The difference between abstract classes and interfaces

abstract class ：（Abstract）

1, Concept ： If a class defines a method , But there is no specific code to execute , Then this method is called abstract method , For abstract methods abstract To modify , Because the abstract method cannot be executed , Then this class is also called abstract class . use abstract The decorated class is an abstract class .

eg1:

// abstract class
abstract class Person {
    // Abstract method
    public abstract void say();
}

characteristic ：1. Abstract classes are not instantiated

eg2:Person p = new Person(); // Compile error

2. Abstract methods have no cube

eg3:public abstract void run();

Interface ：

1, Concept : Interface is an abstraction of behavior , It's a collection of abstract methods , Using interfaces can achieve the purpose of architectural design of separating behavior definition and business implementation , And the multiple implementation and multiple inheritance of the interface , More flexible in architecture design , Easy to expand . An interface is a pure abstract code structure that is more abstract than an abstract class , Cannot contain member variables , Contains only constants , Abstract method , Default method, etc .

difference ：

1. Defining abstract methods in an interface can be done without abstract keyword , And the abstract method must add .

2. There are no ordinary methods in the interface , Construction method not allowed , Declared constants cannot be modified , And abstract classes can have ordinary methods , Allow construction methods , You can define member variables .

3. Multiple inheritance is allowed in the interface , Use implements Multiple interfaces , An abstract class can only inherit one class .

4. Can only be used in the interface public Access modifier , Abstract classes can use arbitrary access modifiers .