面向对象13：instanceof和类型转换

instanceof:

• 比较转换，要有父子关系才会返回ture或者false

父类

public class Person {
}

子类Student

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

}

子类Teacher

public class Teacher extends Person{

}

比较转换

• 要有父子关系才会返回ture或者false

//instanceof:会比较转换，要有父子关系才会返回ture或者false

public class Application {
    public static void main(String[] args) {
        //Object>String
        //Object>Person>Student
        //Object>Person>Teacher
        Object object = new Student();

        //System.out.println(x instanceof y);//代码编译能否通过，看x和y是否有父子继承关系

        //判断类型是否相似
        System.out.println(object instanceof Student);//Ture
        System.out.println(object instanceof Person);//Ture
        System.out.println(object instanceof Object);//Ture
        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);//Ture
        System.out.println(person instanceof Person);//Ture
        System.out.println(person instanceof Object);//Ture
        System.out.println(person instanceof Teacher);//False
        //System.out.println(person instanceof String);//编译报错，两者是同级关系不符合
        System.out.println("===============================");
        Student student = new Student();
        System.out.println(student instanceof Student);//Ture
        System.out.println(student instanceof Person);//Ture
        System.out.println(student instanceof Object);//Ture
        //System.out.println(student instanceof Teacher);//False
        //System.out.println(student instanceof String);//False
        //注意：比较转换，要有父子关系才会返回ture或者false
    }
}

强制类型转换

• 父类转换成子类高转低，语法：（父类）子类

Person obj = new Student();

//student将Person父类转换成子类Student类型，就可以用子类的方法
//表示1
Student student = (Student)obj;
student.go();
//表示2
((Student)obj).go();

• 子类转换成父类：自动类型转换

//子类转换成父类会丢失子类独有的方法和重写方法
Student student2 = new Student();
student2.go();
Person person = student2;
//person.go();

• 代码

//（类名）实例
public class Application2 {
    public static void main(String[] args) {
        //类型之间的转换：前面学过64位类型转成32位列如：double型转int需要强制转换
        //这里父类代表高的，子类代表低的，高转低要强制
        Person obj = new Student();

        //student将Person父类转换成子类Student类型，就可以用子类的方法
        //表示1
        Student student = (Student)obj;
        student.go();
        //表示2
        ((Student)obj).go();


        //子类转换成父类会丢失子类独有的方法和重写方法
        Student student2 = new Student();
        student2.go();
        Person person = student2;
        //person.go();

    }
}

小结

1. 父类引用指向子类的对象
2. 把子类转换为父类，向上转换
3. 父类转换成子类，向下转换：强制转换 （父类）子类
4. 方便方法的调用，减少重复的代码！简洁思维
5. 封装，继承，多态！ 抽象类和接口