java中泛型上限，下限应用

import java.util.*;
class Person implements Comparable<Person>{
   String name;
   int age;
   Person(){
       name = "";
       age = 0;
   }
   Person(String name, int age){
       this.name = name;
       this.age = age;
   }
   public String toString(){
       return name + "...." + age;
   }
   
 
   public int compareTo(Person o){
       
       if(name.compareTo(o.name)==0)
          return o.age - age;
       return o.name.compareTo(name);
   }
}

class Student extends Person{
     int score;
     public Student(){
         super();
     }
     public Student(String name, int age, int score){
         super(name, age);
         this.score = score;
     }
     
     public String toString(){
         return  name + "...." + age + "...." + "score: " + score;
     }
}

class Worker extends Person{
     int salary;
     public Worker(){
         super();
     }
     public Worker(String name, int age, int salary){
         super(name, age);
         this.salary = salary;
     }
     
     public String toString(){
         return  name + "...." + age + "...." + "salary: " + salary;
     }
}

class ComparatorPerson implements Comparator<Person>{
     public int compare(Person o1, Person o2){
         if(o1.name.compareTo(o2.name)==0)
            return o1.age - o2.age;
         return o1.name.compareTo(o2.name);
     }
}
     
public class Test{
   public static void main(String[] args){
       ArrayList<Person> arr = new ArrayList<Person>();
       arr.add(new Person("hujunzheng", 22));
       arr.add(new Person("caihaibin", 21));
       arr.add(new Person("huangweidong", 22));
       
       ArrayList<Student> arr1 = new ArrayList<Student>();
       arr1.add(new Student("hujunzheng", 22, 500));
       arr1.add(new Student("caihaibin", 21, 444));
       arr1.add(new Student("huangweidong", 22, 777));
       
       ArrayList<Worker> arr2 = new ArrayList<Worker>();
       arr2.add(new Worker("789", 22, 500));
       arr2.add(new Worker("465", 21, 444));
       arr2.add(new Worker("798", 22, 777));
       
       //public boolean addAll(Collection<? extends E> c)// 泛型的上限：用于存储的数据
       // 如果不是<? extends E> 而只是<E> 那么E的子类型就不能加载进来！因为Collection<Person> 和 ArrayList<Student>泛型管理的不是同一数据类型
       // 而 <? extends E> 要求传进来的对象实例管理的泛型只要是 E 或者是 E的子类都行！
       arr.addAll(arr1);
       arr.addAll(arr2);
       
       Object[] oo = null;
       Arrays.sort(oo=arr.toArray());
       for(Object o : oo)
          System.out.println(o);
          
       //public TreeSet(Comparator<? super E> c); 传递的是比较泛型为 E 或者是 E的父类的比较器
       //在进行 E类型两个数据比较的时候， 用 E类 或者 E类型的父类型进行接收，并使用 ？类型的定义的比较方式！
       
       System.out.println("泛型上限，下限...................................................");
       TreeSet<Worker> tw = new TreeSet<Worker>(new ComparatorPerson());// E 是 Worker， 传递的是其父类Person的比较器
       tw.addAll(arr2);
       
       for(Iterator it = tw.iterator(); it.hasNext(); )
          System.out.println(it.next());
       
       TreeSet<Student> ts= new TreeSet<Student>(new ComparatorPerson());// E 是 Student， 传递的是其父类Person的比较器
       ts.addAll(arr1);
       
       for(Object o : ts.toArray())
          System.out.println(o);
       //上述的Worker 和 Student在TreeSet中排序时使用的比较器都是Person类实现的比较器
   }
}
　　

ArrayList<Person> alist = new ArrayList<Person>();
ArrayList<? extends Person> list = new ArrayList<Student>();
alist.addAll(list);

Person p = new Person();

Student s = new Student();

//compile error

list.add(p);或者 list.add(s);

ArrayList<? super Person> list = new ArrayList<Person>();
Student s = new Student(); 
list.add(s);
s = (Student) list.get(0);