【java基础】instanceof和类型转换

• instanceof 是 Java 的保留关键字。它的作用是测试它左边的对象是否是它右边的类的实例，返回 boolean 的数据类型。

public class Application {
    public static void main(String[] args) {
        //Object > String
        //Object > Person > Teacher
        //Object > Person > Student
        Object object = new Student();
        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 > Student
        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); //false
        //Object > String
        //Object > Person > Teacher
        //String和Person是同级，无联系
        //System.out.println(person instanceof String);编译报错

    }
}

Person.java

public class Person {
}

Student.java

public class Student extends Person{
}

Teacher.java

public class Teacher extends Person{
}

• 父类引用子类指向子类的对象
• 把子类转换为父类，向上转型，直接转换就行
• 把父类转换为子类，向下转型，需要强制转换，丢失一些方法
• 方便方法的调用，减少重复的代码！是代码简洁
  Application.java

public class Application {
    public static void main(String[] args) {
        //类型之间的转换： 父  子
        //高                     低
        Person obj = new Student();
        //强制转换
        //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 student = (Student) obj;
        student.go();
        //一步强转
        ((Student) obj).go();

        //子类转换为父类，可能丢失自己本来的一些方法
        Student student1 = new Student();
        student1.go();
        Person person = student1;
    }

}

Person.java

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

Student.java

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

    }
}