Java8之前的分组
代码如下：
代码语言：javascript复制 public static List<User> getUserList(){
List<User> userList = new ArrayList<>();
userList.add(new User("小明",11,"北京",66));
userList.add(new User("小红",12,"上海",99));
userList.add(new User("小詹",14,"杭州",77));
userList.add(new User("小龙",16,"伦敦",55));
userList.add(new User("小斯",9,"杭州",33));
userList.add(new User("小詹",9,"上海",33));
userList.add(new User("小布",12,"伦敦",88));
userList.add(new User("小布",12,"上海",55));
return userList;
}
/**
* java8之前对象集合根据某个字段分组
*/
@Test
public void sortUserListByAge(){
List<User> userList = getUserList();
Map<String, List<User>> groupByUserCityMap = new HashMap<>();
for (User user : userList) {
List<User> tmpList = groupByUserCityMap.get(user.getCity());
if (tmpList == null) {
tmpList = new ArrayList<>();
tmpList.add(user);
groupByUserCityMap.put(user.getCity(), tmpList);
} else {
tmpList.add(user);
}
}
System.out.println("按照城市分组后结果：" groupByUserCityMap);
}输出如下：
代码语言：javascript复制按照城市分组后结果：{上海=[User(name=小红, age=12, city=上海, score=99),
User(name=小詹, age=9, city=上海, score=33), User(name=小布, age=12, city=上海, score=55)],
伦敦=[User(name=小龙, age=16, city=伦敦, score=55), User(name=小布, age=12, city=伦敦, score=88)],
杭州=[User(name=小詹, age=14, city=杭州, score=77), User(name=小斯, age=9, city=杭州, score=33)],
北京=[User(name=小明, age=11, city=北京, score=66)]}Java8的分组
代码如下：
代码语言：javascript复制 /**
* java8根据某个字段分组
*/
@Test
public void java8GroupUserList(){
List<User> userList = getUserList();
Map<String, List<User>> groupByUserNameMap = userList.stream().collect(Collectors.groupingBy(User::getName));
System.out.println("按照姓名分组后结果：" groupByUserNameMap);
}输出如下：
代码语言：javascript复制按照姓名分组后结果：{小龙=[User(name=小龙, age=16, city=伦敦, score=55)],
小詹=[User(name=小詹, age=14, city=杭州, score=77), User(name=小詹, age=9, city=上海, score=33)],
小明=[User(name=小明, age=11, city=北京, score=66)],
小红=[User(name=小红, age=12, city=上海, score=99)],
小斯=[User(name=小斯, age=9, city=杭州, score=33)],
小布=[User(name=小布, age=12, city=伦敦, score=88), User(name=小布, age=12, city=上海, score=55)]}
