20230619 java.util.stream.Collectors

介绍

• java.util.stream.Collectors
• public final class Collectors

API

static

生成集合

• toCollection
• toList
• toUnmodifiableList
• toSet
• toUnmodifiableSet

生成 Map

• toMap
• toUnmodifiableMap
• toConcurrentMap

生成字符串

• joining
  • Collector<CharSequence, ?, String> joining()
  • Collector<CharSequence, ?, String> joining(CharSequence delimiter)
  • Collector<CharSequence, ?, String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

生成 (Int|Long|Double)SummaryStatistics 汇总对象

• summingInt
• summingLong
• summingDouble

平均值

• averagingInt
• averagingLong
• averagingDouble

约简

• reducing
  • <T> Collector<T, ?, Optional<T>> reducing(BinaryOperator<T> op)
  • <T> Collector<T, ?, T> reducing(T identity, BinaryOperator<T> op)
  • identity 幺元
• counting
• minBy
• maxBy

群组

• groupingBy
• groupingByConcurrent
  • 产生 Map ，键是 K ，值是 List<T>

分区

• partitioningBy
  • 产生 Map ，键是 Boolean ，值是 List<T>

统计

• summarizingInt
• summarizingLong
• summarizingDouble

再处理

• collectingAndThen
  • <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
  • 在收集器后面添加了一个最终处理步骤
• mapping
  • <T, U, A, R> Collector<T, ?, R> mapping(Function<? super T, ? extends U> mapper, Collector<? super U, A, R> downstream)
  • 反 collectingAndThen
  • 将收集到的元素传给下游处理器
• filtering
• flatMapping
• teeing
  • <T, R1, R2, R> Collector<T, ?, R> teeing(Collector<? super T, ?, R1> downstream1, Collector<? super T, ?, R2> downstream2, BiFunction<? super R1, ? super R2, R> merger)
  • 将两个收集器结果合并为一个