java收集器Collector怎么使用

本篇内容主要讲解“java收集器Collector怎么使用”，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习“java收集器Collector怎么使用”吧!

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.ArrayListSet<Integer>set=integerList.stream().map(Person::getAge).collect(Collectors.toSet());System.out.println(set);//[2, 3]System.out.println(set.getClass());//class java.util.HashSetLinkedList<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);//abcdeeStringset = integerList.stream().map(Person::getName).collect(Collectors.joining(","));System.out.println(set);//a,b,c,d,e,eStringlinkedList = 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());//6System.out.println(number.getMin());//1.0System.out.println(number.getSum());//21.0System.out.println(number.getAverage());//3.5number.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]

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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