Object oriented: the interaction between people, clothes, washing powder and washing machine is completed. People don't need to pay attention to how the washing machine washes clothes and dries clothes

Object oriented is a way of thinking and a kind of thought. For example: concepts and examples. Theory and practice. Name and reality, etc. Its advantage is to make complex things simple, just face an object. Object oriented design holds an important experience: who owns the data and who provides external methods to operate the data (private) (the passive party is the owner of the data and the active party is the executor). During development: find objects, build objects, use objects, and maintain the relationship between objects

// Create class
class <class_name>{  
    field;//Member properties
    method;//Member method
}
// Instantiate object
<class_name> <Object name> = new <class_name>();

class TestDemo{
    public int a;
    public static int count;
    
    public static void change() {
        count = 100;
        //a = 10; error non static data members cannot be accessed
   }
}
public class Main{
   public static void main(String[] args) {
        TestDemo.change()；//Can be called without creating an instance object
        System.out.println(TestDemo.count);   
   }
}

Compile and run the code, and the output is as follows:

100

class Person { 
     private String name = "Zhang San"; 
     private int age = 18; 
 
     public void show() { 
     System.out.println("My name is" + name + ", this year" + age + "year"); 
     } 
} 
class Test { 
     public static void main(String[] args) { 
     Person person = new Person(); 
     //person.name = "Zhang San"// error
     person.show(); 
     } 
}

3.getter and setter methods

4, Construction method

5, Code block

1. The initialization methods of fields are: (1) local initialization, (2) initialization using construction methods, and (3) initialization using code blocks. The first two methods have been summarized earlier. Next, we introduce the third method, which uses code block initialization

2. According to the location and keywords defined by the code block, it can be divided into four types: local code block (ordinary code block), static code block, construction code block and synchronous code block

(1) Native code block: a code block defined in a method

public class Main{ 
    public static void main(String[] args) { 
        {                          //Directly use {} definition, common method block
            int x = 10 ; 
            System.out.println("x1 = " +x); 
        } 
        int x = 100 ; 
        System.out.println("x2 = " +x); 
    } 
}

Compile and run the code, and the output is as follows:

x1 = 10

x2 = 100 

This usage is rare

(2) Construct code block

6, Supplement

1.toString method:

(1) The toString method will be called automatically at println

(2) The operation of converting an object into a string is called serialization

(3) ToString is the method provided by the Object class. The Person class created by ourselves inherits from the Object class by default. You can override toString method to implement our own version of conversion string method

(4) Shortcut key of toString method for IDEA to quickly generate Object: alt+f12(insert)

You don't need shortcut keys. Right click to find generate, then find toString(), and then click ok

class Person { 
    private String name; 
    private int age; 
    public Person(String name,int age) { 
    this.age = age; 
    this.name = name; 
    } 
    public void show() { 
       System.out.println("name:"+name+" " + "age:"+age); 
    } 
} 
public class Main { 
    public static void main(String[] args) { 
    Person person = new Person("caocao",19); 
    person.show(); 
     //We found that the hash value of an address is printed here. The reason: the toString method of Object is called
    System.out.println(person); 
    } 
}

Compile and run the code, and the output is as follows:

Override toString method:

class Person { 
     private String name; 
     private int age; 
     public Person(String name,int age) { 
     this.age = age; 
     this.name = name; 
 } 
     public void show() { 
         System.out.println("name:"+name+" " + "age:"+age); 
     } 
     //Override the toString method of Object
     @Override//@Override is called "Annotation" in Java. Here @ override means that the toString method implemented below is a method that overrides the parent class 
     public String toString() { 
         return "Person{" + 
                "name='" + name + '\'' + 
                ", age=" + age + 
                '}'; 
    } 
} 
public class Main { 
    public static void main(String[] args) { 
        Person person = new Person("caocao",19); 
        person.show(); 
        System.out.println(person); 
    } 
}

Compile and run the code, and the output is as follows: