Abstract classes and interfaces

One 、 abstract class

abstract The class modified by keyword is called abstract class , The decorated method is called abstract method , Abstract methods only have the declaration of methods , There is no method body  . It cannot be instantiated , Can only be used as a superclass of subclasses . Abstract classes are templates used to create subclasses at the inheritance level .

characteristic

1、 Abstract class cannot be instantiated （ Out of commission new Keyword instantiation object ）, Can only be inherited ;

2、 What contains abstract methods must be abstract classes , But abstract classes don't necessarily have abstract methods ;

3、 The modifier of an abstract method can only be public perhaps protected, The default is public;

4、 Abstract methods in abstract classes are only method declarations , There is no way to achieve ;

5、 Subclasses override superclasses （ abstract class ） All the abstract methods of , Then the subclass is a class , Otherwise, it is still an abstract class ;

6、 Abstract classes can contain properties 、 Method 、 Construction method , The goal is for subclasses to call .

Two 、 Interface

An interface is a declaration of a series of methods , Is a collection of method features , An interface has only the characteristics of a method, but no method implementation , So these methods can be implemented by different classes in different places , And these implementations can have different behaviors （ function ）.

characteristic

1、 Interfaces can contain variables 、 Method ; The variable is implicitly specified as public,static,final, Method is implicitly specified as public,abstract（JDK1.8 Before ）;

2、 Interface supports multiple inheritance , That is, an interface can extends Multiple interfaces , Indirectly solved Java Single inheritance of classes in ;

3、 A class can implement multiple interfaces ;

4、JDK1.8 New features have been added to the interface ：

4.1、 The default method （default method）：JDK 1.8 Allows you to add non abstract method implementations to an interface , But must use default Keyword modification ; Defined default Methods can be implemented without subclasses , But it can only be called by the object implementing the subclass ; If a subclass implements multiple interfaces , And these interfaces contain the same default methods , The subclass must override the default method ;

4.2、 Static methods （static method）：JDK 1.8 Allowed in static Keyword modifies a method , And provide implementation , Called interface static methods . Interface static methods can only be called through interfaces （ The interface name . Static method name ）.

3、 ... and 、 Similarities between abstract classes and interfaces

1、 Can't be instantiated .

2、 The implementation class of an interface or subclass of an abstract class can only be instantiated after the method in the interface or abstract class is implemented .　

Four 、 The difference between abstract classes and interfaces

1. Abstract class should be inherited by subclass , Interface to be implemented by class .

2. Interface can only make method declaration , Method declaration can be made in abstract classes , It can also be realized by methods .

3. The variables defined in the interface can only be public static constants , Variables in an abstract class are ordinary variables .

4. Interface is the result of design , Abstract classes are the result of refactoring .

5. Abstract classes and interfaces are used to abstract concrete objects , But the interface has the highest level of abstraction .

6. Abstract classes can have specific methods and properties , Interfaces can only have abstract methods and immutable constants .

7. Abstract classes are mainly used to abstract classes , Interface is mainly used to abstract functions .

8. There are construction methods in abstract classes , There is no constructor in the interface .