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());}}

下面是测试类Test：

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;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 user1list.add(user2);list.add(user1);Collections.sort(list);for(User u:list){System.out.println(u.getName());}}}

输出为：

a

b

2.使用静态内部类实现Comparator接口,Comparator接口位于java.util包下

import java.util.*;
public class Main{
public static void main(String args[]){
ArrayList al=new ArrayList();
al.add(new Student(2,"aa"));
al.add(new Student(1,"bb"));
al.add(new Student(3,"dd"));
al.add(new Student(3,"cc"));
Collections.sort(al,new StudentComparator());
Iterator it=al.iterator();
while(it.hasNext()){
System.out.println(it.next());
}
}
}
class Student {
int id;
String name;
Student(int id,String name){
this.id=id;
this.name=name;
}
public String toString(){
return "id="+this.id+",name="+this.name;
}
}
class StudentComparator implements Comparator{
public int compare(Object o1,Object o2){
Student s1=(Student)o1;
Student s2=(Student)o2;
int result=(s1.id>s2.id)?1:((s1.id==s2.id)?0:-1);
if(0==result){
result=s1.name.compareTo(s2.name);
}
return result;
}
}

3.补充：我遇到的情况是直接List<String> list=new ArrayList();

在这种情况下，我们如果调用Collections.sort(list);方法，它会对list里字符串从左往右的字母的ascii值进行排序，不用实现 Comparator 接口。

public class Connectionssort {public static void main(String[] args) {// TODO Auto-generated method stubList<String> list = new ArrayList();int j=0;JDBManager db=new JDBManager(); ResultSet rs;String q = "select * from securityEvent";try {rs = db.executeQuery(q);while (rs.next()) {list.add(rs.getString("grade"));}Collections.sort(list);while (j < list.size()) {System.out.println(list.get(j).toString());j++;}}catch (Exception e) {// TODO: handle exception}
}
}

实验结果如下：

A

A

B

B

B

C

C

C

D