一、对集合按照单个属性分组

List items =
Arrays.asList(“apple”, “apple”, “banana”,
“apple”, “orange”, “banana”, “papaya”);
// 分组
Map result1 = items.stream().collect(
Collectors.groupingBy(
Function.identity()
)
);
//{papaya=[papaya], orange=[orange], banana=[banana, banana], apple=[apple, apple, apple]}
System.out.println(result1);
// 分组计数
Map result2 = items.stream().collect(
Collectors.groupingBy(
Function.identity(), Collectors.counting()
)
);
// {papaya=1, orange=1, banana=2, apple=3}
System.out.println(result2);
Map finalMap = new LinkedHashMap<>();
//分组, 计数和排序
result2.entrySet().stream()
.sorted(Map.Entry.comparingByValue().reversed())
.forEachOrdered(e -> finalMap.put(e.getKey(), e.getValue()));
// {apple=3, banana=2, papaya=1, orange=1}
System.out.println(finalMap);

case2:
User user1 = new User(“zhangsan”, “beijing”, 10);
User user2 = new User(“zhangsan”, “beijing”, 20);
User user3 = new User(“lisi”, “shanghai”, 30);
List list = new ArrayList();
list.add(user1);
list.add(user2);
list.add(user3);
Map collect = list.stream()
.collect(
Collectors.groupingBy(
User::getName/, Collectors.counting()/
)
);
//{lisi=[User{age=30, name=’lisi’, address=’shanghai’}],
// zhangsan=[User{age=10, name=’zhangsan’, address=’beijing’}, User{age=20, name=’zhangsan’, address=’beijing’}]}
System.out.println(collect);

二、集合按照多个属性分组

1.多个属性拼接出一个组合属性
public static void main(String[] args) {
User user1 = new User(“zhangsan”, “beijing”, 10);
User user2 = new User(“zhangsan”, “beijing”, 20);
User user3 = new User(“lisi”, “shanghai”, 30);
List list = new ArrayList();
list.add(user1);
list.add(user2);
list.add(user3);
Map collect = list.stream().collect(Collectors.groupingBy(e -> fetchGroupKey(e)));
//{zhangsan#beijing=[User{age=10, name=’zhangsan’, address=’beijing’}, User{age=20, name=’zhangsan’, address=’beijing’}],
// lisi#shanghai=[User{age=30, name=’lisi’, address=’shanghai’}]}
System.out.println(collect);
}
private static String fetchGroupKey(User user){
return user.getName() +”#”+ user.getAddress();
}

2.嵌套调用groupBy
不过感觉这样把问题搞复杂了
User user1 = new User(“zhangsan”, “beijing”, 10);
User user2 = new User(“zhangsan”, “beijing”, 20);
User user3 = new User(“lisi”, “shanghai”, 30);
List list = new ArrayList();
list.add(user1);
list.add(user2);
list.add(user3);
Map> collect
= list.stream().collect(
Collectors.groupingBy(
User::getAddress, Collectors.groupingBy(User::getName)
)
);
System.out.println(collect);