instanceof
说明：
本系列博客是本人在工作中遇到的一些问题的整理,其中有些资料来源网络博客,有些信息来自出版的书籍,掺杂一些个人的猜想及验证,总结,主要目的是方便知识的查看,并非纯原创。本系列博客会不断更新。原创不容易,支持原创。对于参考的一些其他博客,会尽量把博客地址列在博客的后面,以方便知识的查看。
instanceof是Java的一个二元操作符，用于判断关键字左边的对象是否是它右边的类型的实例，返回值为boolean类型。
看下如下情形,类A为接口,类B实现接口A,类C继承类B,类D没有显式继承任何类,也没有实现接口:
/**
* Created with IntelliJ IDEA.
* User: [email protected]
* Date: 2014-08-17 20:24
* Description:接口
*/
public interface A {
}
/**
* Created with IntelliJ IDEA.
* User: [email protected]
* Date: 2014-08-17 20:25
* Description:父类,实现接口A
*/
public class B implements A{
}
/**
* Created with IntelliJ IDEA.
* User: [email protected]
* Date: 2014-08-17 20:25
* Description:子类,继承B
*/
public class C extends B{
}
/**
* Created with IntelliJ IDEA.
* User: [email protected]
* Date: 2014-08-19 11:04
* Description:
*/
public class D {
}
看下测试类：图中有红线的是编译不通过的,注释掉
/**
* Created with IntelliJ IDEA.
* User: [email protected]
* Date: 2014-08-17 20:26
* Description:
* 测试java中的instanceof关键字
* A为接口
* 类B实现接口A
* 类C继承类B
*/
public class InstanceofTest {
public static void main(String[] args) {
//创建对象
A ab = new B();
A ac = new C();
B bb = new B();
B bc = new C();
C cc = new C();
D dd = new D();
//..instanceof A
System.out.println("ab instanceof A = " + (ab instanceof A));
System.out.println("ac instanceof A = " + (ac instanceof A));
System.out.println("bb instanceof A = " + (bb instanceof A));
System.out.println("bc instanceof A = " + (bc instanceof A));
System.out.println("cc instanceof A = " + (cc instanceof A));
System.out.println("dd instanceof A = " + (dd instanceof A));
System.out.println("null instanceof A = " + (null instanceof A));
System.out.println("-------------------------------------");
//..instanceof B
System.out.println("ab instanceof B = " + (ab instanceof B));
System.out.println("ac instanceof B = " + (ac instanceof B));
System.out.println("bb instanceof B = " + (bb instanceof B));
System.out.println("bc instanceof B = " + (bc instanceof B));
System.out.println("cc instanceof B = " + (cc instanceof B));
//System.out.println("dd instanceof B = " + (dd instanceof B));
System.out.println("null instanceof B = " + (null instanceof B));
System.out.println("-------------------------------------");
//..instanceof C
System.out.println("ab instanceof C = " + (ab instanceof C));
System.out.println("ac instanceof C = " + (ac instanceof C));
System.out.println("bb instanceof C = " + (bb instanceof C));
System.out.println("bc instanceof C = " + (bc instanceof C));
System.out.println("cc instanceof C = " + (cc instanceof C));
//System.out.println("dd instanceof C = " + (dd instanceof C));
System.out.println("null instanceof C = " + (null instanceof C));
System.out.println("-------------------------------------");
//..instanceof D
System.out.println("ab instanceof D = " + (ab instanceof D));
System.out.println("ac instanceof D = " + (ac instanceof D));
//System.out.println("bb instanceof D = " + (bb instanceof D));
//System.out.println("bc instanceof D = " + (bc instanceof D));
//System.out.println("cc instanceof D = " + (cc instanceof D));
System.out.println("dd instanceof D = " + (dd instanceof D));
System.out.println("null instanceof D = " + (null instanceof D));
}
}
看下测试结果:
ab instanceof A = true
ac instanceof A = true
bb instanceof A = true
bc instanceof A = true
cc instanceof A = true
dd instanceof A = false
null instanceof A = false
-------------------------------------
ab instanceof B = true
ac instanceof B = true
bb instanceof B = true
bc instanceof B = true
cc instanceof B = true
null instanceof B = false
-------------------------------------
ab instanceof C = false
ac instanceof C = true
bb instanceof C = false
bc instanceof C = true
cc instanceof C = true
null instanceof C = false
-------------------------------------
ab instanceof D = false
ac instanceof D = false
dd instanceof D = true
null instanceof D = false
不难看出对象的实际类型(注意是实际类型,即new关键字后的类型,而不是引用的类型)与instanceof关键字后的类型的关系：
1、对象 instanceof 本身类型 = true;如ab instanceof B = true,bb instanceof B = true,ac instanceof C = true,bc instanceof C = true,cc instanceof C = true
2、对象 instanceof 本身类型的父类 = true;如ac instanceof B = true,bc instanceof B = true,
cc instanceof B = true
3、对象 instanceof 本身类型实现的接口 = true;如ab instanceof A = true,ac instanceof A = true,
bb instanceof A = true,bc instanceof A = true,cc instanceof A = true
4、null instanceof 任何类型 = false
使用instanceof时在编译期间,就会判断操作符左边的对象是否在右边的类型继承层次中,如果不在同一个继承层次中编译就会报错。
但是这边有个疑问,dd instanceof A通过了编译,结果为false,具体细节不太清楚,为什么instanceof后的类型为接口能通过编译。
有意思的是,知乎上有个关于instanceof的讨论,有兴趣的可以看下,本人稍微看了下,反正是边看边笑,那感觉就像是学渣看学霸在讲题,呵呵呵呵
http://www.zhihu.com/question/21574535