Java8中Stream 的一些用法

• 2020 年 10 月 20 日
• 筆記
• java8, Stream

數據結構和數據準備

@Data
@AllArgsConstructor
@NoArgsConstructor
static class StreamItem {
    Integer id;
    String name;
    Boolean sex;
}

List<StreamItem> list = Lists.newArrayList(new StreamItem(1, "a", true), new StreamItem(2, "b", false), 
new StreamItem(3, "c", true), new StreamItem(1, "d", false), new StreamItem(4, "e", true));

map

List<String> list2 = list.stream().map(StreamItem::getName).collect(Collectors.toList());
System.out.println(list2);

[a, b, c, d, e]

groupingBy

Map<Integer, List<StreamItem>> list2 = list.stream().collect(Collectors.groupingBy(StreamItem::getId));
System.out.println(list2);

{1=[StreamStudy.StreamItem(id=1, name=a, sex=true), StreamStudy.StreamItem(id=1, name=d, sex=false)], 2=[StreamStudy.StreamItem(id=2, name=b, sex=false)], 
3=[StreamStudy.StreamItem(id=3, name=c, sex=true)], 4=[StreamStudy.StreamItem(id=4, name=e, sex=true)]}

toMap

Map<Integer, String> list2 = list.stream().collect(Collectors.toMap(StreamItem::getId, StreamItem::getName, (k1, k2) -> k2));
System.out.println(list2);

{1=d, 2=b, 3=c, 4=e}

filter

List<StreamItem> list2 = list.stream().filter(StreamItem::getSex).collect(Collectors.toList());
System.out.println(list2);

[StreamStudy.StreamItem(id=1, name=a, sex=true), StreamStudy.StreamItem(id=3, name=c, sex=true), StreamStudy.StreamItem(id=4, name=e, sex=true)]

sum

int sum = list.stream().mapToInt(StreamItem::getId).sum();
System.out.println(sum);

sum = list.stream().map(StreamItem::getId).reduce(Integer::sum).get();
System.out.println(sum);

sum = list.stream().map(StreamItem::getId).reduce(0, Integer::sum);
System.out.println(sum);

BigDecimal bigDecimal = list.stream()
        .map(streamItem -> new BigDecimal(streamItem.getId()))
        .reduce(BigDecimal.ZERO, BigDecimal::add);
System.out.println(bigDecimal);

11
11
11
11

參考

使用Stream快速對List進行一些操作
Java8之Consumer、Supplier、Predicate和Function攻略