1.List中的对象实现Comparable接口：
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=pareTo(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
