1.父类的引用指向子类对象
2.把子类转化为父类，向上转型
3.把父类转化为子类，向下转型，强制转换
4.方便方法的调用，减少重复的代码
instanceof
Object object=new Student();
//Object>Person>Student
//Object>Person>Teacher
//Object>String
//System.out.println(x instanceof Y);//能不能编译通过，就看存不存在父子关系
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);
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);
//System.out.println(student instanceof String);
static关键字总结
-
静态代码块
package com.oop.demo07;
public class Person {
{//2
//(代码块)匿名代码块
System.out.println("匿名代码块");
}
static {//1
System.out.println("静态代码块");
}
public Person() {//3
System.out.println("构造方法");
}
public static void main(String[] args) {
Person person1 = new Person();
System.out.println("===============================");
Person person2 = new Person();
}
}
-
方法加了静态修饰，可以直接调用方法
public class Student {
/*private static int age;
private double score;*/
public static void go(){
System.out.println("我们去吃饭");
}
public void run(){
System.out.println("我在跑");
}
public static void main(String[] args) {
/* Student s1 = new Student();
System.out.println(Student.age);
System.out.println(s1.score);
System.out.println(s1.age);*/
go();
Student.go();
Student student=new Student();
student.run();
student.go();
}
-
静态导入包
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);
}
}