java List 排序 Collections.sort()



用Collections.sort方法对list排序有两种方法

 第一种是list中的对象实现Comparable接口，如下：

 /**
  * 根据order对User排序
 */
 public class User implements Comparable<User>{
      private String name;
      private Integer order;
      public String getName() {
          return name;
      }
      public void setName(String name) {
          this.name = name;
      }
      public Integer getOrder() {
          return order;
      }
      public void setOrder(Integer order) {
          this.order = order;
      }
      public int compareTo(User arg0) {
          return this.getOrder().compareTo(arg0.getOrder());
      }
  } 测试一下：
 public class Test{

      public static void main(String[] args) {
          User user1 = new User();
          user1.setName("a");
          user1.setOrder(1);
          User user2 = new User();
          user2.setName("b");
          user2.setOrder(2);
          List<User> list = new ArrayList<User>();
          //此处add user2再add user1
         list.add(user2);
          list.add(user1);
          Collections.sort(list);
          for(User u : list){
              System.out.println(u.getName());
          }
      }
  }

输出结果如下
a
 b

第二种方法是根据Collections.sort重载方法来实现，例如：

 /**
  * 根据order对User排序
 */
 public class User { //此处无需实现Comparable接口
     private String name;
      private Integer order;
      public String getName() {
          return name;
      }
      public void setName(String name) {
          this.name = name;
      }
      public Integer getOrder() {
          return order;
      }
      public void setOrder(Integer order) {
          this.order = order;
      }
  }

  主类中这样写即可(HastSet——>List——>sort进行排序)：
 public class Test {
     public static void main(String[] args) {
         User user1 = new User();
         user1.setName("a");
         user1.setPrice(11);
         User user2 = new User();
         user2.setName("b");
         user2.setPrice(2);

         Set<User> Hset = new HashSet<User>();
         Hset.add(user2);
         Hset.add(user1);

         List<User> list = new ArrayList<User>();
         list.addAll(Hset);


         Collections.sort(list,new Comparator<User>(){
             public int compare(User arg0, User arg1) {
                 return arg0.getPrice().compareTo(arg1.getPrice());
             }
         });
         for(User u : list){
             System.out.println(u.getName());
         }
     }

输出结果如下：

a
b

默认为升序，将。return arg0.getOrder().compareTo(arg1.getOrder());    改为：
return arg1.getOrder().compareTo(arg0.getOrder());   
就成降序的了。