java收集器
这是一条海鱼 人气:0一、收集器Collector
//T:表示流中每个元素的类型。 A:表示中间结果容器的类型。 R:表示最终返回的结果类型。 public interface Collector<T, A, R> { Supplier<A> supplier()//生成容器 BiConsumer<A,T> accumulator()//是添加元素 BinaryOperator<A> combiner()//是合并容器 Function<A,R>finisher()///是输出的结果 Set<Collector.Characteristics> characteristics()//返回Set的Collector.Characteristics指示此收集器的特征。 //返回一个新的Collector由给定的描述supplier, accumulator,combiner,和finisher功能。 static <T,A,R> Collector<T,A,R> of(Supplier<A> supplier, BiConsumer<A,T> accumulator, BinaryOperator<A> combiner, Function<A,R> finisher, Collector.Characteristics... characteristics) //返回一个新的Collector由给定的描述supplier, accumulator和combiner功能。 static <T,R> Collector<T,R,R> of(Supplier<R> supplier, BiConsumer<R,T> accumulator, BinaryOperator<R> combiner, Collector.Characteristics... characteristics) }
二、收集器工厂Collectors
public final class Collectors extends Object
Collectors作为Stream的collect方法的参数,Collector是一个接口,它是一个可变的汇聚操作,将输入元素累计到一个可变的结果容器中;它会在所有元素都处理完毕后,将累积的结果转换为一个最终的表示(这是一个可选操作);
Collectors本身提供了关于Collector的常见汇聚实现,Collectors的内部类CollectorImpl实现了Collector接口,Collectors本身实际上是一个
工厂。
2.1 变成ConcurrentMap
//返回将Collector元素累积到其中 ConcurrentMap的并发函数,其键和值是将提供的映射函数应用于输入元素的结果。 static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper) //返回将Collector元素累积到其中 ConcurrentMap的并发函数,其键和值是将提供的映射函数应用于输入元素的结果。 static <T,K,U> Collector<T,?,ConcurrentMap<K,U>> toConcurrentMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction) //返回将Collector元素累积到其中 ConcurrentMap的并发函数,其键和值是将提供的映射函数应用于输入元素的结果。 static <T,K,U,M extends ConcurrentMap<K,U>> Collector<T,?,M> toConcurrentMap( Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier )
2.2 变成Map
static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper) //1、当key重复时,会抛出异常:java.lang.IllegalStateException: Duplicate key //2、当value为null时,会抛出异常:java.lang.NullPointerException
案例:
List<Person>integerList=newArrayList<>(); integerList.add(new Person("a",3)); integerList.add(new Person("b",3)); integerList.add(new Person("c",3)); integerList.add(new Person("d",2)); integerList.add(new Person("e",2)); integerList.add(new Person("f",2)); Mapmap=integerList.stream().collect(Collectors.toMap(Person::getName,Person::getAge)); System.out.println(map);//{a=3, b=3, c=3, d=2, e=2, f=2}
//第三个参数用在key值冲突的情况下:如果新元素产生的key在Map中已经出现过了,第三个参数就会定义解决的办法。 static <T,K,U> Collector<T,?,Map<K,U>> toMap( Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",3)); integerList.add(new Person("b",3)); integerList.add(new Person("c",3)); integerList.add(new Person("d",2)); integerList.add(new Person("e",2)); integerList.add(new Person("e",3)); Collections.sort(integerList,comparator); System.out.println(integerList);*/ Map map =integerList.stream().collect(Collectors.toMap(Person::getName,Person::getAge,(a,b)->a+b)); System.out.println(map);//{a=3, b=3, c=3, d=2, e=5}
//返回将Collector元素累积到 Map其键中的值,其值是将提供的映射函数应用于输入元素的结果。 static <T,K,U,M extends Map<K,U>> Collector<T,?,M> toMap( Function<? super T,? extends K> keyMapper, Function<? super T,? extends U> valueMapper, BinaryOperator<U> mergeFunction, Supplier<M> mapSupplier)
2.3 变成Collection
static <T> Collector<T,?,List<T>> toList() static <T> Collector<T,?,Set<T>> toSet() //自定义 static <T,C extends Collection<T>> Collector<T,?,C> toCollection(Supplier<C> collectionFactory)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",3)); integerList.add(new Person("b",3)); integerList.add(new Person("c",3)); integerList.add(new Person("d",2)); integerList.add(new Person("e",2)); integerList.add(new Person("e",3)); List<Integer> list= integerList.stream().map(Person::getAge).collect(Collectors.toList()); System.out.println(list);//[3, 3, 3, 2, 2, 3] System.out.println(list.getClass());//class java.util.ArrayList Set<Integer>set=integerList.stream().map(Person::getAge).collect(Collectors.toSet()); System.out.println(set);//[2, 3] System.out.println(set.getClass());//class java.util.HashSet LinkedList<Integer>linkedList=integerList.stream().map(Person::getAge).collect(Collectors.toCollection(LinkedList::new)); System.out.println(linkedList);//[3, 3, 3, 2, 2, 3] System.out.println(linkedList.getClass());//class java.util.LinkedList
2.4 变成String
static Collector<CharSequence,?,String> joining() //delimiter分隔符连接 static Collector<CharSequence,?,String> joining(CharSequence delimiter) //prefix前缀 //suffix后缀 static Collector<CharSequence,?,String> joining(CharSequence delimiter, CharSequence prefix, CharSequence suffix)
案例:
List<Person> integerList = newArrayList<>(); integerList.add(new Person("a",3)); integerList.add(new Person("b",3)); integerList.add(new Person("c",3)); integerList.add(new Person("d",2)); integerList.add(new Person("e",2)); integerList.add(new Person("e",3)); Stringlist = integerList.stream().map(Person::getName).collect(Collectors.joining()); System.out.println(list);//abcdee Stringset = integerList.stream().map(Person::getName).collect(Collectors.joining(",")); System.out.println(set);//a,b,c,d,e,e StringlinkedList = integerList.stream().map(Person::getName).collect(Collectors.joining(",","(",")")); System.out.println(linkedList);//(a,b,c,d,e,e)
2.5 计算最值
static <T> Collector<T,?,Optional<T>> maxBy(Comparator<? super T> comparator) static <T> Collector<T,?,Optional<T>> minBy(Comparator<? super T> comparator)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("b",2)); integerList.add(new Person("c",3)); integerList.add(new Person("d",4)); integerList.add(new Person("e",5)); integerList.add(new Person("e",6)); Optional<Person> person = integerList.stream().collect(Collectors.maxBy(Comparator.comparing(Person::getAge))); System.out.println(person.get());//Person{name='e',age='6'}
2.6 平均值
static <T> Collector<T,?,Double> averagingDouble(ToDoubleFunction<? super T> mapper) static <T> Collector<T,?,Double> averagingInt(ToIntFunction<? super T> mapper) static <T> Collector<T,?,Double> averagingLong(ToLongFunction<? super T> mapper)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("b",1)); integerList.add(new Person("c",1)); integerList.add(new Person("d",1)); integerList.add(new Person("e",1)); integerList.add(new Person("e",1)); double number=integerList.stream().collect(Collectors.averagingDouble(Person::getAge)); System.out.println(number);//1.0
2.7 统计数据
static <T> Collector<T,?,DoubleSummaryStatistics> summarizingDouble(ToDoubleFunction<? super T> mapper) static <T> Collector<T,?,IntSummaryStatistics> summarizingInt(ToIntFunction<? super T> mapper) static <T> Collector<T,?,LongSummaryStatistics> summarizingLong(ToLongFunction<? super T> mapper)
DoubleSummaryStatistics,IntSummaryStatistics,LongSummaryStatistics 用于收集统计数据(如计数,最小值,最大值,总和和平均值)的状态对象。
此实现不是线程安全的。但是,Collectors.toXXXStatistics()在并行流上使用是安全的 ,因为并行实现Stream.collect() 提供了必要的分区,隔离和合并结果,以实现安全有效的并行执行。
他们的方法如下:
void accept(int value)//添加一个值 void combine(IntSummaryStatistics other)//将另一个的状态合并IntSummaryStatistics到这个状态中。 double getAverage()//算术平均值,如果没有记录值,则返回零。 long getCount()//返回记录的值的计数。 int getMax()//返回记录的最大值,或者Integer.MIN_VALUE没有记录值。 int getMin()//返回记录的最小值,或者Integer.MAX_VALUE没有记录值。 long getSum()//返回记录的值的总和,如果没有记录值,则返回零。 String toString()//返回对象的字符串表示形式。
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("b",2)); integerList.add(new Person("c",3)); integerList.add(new Person("d",4)); integerList.add(new Person("e",5)); integerList.add(new Person("e",6)); DoubleSummaryStatistics number = integerList.stream().collect(Collectors.summarizingDouble(Person::getAge)); System.out.println(number.getMax());//6 System.out.println(number.getMin());//1.0 System.out.println(number.getSum());//21.0 System.out.println(number.getAverage());//3.5 number.accept(100); System.out.println(number.getMax());//100.0
2.8 求和
static <T> Collector<T,?,Double> summingDouble(ToDoubleFunction<? super T> mapper) static <T> Collector<T,?,Integer> summingInt(ToIntFunction<? super T> mapper) static <T> Collector<T,?,Long> summingLong(ToLongFunction<? super T> mapper)
2.9 reducing函数
//op 缩减的函数 static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op) //identity储存器初始值 static <T> Collector<T,?,T> reducing(T identity, BinaryOperator<T> op) //mapper作用的数值 static <T,U> Collector<T,?,U> reducing(U identity, Function<? super T,? extends U> mapper, BinaryOperator<U> op)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("b",0)); integerList.add(new Person("c",0)); integerList.add(new Person("d",0)); integerList.add(new Person("e",0)); integerList.add(new Person("e",0)); Integernumber = integerList.stream().collect(Collectors.reducing(1,Person::getAge,(a,b)->a+b)); System.out.println(number);//2
2.10 计数
//返回Collector类型的接受元素,T用于计算输入元素的数量。 static <T> Collector<T,?,Long> counting()
2.11 分组-变成map
//classifier分组依据函数 static <T,K> Collector<T,?,Map<K,List<T>>> groupingBy(Function<? super T,? extends K> classifier)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("a",2)); integerList.add(new Person("a",3)); integerList.add(new Person("b",4)); integerList.add(new Person("b",5)); integerList.add(new Person("b",6)); Map map =i ntegerList.stream().collect(Collectors.groupingBy(Person::getName)); System.out.println(map); { a=[Person{name='a', age='1'}, Person{name='a', age='2'}, Person{name='a', age='3'}], b=[Person{name='b', age='4'}, Person{name='b', age='5'}, Person{name='b', age='6'}] }
//downstream将小组内对象进行处理 static <T,K,A,D> Collector<T,?,Map<K,D>> groupingBy(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream) //mapFactory中间操作 static <T,K,D,A,M extends Map<K,D>> Collector<T,?,M> groupingBy(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
案例:
List<Person> integerList = newArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("a",2)); integerList.add(new Person("a",3)); integerList.add(new Person("b",4)); integerList.add(new Person("b",5)); integerList.add(new Person("b",6)); Map map= i ntegerList.stream() .collect(Collectors.groupingBy(Person::getName,Collectors.reducing(0,Person::getAge,(a,b)->a+b))); System.out.println(map);//{a=6, b=15} Map map = integerList.stream() .collect(Collectors.groupingBy(Person::getName,TreeMap::new,Collectors.reducing(0,Person::getAge,(a,b)->a+b))); System.out.println(map.getClass());//classjava.util.TreeMap
2.12 分组-变成ConcurrentMap
static <T,K> Collector<T,?,ConcurrentMap<K,List<T>>> groupingByConcurrent(Function<? super T,? extends K> classifier) static <T,K,A,D> Collector<T,?,ConcurrentMap<K,D>> groupingByConcurrent(Function<? super T,? extends K> classifier, Collector<? super T,A,D> downstream) static <T,K,A,D,M extends ConcurrentMap<K,D>> Collector<T,?,M> groupingByConcurrent(Function<? super T,? extends K> classifier, Supplier<M> mapFactory, Collector<? super T,A,D> downstream)
2.13 分割流
//predicate分区的依据 static <T> Collector<T,?,Map<Boolean,List<T>>> partitioningBy(Predicate<? super T> predicate) static <T,D,A> Collector<T,?,Map<Boolean,D>> partitioningBy(Predicate<? super T> predicate, Collector<? super T,A,D> downstream)
2.14 收集器
通过在累积之前将映射函数应用于每个输入Collector元素,使类型的接受元素适应一个接受类型的U元素T。
static <T,U,A,R> Collector<T,?,R> mapping(Function<? super T,? extends U> mapper, Collector<? super U,A,R> downstream)
案例:
List<Person> integerList = new ArrayList<>(); integerList.add(new Person("a",1)); integerList.add(new Person("a",2)); integerList.add(new Person("a",3)); integerList.add(new Person("b",4)); integerList.add(new Person("b",5)); integerList.add(new Person("b",6)); List list = integerList.stream().collect(Collectors.mapping(Person::getName,Collectors.toList())); System.out.println(list);//[a, a, a, b, b, b]
2.15 收集之后继续做一些处理
static <T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
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