向上、下转型

向上转型

Person 父类   Student 子类

Student student = new Student（）

Person p1 = student      ok

Object o1 = student       ok

Object o2 = student       ok

向下转型

Person p = new Student()

Person p2 = new Person()

Student s=(Student) p     ok

Student s2 =(Student) p2   false

 instanceof

System.out.println(s instanceof Student); // true

System.out.println(s2 instanceof Student); // false