instanceof和类型转换

package com.oop;

import com.oop.demo06.Person;
import com.oop.demo06.Student;
import com.oop.demo06.Teacher;

public class Application {
    public static void main(String[] args) {

        //Object >

        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);//编译报错！
        System.out.println("==================================");
        Student student = new Student();
        System.out.println(student instanceof Student);//true
        System.out.println(student instanceof Person);//true
        System.out.println(student instanceof Object);//true
        //System.out.println(student instanceof Teacher);//编译报错！
        //System.out.println(student instanceof String);//编译报错！
    }
}

1. 父类引用指向子类对象
2. 把子类转换为父类，向上转型
3. 把父类转换为子类，向下转型：强制转换
4. 方便方法的调用，减少重复使用代码！

package com.oop.demo06;

public class Student extends Person{
    public void go(){
        System.out.println("go");
    }
}

package com.oop;

import com.oop.demo06.Person;
import com.oop.demo06.Student;

public class Application {
    public static void main(String[] args) {
        //类型之间的转化：父      子

        /*
        //高                  低
        Person obj = new Student();

        //student将这个对象转换为Student类型，我们就可以使用Student类型的方法了！
        ((Student)obj).go();
         */
        Student student = new Student();
        student.go();
        Person person = student;
    }
}

static关键字详解

package com.oop.demo07;

public class Person {

    //2:赋初值~
    {
        System.out.println("匿名代码块");
    }

    //1:只执行一次~
    static {
        System.out.println("静态代码块");
    }

    //3
    public Person() {
        System.out.println("构造方法");
    }

    public static void main(String[] args) {
        Person person1 = new Person();
        System.out.println("====================");
        Person person2 = new Person();
    }
}

运行结果

静态代码块
匿名代码块
构造方法
====================
匿名代码块
构造方法

package com.oop.demo07;

//static
public class Student {
    private static int age;//静态的变量   多线程！
    private double score;//非静态的变量

    public void run(){

    }
    public static void go(){

    }
    public static void main(String[] args) {
        Student s1 = new Student();

        System.out.println(Student.age);
        System.out.println(s1.age);
        System.out.println(s1.score);
    }
}

package com.oop.demo07;

//静态导入包~
import static java.lang.Math.random;
import static java.lang.Math.PI;

public class Test {

    public static void main(String[] args) {
        System.out.println(random());
        System.out.println(PI);
    }
}

final之后断子绝孙

笔记出处——狂神老师