x instanceof A : 检测x是否是类A的实例对象，或者A的子类创建的实例对象，返回值为boolean型。 –要求x所属的类与类A必须是子类和父类的关系，否则编译错误 –如果x属于类A的子类B，x instanceof A 返回值也为true
使用情景：为了避免在向下转型时出现ClassCastException的异常，通常在向下转型之前，先进行instanceof的判断，若返回true，就可以向下转型。
如果x instanceof A 返回true，且x instanceof C 也返回true，则类A是类C的父类，或者类C是类A的父类
class Person { String name; int age; int id = 1001; public void eat() { System.out.println("person：eat"); } public void walk() { System.out.println("person：walk"); } } class Man extends Person { boolean isSmoking; int id = 1002; public void earnMoney() { System.out.println("man:earn money"); } public void eat() { System.out.println("man:eat"); } public void walk() { System.out.println("man:walk"); } } class Woman extends Person { boolean isBeauty; public void earnMoney() { System.out.println("woman:earn money"); } public void eat() { System.out.println("woman:eat"); } public void walk() { System.out.println("woman:walk"); } } public class InstanceofTest { public static void main(String[] args) { Person p1 = new Person(); p1.eat(); Man man = new Man(); man.eat(); man.earnMoney(); Person p2 = new Man(); p2.eat(); System.out.println("**************"); if(p2 instanceof Man) { Person p10 = (Man)p2; ((Man) p10).earnMoney(); } if(p2 instanceof Person) { Person p11 = (Man)p2; p11.eat(); } if(p2 instanceof Object) { Person p12 = (Man) p2; p12.walk(); } } } /* person：eat man:eat man:earn money man:eat ************** man:earn money man:eat man:walk */容易出现的问题 问题一，编译时通过，运行时不通过 1.
Person p3 = new Woman(); Man m3 = (Man)p3;2，
Person p4 = new Person(); Man m4 = (Man)p4;问题二，编译通过，运行也通过
Object obj = new Woman(); Person p5 = (Person)obj;问题三，编译不通过
Man m5 = new Woman();