Day013 instanceof和类型转换

Posted 2021-05-13 南方风

instanceof

​ instanceof是Java的一个二元操作符，和==， >， <是同一类东东。由于它是由字母组成的，所以也是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类型的数据。

假设有三个类，Person、Teacher、Student，Student和Teacher继承人

public class Person {
}

public class Student extends Person {
}

public class Teacher extends Person{
}

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
}

输出结果

true
true
true
false
false	

换成Person引用类型看一下

  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 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
        //System.out.println(person instanceof String);//编译报错

    }

输出结果

true
true
true
false
false
=======================
true
true
true
false

把Person再换成Student

 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 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
        //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
    }

输出结果：

true
true
true
false
false
=======================
true
true
true
false
=======================
true
true
true

结论：

x instanceof y 是否能编译通过取决于x的引用类型是否与y是父子关系

---

类型转换

回顾基本类型转换

低------------------------------------------------------>高

byte,short,char->int->long->float->double

低转高自动转换，高转低需要强制转换

引用类型转换

父类（高） 子类（低）

因此父类转子类是高转低，需要强制转换；子类转为父类型是低转高，可以自动转换。

现在在person类里面加run方法，student里加go方法

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

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

 public static void main(String[] args) {
	Person obj=new Student();//低转高自动转换
	//obj.go();编译报错
 }

可以发现obj.go()报错了，因为自动把Student类型转换为了Person类型，go是Sudent类的方法，要想使用Student的方法，需要把Person类型强制转换为Student类型

 public static void main(String[] args) {
 	Person obj=new Student();//低转高自动转换
    //obj.go();编译报错
    Student student=(Student) obj;
    student.go();
 }

输出结果

go

再来看一个例子

 //自动类型转换
 Student student = new Student();
 student.go();
 Person person=student;
 //person.go();//编译报错

输出结果

go

发现子类Student转为父类Person之后，就不能调go()方法了

结论 ：父类（高）转为子类（低）需要强制转换；子类（低）转父类（高）可以自动转换，子类转为父类可能会丢失一些子类的方法。

---

总结

1. 子类转换为父类，向上转型，自动转换，可能会丢失子类的方法
2. 父类转换为子类，向下转型，强制转换
3. 类型转换的好处：方便方法的调用，减少重复的代码。

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