Abstract class
1. Abstract class Overview
In Java, a method that has no method body is defined as an abstract method, and a class that has abstract methods must be defined as an abstract class.
2. Characteristics of abstract classes
- Abstract classes and abstract methods must be decorated with the abstract keyword
Public abstract class class name {}
Public abstract void method name ()
- An abstract class does not necessarily have abstract methods, but a class with abstract methods must be an abstract class.
- Abstract classes can only be instantiated by subclass objects in a polymorphic manner. This is called abstract class polymorphism
- A subclass of an abstract class can either override an abstract method in an abstract class or be an abstract class.
3. Membership characteristics of abstract classes
- Member variables
A member variable can be a variable or a constant.
- A constructor
Abstract classes have constructors, but cannot be instantiated. Constructors are primarily used to initialize subclasses’ access to data from their parent classes.
- Members of the method
You can have abstract methods that qualify subclasses to perform certain actions
There is also a non-abstract approach: improve code reuse
2. Interface
1. Overview of interfaces
The interface is a common specification standard, as long as it meets the specification standard, everyone can use it. Interfaces in Java are more about abstraction of behavior
2. Interface features
- The interface is decorated with the keyword interface
Public interface Interface name {}
- Class implementation interfaces are represented by implements
Public class name implements interface name {}
- Instantiation of the interface
Interfaces cannot be instantiated directly. They can only be instantiated in a polymorphic way by implementing class objects. This is called interface polymorphism.
- The implementation class of the interface
Like subclasses of abstract classes, the implementing class of an interface either overrides all the abstract methods in the interface or is an abstract class.
3. Interface member features
- Member variables
Must be constant and has the default modifier: public static fianal
- A constructor
There are no constructors for interfaces, because interfaces are primarily abstractions of behavior and do not exist concretely. A class that has no parent class inherits from object by default.
- Members of the method
A member method in an interface can only be abstract because it has a default modifier: public Abstract
The difference between abstract classes and interfaces
1. Grammatical differences
- Members of the difference between
| Members of the difference between | |
|---|---|
| An abstract class | Variable, constant; There are construction methods; There are abstract methods and non-abstract methods |
| interface | Constants, abstract methods |
- Relationship between the difference
| Relationship between the difference | |
|---|---|
| Class with the class | Inheritance, single inheritance |
| Classes and interfaces | Implementation, can be single implementation, can also be more implementation |
| Interfaces and interfaces | Inheritance, single inheritance, multiple inheritance |
2. Differences in design concepts
Interfaces are designed to “constrain” the behavior of classes, that is, to provide a mechanism to force different classes to behave the same way. Interfaces only restrict behavior, not how it can be implemented, so they mostly abstract behavior. Abstract classes are designed for code reuse. Abstract classes are formed by analyzing objects and refining their internal commonalities to represent the essence of objects. Therefore, abstract classes are abstractions of classes, including their attributes and behaviors. As a result, developers have different motivations for using them. Developers inherit abstract classes in order to use their properties and behavior, and developers implement interfaces only in order to use their behavior.
In short, abstract classes are abstractions of things, while interfaces are abstractions of behavior.