stream流笔记

stream流笔记

map 转换元素

List<Long> ids = list.stream().map(DictionaryMetaPO::getId).collect(Collectors.toList());

Collectors.joining 拼接字符串

String s = list.stream().map(DictionaryMetaPO::getValue).collect(Collectors.joining("-"));

sorted 排序

// e1-e2升序，e2-e1降序
List<DictionaryMetaPO> list = list.stream().sorted((e1, e2) -> e2.getType() - e1.getType()).collect(Collectors.toList());

Collectors.toMap 收集元素成Map

Map<Long, String> map = list.stream().collect(Collectors.toMap(DictionaryMetaPO::getId, DictionaryMetaPO::getValue, (u, v) -> u));

Collectors.groupingBy() 分组

// 根据字段分组
Map<Integer, List<DictionaryMetaPO>> map = list.stream().collect(Collectors.groupingBy(DictionaryMetaPO::getType));

//根据字段分组后，处理分组后数据
Map<Integer, List<String>> map = list.stream().collect(Collectors.groupingBy(DictionaryMetaPO::getType,
                Collectors.mapping(DictionaryMetaPO::getValue, Collectors.toList())));

元素拆分出流 flatMap

List<List<Long>> list;
list.stream().flatMap(ids -> {
  return ids.stream().map(e -> new AbstractMap.SimpleEntry<>(e, "value"));
}).collect(Collectors.toMap(Map.Entry::getKey, Map.Entry::getValue, (a, b) -> a));