例如:
public class Person{
public void run(){}
}
public class Student extends Person{
}
public class Teacher extends Person{
}
例如:
main:
Object o = new Student();
System.out.println(o instanceof Student);//true
System.out.println(o instanceof Person);//true
System.out.println(o instanceof Object);//true
System.out.println(o instanceof Teacher);//false
System.out.println(o instanceof String);//false
---------------------------
Person o = new Student();
System.out.println(o instanceof Student);//true
System.out.println(o instanceof Person);//true
System.out.println(o instanceof Object);//true
System.out.println(o instanceof Teacher);//false
//编译报错
System.out.println(o instanceof String);
---------------------------
Student o = new Student();
System.out.println(o instanceof Student);//true
System.out.println(o instanceof Person);//true
System.out.println(o instanceof Object);//true
//编译报错
System.out.println(o instanceof Teacher);
//编译报错
System.out.println(o instanceof String);
注1:
System.out.println(x instanceof Y);
该代码能否编译通过,主要是看声明变量x的类型和Y是否存在子父类的关系.有子父类关系就编译通过,没有子父类关系就是编译报错
注2:
System.out.println(x instanceof Y);
输出结果是true还是false,主要是看变量x所指向的对象实际类型是不是Y类型的子类型.
例如:
main:
Object o = new Person();
System.out.println(o instanceof Student);//false
System.out.println(o instanceof Person);//true
System.out.println(o instanceof Object);//true
System.out.println(o instanceof Teacher);//false
System.out.println(o instanceof String);//false
类型转换
public class Person{
public void run(){}
}
public class Student extends Person{
public void go(){}
}
public class Teacher extends Person{
}
1)为什么要类型转换
//编译报错,因为p声明的类型Person中没有go方法
Person p = new Student();
p.go();
//需要把变量p的类型进行转换
Person p = new Student();
Student s = (Student)p;
s.go();
或者
//注意这种形式前面必须要俩个小括号
((Student)p).go();
2)类型转换中的问题
//编译通过
Object o = new Student();
Person p = (Person)o;
//编译通过
Object o = new Student();
Student s = (Student)o;
//编译通过,运行报错
Object o = new Teacher();
Student s = (Student)o;
即:
X x = (X)o;
运行是否报错,主要是变量o所指向的对象实现类型,是不是X类型的子类型,如果不是则运行就会报错。