Abstract classes, interfaces and common keywords

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4、 Abstract classes and interfaces

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4.1、 abstract class

use abstract The class modified by keyword is abstract class , use abstract The method of keyword modification is abstract method . Abstract methods only define structures , Without concrete realization , The implementation of the class is handed over to its descendants . A class with abstract methods must be an abstract class , But an abstract class can have ordinary methods or use abstract methods .

An abstract class can't instantiate an object , Other functions of the class still exist , Member variables 、 Member methods and construction methods are accessed in the same way as ordinary classes . So abstract classes need to have Descendant classes inherit and implement all their abstract methods , Subclasses that inherit abstract classes must implement all abstract methods of the parent class , Otherwise, it should also be set as an abstract class . Because the abstract method itself has no meaning , Therefore, abstract methods must be rewritten to implement （ All abstract methods of a class must be implemented ）.

The significance of the existence of abstract classes lies in their Designability 、 Reusability and expansibility . In terms of the actual requirements of development , Ordinary classes try not to inherit another ordinary class , But to inherit abstract classes .

// Define animals as abstract classes , There are abstract methods 
abstract class Animals {
    
    abstract void sound();
    
}
//Dog Implements the abstract class , You must implement the abstract methods of the abstract class 
class Dog extends Animals{
    
    @Override
    public void sound() {
    
        System.out.println(" Barking of a dog ");
    }
}

1、final When decorating a method, the representation method cannot be overridden by subclasses , Abstract methods must be overridden by subclasses , So it can't be used together .
2、static It's a static method , Abstract methods need to be inherited and implemented by subclasses ,static It has nothing to do with Subclasses .
3、native It means that this method should be implemented in another platform dependent programming language , There is no subclass inheritance problem , So it can't be used .
4、synchronized It's thread synchronization , and abstract Abstract methods only declare that they do not implement , So there is no synchronization , Cannot be used together . Can be used in subclasses that implement this method .

4.2、 Interface

Interface （interface） And abstract classes （abstract） It means something like , But we know that , A class can only inherit one parent class , Then this has great limitations at some times . So the concept of interface appears ,Java Specify that a class can implement multiple interfaces . Single inheritance and multiple implementation .

The interface is mainly to formulate specifications , There is no specific method inside , Interface cannot be instantiated , Unless defined as an abstract class , Otherwise, once the interface is implemented, all methods in the interface must be implemented .

Interface is to provide services , Like a socket at home , It just provides a specification , I don't care about the use of electrical appliances , As long as the electrical appliance conforms to the interface of this socket , You can use the socket , Turn on the power .

• Interfaces cannot be used to instantiate objects , There is no internal construction method . A class that implements an interface , All methods described in the interface must be implemented , Otherwise, it must be declared as an abstract class . in addition , stay Java in , The interface type can be used to declare a variable , They can be a null pointer , Or be bound to an object implemented by this interface .
• JDK 1.8 in the future , There can be static methods and method bodies in the interface .JDK 1.8 in the future , Interfaces allow methods that contain concrete implementations , This method is called " The default method ", The default method is default Keyword modification .
• Member variables in an interface can only be public static final type
• Interface is not inherited by class , But to be implemented by class . A class can implement multiple interfaces

// Interface declaration 
[public] interface  The interface name  [extends  Other interfaces ]{
    
	// Variable 
    // Method 
}

Take computers for example , There are many interfaces on the computer , among USB Interface , keyboard 、 mouse 、 Headphones, etc , As long as meet USB The specification of the interface can be connected to the computer . In contrast, the function of the interface is similar to USB, Once defined, as long as it conforms to the specification of the interface , You can use

• Define a computer

  public class Computer{
        
      // The computer defines a USB The interface of , As long as meet USB The interface can work on the computer 
      private USB usb;
      public void setUsb(USB usb){
        
          this.usb = usb;
      }
      public void wook(){
        
          if(usb != null){
        
              System.out.println("USB The device is working ");
              usb.write();
          }
      }
  }
  

• Definition USB Interface

  public interface USB {
        
      void read();	// How to read and write 
      void write();
  }
  

• Define keyboard and mouse , Realization USB Interface

  // Define a keyboard and a mouse , accord with USB Specification of the interface 
  public class Keyboard implements USB {
        
      public void Knock(){
        
          System.out.println(" Tap the keyboard ");
      }
      @Override
      public void read() {
        }
      @Override
      public void write() {
        
          this.Knock();
      }
  }
  
  public class Mouse implements USB {
        
      public  void rClick(){
        
          System.out.println(" Click on the right ");
      }
      public void lClick(){
        
          System.out.println(" Click the left button ");
      }
      public void read() {
        
      }
      public void write() {
        
          this.rClick();
          this.lClick();
      }
  }
  

• The keyboard and mouse work through the computer

4.3、 difference

The difference between interface and class :

1、 Interfaces cannot be used to instantiate objects .
2、 Interface has no constructor .
3、 All methods in an interface must be abstract methods .
4、 Interface cannot contain member variables , except  static  and  final  Variable .
5、 Interface is not inherited by class , But to be implemented by class .
6、 Interface supports multiple inheritance .

The difference between interface and abstract class

1、 A class can only inherit one abstract class , A class can implement multiple interfaces .
2、 Member variables in an interface can only be  public、static、final Type of , Abstract classes can define various types of instance variables and static variables 
3、 Interfaces mainly play a role in system architecture design , Used to define the communication contract between modules . Abstract classes are mainly used in code implementation . Can achieve code reuse .

Interfaces are mainly used to constrain the behavior of classes , It only restricts the existence of class behavior , There is no provision for the specific realization of the behavior . Interfaces only define behavior , How to implement the body of a class , The interface doesn't care . yes like a The relationship between . Therefore, the interface is mainly used for the communication contract between different modules of the architecture .

Abstract classes are mainly used to realize code reuse , When parts of some methods of more than two classes have the same implementation , These same behaviors are usually abstracted out , Make an abstract class , After inheriting these functions, you can also expand your own functions . Abstract the essence of a class , A subclass is at least one parent ,is a.

5、 Several commonly used keywords

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5.1、final、finally、finalize

1、final Used to declare properties 、 Method 、 class . They represent immutable attributes 、 Methods cannot be overridden 、 Class cannot be inherited . When an inner class accesses a local variable , Local variables must have final modification .
2、finally Is to cooperate when handling exceptions try、catch Use it together ,finally The code block in the is bound to execute .
3、finalize yes Object class , When objects are recycled , The garbage collection processor calls this method .JVM There is no guarantee that this method will always be called .

5.2、static

Can I static Call non - within a modified method static Method ？

answer ： Can not be ,static The modified method is static , It belongs to the class , When a class is loaded, it is allocated space , Call... Directly with the class name . Instead of static Property method belongs to the object , The system allocates space for objects only when they are created , Call with method name .
therefore static Modified static methods when calling non static methods , The object may not have been created yet , So at this time, call non static There will be anomalies .

5.3、 Permission modifier

5.4、 super、this

stay Java In language this Represents a reference to an object of this class , General implicit member variables and methods used to reference objects . Simply speaking this It is used to call its own properties and methods .

this and super keyword

this： Refers to the current object itself ( Who uses this It means who , Just as we said in the display ), You can call your own properties and methods ( No static , Static corresponds to a class, not an object ).java The principle of proximity , There are local variables and methods with the same name , Direct calls are local variables , add this You can call properties .

super： Reference to parent space , That is, the reference to the parent class space ( Does not represent an object , It just represents a piece of memory in an object ),super You can call the property method of the parent class . There is a member with the same name in the child and parent class （ Including variables and methods ） when , In a subclass, the default is to access the members of the subclass , Can pass super Keyword specifies the member accessing the parent class ;

super And this Keywords can not appear at the same time to call other constructors in the same constructor . Because both statements need the first one .

5.5、instanceof