Instanceof And type conversion

Instanceof

The main function

package oop;

import oop.Demo05.Person;
import oop.Demo05.Student;
import oop.Demo05.Teacher;

// There is only one specification for a project main Method 
public class Application {
    
    public static void main(String[] args) {
    
        /*  Be careful ：  The students and teachers here because in the code ,  Students and teachers have no code relationship ,  Just inherited the human class  */

        //Object > String
        //Object > Person > Student
        //Object > Person > Teacher
        // This is another route from  Student  It doesn't matter, so it is false
        // This is to determine whether an object is an instance of an object or any of its base classes , Here  object  It's actually  Student  Class 
        Object object = new Student();

        System.out.println(object instanceof Student);  //true
        System.out.println(object instanceof Person);  //true
        System.out.println(object instanceof Object);  //true
        System.out.println(object instanceof Teacher);  //false
        System.out.println(object instanceof String);  //false


        System.out.println("===============================================");

        Person person = new Student();

        System.out.println(person instanceof Student);  //true
        System.out.println(person instanceof Person);  //true
        System.out.println(person instanceof Object);  //true
        System.out.println(person instanceof Teacher);  //false
        //System.out.println(person instanceof String); // Compile and report errors , because  Person  and  String  Is at the same level ,
                                                         //  There is no connection between them , Two lines 


        /* //System.out.println(x instanceof y);  If  x  and  y  There is an inheritance relationship , Then at least the compiler will not report an error ,  But if at this time  x  yes  y Parent class of ,  because  instanceof  The comparison is to go back up one by one for comparison  */
    }
}

Parent class

package oop.Demo05;

public class Person {
    

    public void run(){
    
        System.out.println("run");
    }
}

Subclass Student

package oop.Demo05;

public class Student extends Person{
    


}

Subclass Teacher

package oop.Demo05;

public class Teacher extends Person{
    

}

Type conversion

The main function

package oop;

import oop.Demo05.Person;
import oop.Demo05.Student;
import oop.Demo05.Teacher;

// There is only one specification for a project main Method 
public class Application {
    
    public static void main(String[] args) {
    
        // Conversion between types ： Basic type conversion   High and low  64 32 16 8
        // Conversion between types ： Father   Son 


        //Person  Class height  Studnet  Class low 
        // So the conversion from low to high does not need to be forced 
        Person qwe = new Student();
        /*qwe.go();// because  Person  There is no  go  Method , // Student  Class  go  Method , So no  */


        // So now we will  qwe  This object is converted to  Student  type ,
        // Can  Student  Method of type 
        // So this is a high to low , A cast is required 
        Student student = (Student) qwe;
        student.go();
        // or 
        ((Student)qwe).go();



        // Low to high （ Converting a subclass to a parent class may lose some of the original methods ）
        Student student1 = new Student();
        student1.go();
        Person person = student1;
        //person.go();// Lost method , So wrong reporting 
    }
}
/* 1. The reference of the parent class points to the object of the child class  2. Convert a subclass to a parent , Upward transformation  3. Convert a parent class to a child class , Move down ： Coercive transformation （ Will lose precision , Is not very good ） 4. Convenient method call , Reduce duplicate code   abstract ： encapsulation 、 Inherit 、 polymorphic  */

Parent class

package oop.Demo05;

public class Person {
    

    public void run(){
    
        System.out.println("run");
    }
}

Subclass Student

package oop.Demo05;

public class Student extends Person{
    

    public void go(){
    
        System.out.println("go");
    }

}

Subclass Teacher

package oop.Demo05;

public class Teacher extends Person{
    

}

@ Meet frenzy