class Person{
   int age;                      //Member variable age
   public Person(int age){       //Local variable age
      this.age=age;              //Assign the value of the local variable age to the member variable age
  } 
}

As in the above example, if "age" is used in the construction method, local variables are accessed, and if "this.age" is used, member variables are accessed.

(2) Calling member methods with the this keyword

As follows:

class Person{
 public void openMouth(){
     ....
  }
 public void speak(){
     this.openMouth();
  }
}

Through the above example, we used the this keyword to call the openMouth() method. It should be noted that the this keyword here can be omitted without writing, and the effect is the same. Therefore, we won't repeat it, but it means that it can be called in this way.

(3) this keyword calls the constructor

The construction method is automatically called by the Java virtual machine when instantiating the object. In the program, other construction methods can be called in the form of "this [parameter 1, parameter 2,...]" in one construction method.

class Person{
   public Person(){
       System.out.println("The parameterless constructor was called...");
  }
   public Person(int age){
       this();
       System.out.println("The constructor with parameters was called...");
 }
}

public class testdemo{
    public static void main(String[] args){
      Person p=new Person(18);
  }
}

//Operation results
 The parameterless constructor was called...
The constructor with parameters was called...

Analyze the above code: when instantiating the Person object, the constructor with parameters is called, and the constructor without parameters is called through this() in the constructor with parameters. Therefore, the running results show that both constructors have been called.

When using this to call the constructor of a class, you should pay attention to the following points:

1. this can only be used in constructor to call other constructor methods, not in other member methods.

2. In a construction method, the statement that uses this to call the construction method must be the first execution statement of the method and can only appear once.

2. static keyword

First, the static keyword can be used to——

(1) Modifier attribute

Java static attributes are related to classes, not specific instances. In other words, different instances of the same class can share the same static attribute.

When the attribute is modified by static, it is shared by all classes and does not belong to an object. The access method is: class name. Attribute.

5, Class encapsulation

1. How to realize encapsulation

Class encapsulation means that the state information of the object is hidden inside the object, and the external program is not allowed to directly access the internal information of the object, but to access the internal information through the methods provided by the class.

Its specific implementation process is to privatize the attributes in the class, that is, to modify them with the private keyword. If the outside world wants to access private properties, it needs to provide some common methods decorated with public.

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.show(); 
 } 
} 

// results of enforcement
 My name is Zhang San, He is 18 years old

• At this point, the field has been decorated with private. The caller of class cannot use it directly, but needs to use the show method. At this point, the user of the class does not have to understand the implementation details of the Person class.
• At the same time, if the implementer of the class modifies the name of the member variable, the caller of the class does not need to make any modification (because the caller of the class can't access variables such as name and age), so the code maintenance cost is greatly reduced.