Concepts of objects and classes

Java Summary of knowledge points ： You can enter from here if you want to see

1、 object-oriented programming

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​ java The core idea is object-oriented （OOP Object-Oriented Programming）. Object oriented is suitable for dealing with complex problems , Suitable for dealing with multi person cooperation . For a problem , The first is to classify the problems , Then think about the class alone , Finally, process oriented analysis is carried out for each class .

​ The essence of object-oriented programming is ： Organize code as a class , Encapsulating data in the form of objects .
A program is seen as a collection of objects , The object is the smallest unit . A set of related objects form a complex system .

1.1 The concept of object

​ So what is an object ？

​ Anything in real life can be regarded as an object , We usually use the static part （ attribute ） And the dynamic part （ Behavior ）, To understand things .
All things are objects , Entities and concepts in the problem domain can be abstracted as objects . In the field of school ： Student 、 teacher 、 Course 、 Classes and so on are all objects .
- Objects are unique in a system

• Objects have properties and behaviors ： Static part （ attribute ） And the dynamic part （ Behavior ）
  ​ How do we describe a person in reality ？ Usually through his height, fat and thin 、 Gender 、 Age and other attributes . In addition, it usually describes what he is currently doing ： speak 、 running 、 Crying and other behaviors . Through these aspects, we can fully understand a person .
• Object has state ： After a certain behavior occurs, the value of the attribute may be changed .
• Objects are the concrete implementation of a class .
  ​

 How do we describe a student who is studying in the computer A？
1、 First, we abstract the student entity as an object , Then consider the attributes and specific behaviors of students .
2、 attribute ： Student number 、 full name 、 Age 、 Gender 、 Class, etc .	 Behavior ： Study 、 game 、 Sports and so on 
3、 Specific to this student through attributes A.
4、 Describe the students A Is learning .
 Therefore, by analyzing the static part and dynamic part of students, we can determine a specific student object .

1.2 The concept of class

​ Class can actually be seen as a template , And the object is the entity created through this template . In reality, we print paper money , You need to make a template first （ class ）, Then the paper money is quickly generated through the template （ object ）.

​ So the essence of a class is the carrier that encapsulates the properties and behaviors of an object , An object is an instance of a class . A class is a general term for a series of objects with the same characteristics . The properties of the object are defined in the form of member variables in the class , And behavior is defined as specific methods .

2、 Classes and objects

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2.1、 package

stay java In, we need to create different classes , Create objects based on classes to write programs , So when our program gets bigger and bigger , If you put all the classes together , The difficulty of management can be imagined , I think anyone who sees a lot of classes together will have a headache .

So in java Package is introduced in (package) The concept of . It is similar to a folder , Multiple namespaces can be provided .

Bag 3 The two functions are as follows ：

• It is convenient for us to distinguish classes with the same name
• It is more convenient to manage and maintain classes
• It is more convenient to control the access rights of classes

The first line in the code is the package definition statement , By keyword package. The following import statement of follow-up package , Keyword is import

package  Package name (com.*****);	// first line , And only one 
import  Package name + Class name ;		// stay package after , There can be multiple 

2.2 、 class

​ class （Class） It is the carrier that encapsulates the properties and behaviors of objects , Is one for creating java Templates for objects , It abstracts the common features and behaviors of objects , Use member variables to represent the properties of things , Describe the behavior in a specific way , So as to form a class . adopt new Keyword instantiates a class , That is to create an object with such characteristics and behavior .

​ We define a student's class ：

2.2.1 attribute

​ Java The properties of an object are member variables , Member variables can be set to various data types , The initial value can be assigned , You can also assign no value （ Have default values ）. Usually, attributes use keywords private Set up , Expressed as private , Other classes are not visible , Provide some operation methods through .

​

2.2.2 Method

​ There are generally two methods ： Construction methods and common methods .

1、 Construction method

​ Constructor( Constructors ) Is a special method in a class , Execute only once when the object is created , Some operations used for initialization , Class will have a parameterless constructor by default , You can also define , But once you define yourself , The default constructor will be overridden . The construction method can be overridden , The corresponding construction method will be selected according to different parameters passed .

• characteristic
  • No return value , nothing void
  • The name of the method is exactly the same as the name of the class
• effect
  • It is mainly used to initialize properties

public class Student{
    
    private String name;
    // Construction method . No return value , The method name and type are the same 
    public Student(String name){
    
        this.name = namel
    }
    // The construction method with parameters is defined , Also want to use a parameterless construct , You have to define it manually 
    public Student(){
    
        
    }
}

2、 Common method

• Method definition

  // Common method 
   Permission modifier   return type   Method name ( Parameter type    Parameter name ......){
        		// No return value is void, If there is a return value, it is the corresponding data type 
       Method body ;
      return  Return value ;   // When there is a return value , Use keywords return return 
  }
  /** 1、 Modifier ： It's optional , Tell the compiler how to call the method . Defines the access type of the method . 2、 return type  ： Method may return a value . Or there may be no return value . When there is a return value, it indicates the return value type , And use return return . Use when not in use void keyword  3、 Parameter type ： Parameters passed to the method when the method is called , This parameter can be used in the method body . 4、 Method body ： The method body contains specific statements , Define the function of the method . 5、 keyword return： Code execution to return End directly after , The subsequent code will not execute .return Followed by the corresponding return value  */
  

  Methods pass parameters in addition to ordinary parameters , There is also a variable parameter ：

• Method call

  • The type of parameters passed by the called method must be consistent with the parameters of the method
  • Static methods ： keyword static The method of decoration , When the class is loaded, it will produce
    • Class name . Method name ();
  • Non static methods ： Only when the object is created
    • Instantiate the object first ： Class name Object name = new Class name ();
    • Object name . Method name ();

// Create a class about people , Describe a person by attributes , People have their own specific behavior 
package **;			// Package of class 
import **;			// Import other classes 
//class The keyword is represented as a class ,public For permissions ,Person For class name 
public class Person(){
    
    // attribute , What are the characteristics of a person . People have names 、 With age 、 There's sex 
    private String name;   // full name 
	private int age;	   // Age 
  	private String sex;	   // Gender 
    // Constructors , Each class has a default constructor , You can define , After definition, it defaults to custom 
    public Person(String name,int age,String sex){
    
        this.name = name;
        this.age = age;
        this.sex = sex;
    }
    public Person(){
    }
    
    // Common method , People can talk 
    public void say(){
    
        System.out.printf(" My name is %s, This year, %d year , Gender %s",name,age,sex);
    }
}

2.2.4 Inner class

2.3、 object

2.3.1 Object creation

 Steps to generate an object ：
1、 Statement ： Declare an object , That is, declare the type and name of the object .
2、 Instantiation ： adopt new Keyword to create an object .
3、 initialization ： When using new When you create an object , The constructor will be called to initialize the object .

Syntax for creating objects ： Class name name = new Class name (); Create a student object Student student = new Student(); student It's actually a reference to an object , It doesn't save the object itself , It's the address of the object ( The data of the object is stored in heap memory , Stack memory is the address in heap memory ).

Different objects created by the same class are independent of each other , Allocate memory address when creating , When the object is no longer used , Object by Java Garbage collection and disposal mechanism of virtual machine .

2.3.2 Use of objects

• Access properties ： Property cannot be set to private

  Object name . Property name = value ;

• Calling method

  Object name . Method name ( Parameters );