Java Stream

find Index

IntStream.range(0, users.size())
    .filter(userInd-> users.get(userInd).getName().equals(username))
    .findFirst()
    .getAsInt();

collect 指定 型別

List<String> fields=Arrays.asList("foo", "bar", "baz", "foo");
CopyOnWriteArrayList<String> l =
    fields.stream()
          .distinct()
          .collect(Collectors.toCollection(CopyOnWriteArrayList::new));
 ```  

## collection to map
```java=
 var map= l.stream().collect(Collectors.toMap(e -> e.positionSide, e-> e));

groupingBy to List

```java= Map<BlogPostType, List> postsPerType = posts.stream() .collect(groupingBy(BlogPost::getType));

進階
```java=
List<CacheKBar> list= cacheKBarDao.findByExchangeAndPair(exPair.exchange,exPair.pair);
        return list.stream()
                .collect(Collectors.groupingBy(
                        CacheKBar::getSer,
                        Collectors.mapping(CacheKBar::load, Collectors.toList())
                ));

tags: java