java中泛型上限，下限应用

v 一.程序中无形之中用到的泛型

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
   }
}

v 二.简单的理解

 

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);

一般来讲，定义成<? extends T>的参数通常只能用来从里面取数据，而不能用add方法添加数据。

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

而定义成<? super T>的参数是可以用add方法来进行数据的添加的。它接受的 T类型或者T的父类，为什么Student是Person的子类但是仍能接受呢？因为在加入的时候讲Sutdent进行了向上转型为Object，这就是为什么在取出元素的时候是Object类型了！