來看看Java stream提供的分組 - groupingBy
一. 基本用法 - 接收一個(gè)參數(shù)
它接收一個(gè)函數(shù)作為參數(shù)歉嗓，也就是說可以傳lambda表達(dá)式進(jìn)來。
public static <T, K> Collector<T, ?, Map<K, List<T>>>
groupingBy(Function<? super T, ? extends K> classifier) {
return groupingBy(classifier, toList());
}
UserList.stream().collect(Collectors.groupingBy( 函數(shù) ))甘晤，返回的是一個(gè)map柠辞，key為分組的值鲸阔，value為list订晌，包含組內(nèi)的元素。
1. 簡(jiǎn)單類型分組
@Test
public void test01() {
List<Integer> intList = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 1, 2, 3);
Map<Integer, List<Integer>> collect = intList.stream().collect(Collectors.groupingBy(e -> e%2));
System.out.println(collect);
}
2. 對(duì)象饿自，按一個(gè)屬性分組
這里用到的User::getEducation汰翠，class名字+雙冒號(hào)+方法名龄坪，等同于lambda表達(dá)式e -> e.getEduction()
@Test
public void test02() {
List<User> userList = Arrays.asList(new User(0, "zhangsan", "zhangsan@qq.com", 20, "High School"),
new User(1, "lisi", "lisi@qq.com", 20, "High School"),
new User(2, "wangwu", "wangwu@qq.com", 20, "High School"),
new User(3, "lilei", "lilei@qq.com", 25, "Graduate"),
new User(4, "hanmeimei", "hanmeimei@qq.com", 26, "Graduate"),
new User(5, "lucy", "lucy@qq.com", 25, "Graduate"));
Map<String, List<User>> collect = userList.stream().collect(Collectors.groupingBy(User::getEducation));
System.out.println(collect);
}
3. 對(duì)象，按多個(gè)屬性分組
嵌套分組 (groupingBy 接收兩個(gè)參數(shù))
Map<Integer, Map<String, List<User>>> collect =
userList.stream().collect(Collectors.groupingBy(User::getAge, Collectors.groupingBy(User::getEducation)));
System.out.println(collect.get(20).get("High School"));
4. 按整個(gè)對(duì)象分組
用User::new方法進(jìn)行分組复唤，它會(huì)調(diào)用構(gòu)造方法public User(User user)健田，并依賴于hashcode和equals方法來判斷對(duì)象是否相等，進(jìn)而進(jìn)行分組佛纫。如果有一些特殊的邏輯對(duì)對(duì)象進(jìn)行分組妓局，可以重寫hashcode和equals方法。
Map<User, List<User>> collect = userList.stream().collect(Collectors.groupingBy(User::new));
public User(User user) {
this.id = user.id;
this.name = user.name;
this.email = user.email;
this.age = user.age;
this.education = user.education;
}
5. 處理null key
Map<String, List<User>> collect = userList.stream().collect(Collectors.groupingBy(e -> {
if (e.getEducation() == null || "".equals(e.getEducation())) {
return "";
}
return e.getEducation();
}));
二. 接收兩個(gè)參數(shù) - 對(duì)groupingby結(jié)果集進(jìn)一步處理
public static <T, K, A, D>
Collector<T, ?, Map<K, D>> groupingBy(Function<? super T, ? extends K> classifier,
Collector<? super T, A, D> downstream) {
return groupingBy(classifier, HashMap::new, downstream);
}
1. 計(jì)算平均值
Map<String, Double> collect =
userList.stream().collect(Collectors.groupingBy(User::getEducation, Collectors.averagingInt(User::getAge)));
{High School=20.0, Graduate=25.333333333333332}
2. count
Map<String, Long> collect =
userList.stream().collect(Collectors.groupingBy(User::getEducation, Collectors.counting()));
{High School=3, Graduate=3}
3. 求和
Map<String, Integer> collect =
userList.stream().collect(Collectors.groupingBy(User::getEducation, Collectors.summingInt(User::getAge)));
{High School=60, Graduate=76}
三. 接收三個(gè)參數(shù)
添加了對(duì)結(jié)果Map的生成方式呈宇，默認(rèn)是HashMap
public static <T, K, D, A, M extends Map<K, D>>
Collector<T, ?, M> groupingBy(Function<? super T, ? extends K> classifier,
Supplier<M> mapFactory,
Collector<? super T, A, D> downstream)
例好爬，用TreeMap對(duì)key進(jìn)行排序：
Map<String, Set<String>> namesByEdu =
userList.stream().collect(Collectors.groupingBy(User::getEducation, TreeMap::new, mapping(User::getName,Collectors.toSet())));
System.out.println(namesByEdu);
{Graduate=[lilei, hanmeimei, lucy], High School=[lisi, zhangsan]}
