1, Encapsulation

        1. There can only be one class modified by public in the file. If there is more than one class, an error will be reported:

         2. Four access modifiers:

                

        3. private for encapsulation

① it can be seen from 2 that variable methods modified by private can only be used in this class. How to use them

As follows:

class Student{
    private String name;
    private int sno;
    private String subject;

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getSno() {
        return sno;
    }

    public void setSno(int sno) {
        this.sno = sno;
    }

    public String getSubject() {
        return subject;
    }

    public void setSubject(String subject) {
        this.subject = subject;
    }
}

Note: the get and set methods are used to set and call variables modified by private by creating objects. After the this keyword, just know it first.

② how to call? As follows

public class TextStudent {
    public static void main(String[] args) {
        Student student = new Student();
        student.setName("Wang ");
        student.setSno(2);
        student.setSubject("language");
        System.out.println(student.getName()+student.getSno()+student.getSubject());
 //Wang Zhenya 2 Chinese
    }
}

        4. Construction method

① function: initialize the object, and each instantiation of the object will call the construction method

② format: the method with the same name as the class and no return value type has no return value

③ use and call of construction method

class Car{
    public Car(){
        System.out.println("Welcome to the car home");
    }

public class CarText {
    public static void main(String[] args) {
        Car car = new Car();
        }}

5. Method overload

① concept: there are multiple methods in the same class with the same method name but different parameter lists

② this keyword: This represents this class, which can be understood as Cat cat = new Cat();

The following is this sum=20;

class Cat{
     int sum = 20;
     int s=30;
     public void eat(){
         int sum=10;
         System.out.println(sum);
         System.out.println(s);
         System.out.println(this.sum);
     }
} 

③ use and call

For overloading of construction methods:

class Car{
    public Car(){
        System.out.println("Welcome to the car home");
    }

public Car(String color){
    this.color=color;
}}

public class CarText {
    public static void main(String[] args) {
        Car car = new Car();
        Car car2 = new Car("red");}}

For overloading of functional methods:

class Car{
    public void start(){
        System.out.println("My car started");
    }
    public void start(String color){
        System.out.println("My car started");
    }

public class CarText {
    public static void main(String[] args) {
        Car car = new Car();
        car.start();}}

6. Code block

① Construct code blocks and local code blocks:

Execution order: triggered every time the constructor is called, and it takes precedence over the execution of the constructor

public class TeacherTest {
    public static void main(String[] args) {
        Teacher teacher1 = new Teacher();// The construction code block is called before the construction method
    }
}
class Teacher{
    String name;
    String age;
    public Teacher(){
        System.out.println("good morning");
    }
    {
        System.out.println("I'm building blocks");
    }
    public void cals(){
        System.out.println("I'm teaching");
        {
            System.out.println("I'm a local code block");
            int i=10;
            System.out.println(i);
        }
        //System.out.println(i);,, An error will be reported because local code blocks can be used locally
    }
}

② sequence of code blocks

Construction code block -- > construction method -- > object created successfully -- > local code block

7. Application of this keyword

//this(): call the parameter free constructor of benzene class
 //The statement related to this must be written on the first line
 //This (parameter): call the line parameter constructor of this class
class Dog{
    String name;
    int age;
    public Dog(){
        this(666);
    }
    public  Dog(int age){
        this.age=age;
    }
    
}