instanceof和类型转换

instanceof和类型转换

instanceof

package oop.demo01.demo06;

public class Person {

}

package oop.demo01.demo06;

public class Teacher {
}

package oop.demo01.demo06;

public class Student extends Person {

}

package oop.demo01.demo06;

public class Application {
    public static void main(String[] args) {
        //Object>Person>Student
       Object object= new Student();
       //System.out.println(x instanceof y);//能不能编译通过 即x与y是否存在关系
        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);//编译报错
        //System.out.println(person instanceof String);//编译报错
        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);//编译报错
        //System.out.println(student instanceof String);//编译报错
    }
}

类型转换

package oop.demo01.demo06;

public class Person {
public void run(){
    System.out.println("run");
}
}

package oop.demo01.demo06;

public class Teacher {
}

package oop.demo01.demo06;

public class Student extends Person {
public void go(){
    System.out.println("go");
}
}

package oop.demo01.demo06;

public class Application {
    public static void main(String[] args) {
       //类型之间的转换：父    子
        Person obj=new Student();
        //student将这个对象转换为Student类型，我们就可以使用Student类型的方法了！
        ((Student) obj) .go();//类型转换
        //子类转换为父类，可能丢失自己本来的方法
    }
}

小结：

1. 父类引用指向子类对象。

2. 把子类对象转换为父类，向上转型。

3. 把父类转换成子类，向下转型，强制转换。

4. 方便方法的调用，减少重复的代码！