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。