2.5.2.1 语法结构

public void 方法名(类名<? extends 上限类型(Number或者Number的子类)> 参数 ){
}

举例：

public class Student_1 {
    public void showFlag(Student<? extends Number> flag){
        System.out.println(flag.getFlag());
    }
}

public class Test_3 {
    public static void main(String[] args) {
        Student_1 student_1 = new Student_1();
        Student<Integer> student = new Student<>();
        student.setFlag(20);
        student_1.showFlag(student);
        // 报错，因为String不是Number类型的本类或者子类
        /*Student<String> student1 = new Student<>();
        student1.setFlag("爱摸鱼的TT~");
        student_1.showFlag(student1);*/
        Student<Number> student2 = new Student<>();
        student1.setFlag("爱摸鱼的TT~");
        student_1.showFlag(student2);
    }
}