现在 JAVA 17 和 Java 11 基本上可以和 Java8 平分 JDK 装机比例。下面是我常用的一些 Strem API 操作。除了分组、转换、排序，如果大家还有更多常用的 API 可以一起留言交流。
List 默认分组过后是 Map<Key, List>
List<StreamItem> streamList = Stream.of(
new StreamItem(1, "k1"),
new StreamItem(2, "k1"),
new StreamItem(3, "k2"),
new StreamItem(4, "k2")).collect(Collectors.toList());
System.out.println(streamList);
//1.1分组
Map<String, List<StreamItem>> streamMap1 = streamList.stream().collect(Collectors.groupingBy(StreamItem::getKey));
System.out.println(streamMap1);
//输出：
{k1=[StreamItem{id=1, key='k1', name='i_1|k_k1'}, StreamItem{id=2, key='k1', name='i_2|k_k1'}], k2=[StreamItem{id=3, key='k2', name='i_3|k_k2'}, StreamItem{id=4, key='k2', name='i_4|k_k2'}]}
List 默认分组过后是 Map<Key, Object>
//1.2分组只拿第一个
Map<String, StreamItem> streamMap2 = Stream.of(
new StreamItem(1, "k1"),
new StreamItem(2, "k2")).collect(Collectors.toMap(StreamItem::getKey, Function.identity()));
//如果 key 重复报: java.lang.IllegalStateException: Duplicate key
System.out.println(streamMap2);
//输出：
{k1=StreamItem{id=1, key='k1', name='i_1|k_k1'}, k2=StreamItem{id=2, key='k2', name='i_2|k_k2'}}
分组，在组内排序然后获取最大值，或者最小值
Comparator<StreamItem> idComparator =Comparator.comparing(StreamItem::getId);
Map<String, Optional<StreamItem>> streamMap3 = streamList.stream().collect(Collectors.groupingBy(StreamItem::getKey,
Collectors.reducing(BinaryOperator.maxBy(idComparator))));
System.out.println(streamMap3);
//输出
{k1=Optional[StreamItem{id=2, key='k1', name='i_2|k_k1'}], k2=Optional[StreamItem{id=4, key='k2', name='i_4|k_k2'}]}
这个也是超级实用的 api
List<List<StreamItem>> partitionList = Lists.partition(streamList, 2);
List<StreamItem> streamList1 = partitionList.stream().flatMap(Collection::stream).collect(Collectors.toList());
System.out.println(streamList1);
//输出
[StreamItem{id=1, key='k1', name='i_1|k_k1'}, StreamItem{id=2, key='k1', name='i_2|k_k1'}, StreamItem{id=3, key='k2', name='i_3|k_k2'}, StreamItem{id=4, key='k2', name='i_4|k_k2'}]
排序，默认正序，如果是需要倒序，可以在comparing 方法后面再调用 reversed 方法
//3.1 正序
List<StreamItem> streamList2 = Stream.of(
new StreamItem(3, "k1"),
new StreamItem(1, "k1"),
new StreamItem(2, "k2"))
//倒序：Comparator.comparing(StreamItem::getId).reversed()
.sorted(Comparator.comparing(StreamItem::getId)).collect(Collectors.toList());
System.out.println(streamList2);
去重复后，保留最后写入的值
//4.1 去重复
List<StreamItem> streamList3 = Stream.of(
new StreamItem(3, "k1"),
new StreamItem(1, "k1"),
new StreamItem(2, "k2"))
//如果只需要保留最大的 id 的值，就可以先排序, 时间复杂度考了个人觉得实用 group 更优
.sorted(Comparator.comparing(StreamItem::getId).reversed())
.collect(Collectors.collectingAndThen(Collectors.toCollection(() ->
//利用 set 特征去重
new TreeSet<>(Comparator.comparing(StreamItem::getKey))), ArrayList::new));
System.out.println(streamList3);
//输出：
[StreamItem{id=3, key='k1', name='i_3|k_k1'}, StreamItem{id=2, key='k2', name='i_2|k_k2'}]
