What are the differences between java abstract classes and interfaces?

1. Abstract class

In Java, a class modified by the keyword abstract is called an abstract class; Methods modified by abstract are called abstract methods. Abstract methods only have method declarations and no method bodies.

Abstract classes have the following characteristics:

1. Abstract classes cannot be instantiated, but can only be inherited.

2. A class containing an abstract method must be an abstract class, but an abstract class does not necessarily contain an abstract method (an abstract class can contain ordinary methods).

3. The permission modifier of an abstract method can only be public or protected, and it is public by default.

4. If a class inherits from an abstract class, the subclass must implement the abstract method of the abstract class. If the subclass does not implement the abstract method of the parent class, the subclass must be defined as an abstract class.

5. Abstract classes can contain attributes, methods, and constructors, but constructors cannot be used to instantiate objects and can only be called by subclasses.

2. Interface

The interface can be regarded as a special class and can only be modified with the interface keyword. The interface is provided for others to call. Interfaces in Java have the following characteristics:

1. Interfaces can contain variables and methods. Variables are implicitly designated as public static final, and methods are implicitly designated as public abstract (before JDK 1.8);

2. The interface supports multiple inheritance, that is, one interface can inherit (extends) multiple interfaces, which indirectly solves the problem that classes in Java cannot inherit from multiple interfaces;

3. One class can be implemented at the same time Multiple interfaces. If a class implements an interface, it must implement the abstract method in the interface, otherwise the class must be defined as an abstract class;

4. Two new features have been added to interfaces in JDK 1.8:

(1) Default method: JDK 1.8 allows adding non-abstract method implementations to interfaces, but they must be modified with the default keyword; methods that define default can not be implemented by implementation subclasses. But it can only be called by objects that implement the subclass; if the subclass implements multiple interfaces, and these interfaces contain the same default method, the subclass must override the default method.

(2) Static method: JDK 1.8 allows the use of the static keyword to modify a method and provide an implementation, which is called an interface static method. Interface static methods can only be called through the interface (interface name. static method name).

5. In JDK 1.9, the interface has been enhanced again to implement private method and private static method. Because methods in interfaces can be implemented in JDK 1.8, when we do not want the implemented methods to be exposed to the outside, we can define the methods as private.

3. The difference between interfaces and abstract classes

1. Abstract classes can contain ordinary methods, but interfaces can only contain public abstract methods (before JDK 1.8) ;

2. There are no access restrictions on member variables in abstract classes, but variables in interfaces can only be modified by public static final;

3. One interface can inherit multiple interfaces, But a class can only have one parent class, and a class can implement multiple interfaces;

4. An abstract class is an abstraction of a type of thing, and an interface is an abstraction of behavior. A class inheriting an abstract class represents the "is it" relationship, while a class implementing an interface represents the "is it having" relationship.

php Chinese website, a large number of free Java introductory tutorials, welcome to learn online!

The above is the detailed content of What are the differences between java abstract classes and interfaces?. For more information, please follow other related articles on the PHP Chinese website!