注意
想法及时记录，实现可以待做。
示例介绍
java8提供了stream的流式处理方式，可以非常优雅的处理各种集合操作，远比之前各种循环嵌套要干净清爽许多。
一般处理
public Map<Long, String> getIdNameMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getId, Account::getUsername));
}
收集为自身
public Map<Long, Account> getIdAccountMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getId, account -> account));
}
重复key的处理
public Map<String, Account> getNameAccountMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2));
}
指定具体的收集Map
public Map<String, Account> getNameAccountMap(List<Account> accounts) {
return accounts.stream().collect(Collectors.toMap(Account::getUsername, Function.identity(), (key1, key2) -> key2, LinkedHashMap::new));
}
分组聚合
Map<Department, List<Employee>> byDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment));
聚合为字符串
String joined = things.stream()
.map(Object::toString)
.collect(Collectors.joining(", "));
聚合为累加和
int total = employees.stream()
.collect(Collectors.summingInt(Employee::getSalary)));
分组聚合后，计算和
Map<Department, Integer> totalByDept
= employees.stream()
.collect(Collectors.groupingBy(Employee::getDepartment,
Collectors.summingInt(Employee::getSalary)));
复杂类型的分组聚合
Map<Tuple, List<BlogPost>> postsPerTypeAndAuthor = posts.stream()
.collect(groupingBy(post -> new Tuple(post.getType(), post.getAuthor())));
分组聚合后修改返回值
Map<BlogPostType, Set<BlogPost>> postsPerType = posts.stream()
.collect(groupingBy(BlogPost::getType, toSet()));
Map<BlogPostType, String> postsPerType = posts.stream()
.collect(groupingBy(BlogPost::getType,
mapping(BlogPost::getTitle, joining(", ", "Post titles: [", "]"))));
EnumMap<BlogPostType, List<BlogPost>> postsPerType = posts.stream()
.collect(groupingBy(BlogPost::getType,
() -> new EnumMap<>(BlogPostType.class), toList()));
多个字段分组聚合
Map<String, Map<BlogPostType, List>> map = posts.stream()
.collect(groupingBy(BlogPost::getAuthor, groupingBy(BlogPost::getType)));
并发分组聚合
ConcurrentMap<BlogPostType, List<BlogPost>> postsPerType = posts.parallelStream()
.collect(groupingByConcurrent(BlogPost::getType));
