java8中的函数式数据处理

1、在流中只能遍历一次，否则会提示流已经操作或者关闭的错误

List title = Arrays.asList("Java8", "In", "Action");

        Stream stream = title.stream();

        stream.forEach(System.out::println);

     stream.forEach(System.out::println);

2、操作类型分为中间操作和终端操作

中间操作有

操作	类型	返回类型	操作参数	函数描述符
filter	中间	Stream	Predicate	T -> boolean
map	中间	Stream	Function	T -> R
sorted	中间	Stream	Comparator	(T,T) -> int
limit	中间	Stream
distinct	中间	Stream
skip	中间	Stream

终端操作有

操作	类型	返回类型	操作参数	函数描述符
forEach	终端	void	Consumer	T -> void
count	终端	long
collect	终端	R	Collector
allMatch	终端	boolean	Predicate	T -> boolean
anyMatch	终端	boolean	Predicate	T -> boolean
noneMatch	终端	boolean	Predicate	T -> boolean
findAny	终端	Optional
findFirst	终端	Optional
reduce	终端	Optinal	BinaryOperator	(T,T)->T

3、原始类型流物化

IntStream, DoubleStream, LongStream

映射到数值流，使用mapToInt,mapToDouble, mapToLong

转换为对象流使用boxed

生成范围使用range和rangeClosed，第一个参数表示起始值，第二个参数表示结束值，range不包含结束值，rangeClosed包含

4、创建流

（1）通过值创建流，使用静态方法Stream.of,使用Stream.empty()创建空流

（2）数组创建流，使用静态方法Arrays.stream

（3）由文件创建流，Files.lines

（4）函数创建流，使用静态方法Stream.iterate和Stream.generate

5、收集器

toList

toSet

joining

joining(CharSequence delimiter)

joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)

counting

minBy(Comparator comparator)

maxBy(Comparator comparator)

summingInt(ToIntFunction mapper)

summingLong(ToLongFunction mapper)

summingDouble(ToDoubleFunction mapper)

averagingInt(ToIntFunction mapper)

averagingLong(ToLongFunction mapper)

averagingDouble(ToDoubleFunction mapper)

reducing(T identity, BinaryOperatorop)

reducing(BinaryOperator op)

reducing(U identity, Function mapper, BinaryOperator op)

groupingBy(Function classifier)

groupingBy(Function classifier, Collector downstream)

groupingBy(Function classifier, Supplier mapFactory, Collector downstream)

groupingByConcurrent(Function classifier)

groupingByConcurrent(Function classifier, Collector downstream)

groupingByConcurrent(Functionclassifier, SuppliermapFactory, Collector downstream)

partitioningBy(Predicatepredicate)

partitioningBy(Predicatepredicate, Collector downstream)

collectingAndThen(Collector downstream, Function finisher)

toMap(Function keyMapper, Function valueMapper)

toMap(Function keyMapper, Function valueMapper, BinaryOperation mergeFunction)

toMap(FunctionkeyMapper, Function valueMapper, BinaryOperator mergeFunction, SuppliermapSupplier)

toConcurrentMap(Function keyMapper, Function valueMapper)

toConcurrentMap(Function keyMapper, Function valueMapper, BinaryOperationmergeFunction)

toConcurrentMap(FunctionkeyMapper, Function valueMapper, BinaryOperator mergeFunction, SuppliermapSupplier)

summarizingInt(ToIntFunctionmapper)

summarizingLong(ToLongFunctionmapper)

summarizingDouble(ToDoubleFunctionmapper)