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.LinkedList2.4 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);//1.02.7 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());//100.02.8 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);//22.10 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.TreeMap2.12 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)到此，相信大家对“java收集器Collector怎么使用”有了更深的了解，不妨来实际操作一番吧！这里是编程网网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！
