The difference between abstract classes and interfaces

abstract class

• Abstract method —— Only method declaration , There is no concrete realization .
• Abstract method —— Must use abstract Keyword modification . If a class contains abstract methods , We call this class an abstract class , Abstract classes must be used before classes abstract Keyword modification .
• Because abstract classes contain methods without concrete implementation , So you can't create objects with abstract classes .
• If a class does not contain abstract methods , Just use abstract Adornment is also an abstract class . That is to say, an abstract class does not have to contain abstract methods .
• A class containing abstract methods is called an abstract class , But it doesn't mean that there are only abstract methods in an abstract class , It's the same as a normal class , You can also have member variables and common member methods . Be careful , There are three main differences between abstract classes and ordinary classes ：
  • The abstract method must be public perhaps protected（ Because if it's private, Can't be inherited by subclass , Subclass can't implement this method ）, The default is public.
  • Abstract classes cannot be used to create objects ;
  • If a class inherits from an abstract class , The subclass must implement the abstract method of the parent class . If the subclass does not implement the abstract method of the parent class , The subclass must also be defined as abstract class .

Interface

Interface , It's called interface, In software engineering , Interface generics refer to methods or functions that are called by others

• Interfaces can contain Variables and methods . But be careful , Variables in the interface are implicitly specified as public static final Variable （ And it can only be public static final Variable , use private Modification will report compilation error ）
• The method is implicitly specified as public abstract The method can only be public abstract Method （ Use other keywords , such as private、protected、static、final And other modifications will report compilation errors ）, And all the methods in the interface can't have specific implementation , in other words , The methods in the interface must all be abstract methods . From here, we can see the difference between interface and abstract class , An interface is an extremely abstract type , It's more than an abstract class “ abstract ”, And in general, variables are not defined in the interface .
• To make a class follow a specific set of interfaces, you need to use implements keyword
• It can be seen that , Allow a class to follow multiple specific interfaces . If a non abstract class follows an interface , You have to implement all the methods in this interface . For an abstract class that follows an interface , The abstract methods in this interface can not be implemented .

The difference between abstract classes and interfaces

1. Grammatical differences

• Abstract classes can provide implementation details of member methods , And there is only... In the interface public abstract Method ;
• Member variables in abstract classes can be of various types , The member variables in an interface can only be public static final Type of ;
• Interfaces cannot contain static code blocks and static methods , Abstract classes can have static code blocks and static methods ;
• A class can only inherit one abstract class , A class can implement multiple interfaces .

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