SELECT Count(1) AS total,
'hello' AS filter,
field1 AS field1,
Count(DISTINCT field2) AS total_field2
FROM table
WHERE field = true
AND status = 'ok'
GROUP BY field1
有疑问如何使用Java 8制作地图来存储以下结果。地图键必须是字段field1
,地图值必须是字段total_field2
。
也就是说,我需要使用字段field1
对我的列表进行分组,并计算字段field2
的数量。
对于总字段,我有以下内容:
myList.stream().collect(Collectors.groupingBy(MyObject::getField1, Collectors.counting()))
// this is just counting the records grouped by field1
我的结果是正确的 total_field1: {4=55, 6=31}
对于field2,我需要类似这样的东西,但它只给我一条记录
myList.stream().filter(distinctByKey(MyObject::getField2))
.collect(Collectors.groupingBy(MyObject::getField1, Collectors.counting()));
public static <T> Predicate<T> distinctByKey(Function<? super T, ?> keyExtractor) {
Set<Object> seen = ConcurrentHashMap.newKeySet();
return t -> seen.add(keyExtractor.apply(t));
}
结果 total_Field2: {4=31}
应返回我2个示例记录 total_Field2: {4=31, 6=31}
示例 @Naman
public static <T, A, R> Collector<T, ?, R> filtering(
Predicate<? super T> predicate, Collector<? super T, A, R> downstream) {
BiConsumer<A, ? super T> accumulator = downstream.accumulator();
return Collector.of(downstream.supplier(),
(r, t) -> { if(predicate.test(t)) accumulator.accept(r, t); },
downstream.combiner(), downstream.finisher(),
downstream.characteristics().toArray(new Collector.Characteristics[0]));
}
myList.stream().collect(Collectors.groupingBy(MyObject::getField1, filtering(distinctByKey(MyObject::getField2), Collectors.counting())));
field1
分组,并对field2
进行不重复计数,是吗? - Ryuzaki L