java class can implement multiple interfaces but it can extend only one abstract class.
  
• Interface is absolutely abstract and cannot be instantiated; A Java abstract class also cannot be instantiated, but can be invoked if a main() exists.
  
• In comparison with java abstract classes, java interfaces are slow as it requires extra indirection.