java收集器Collector案例匯總
一、收集器Collector
//T:表示流中每個元素的類型。 A:表示中間結(jié)果容器的類型。 R:表示最終返回的結(jié)果類型。 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()///是輸出的結(jié)果 ?? ?Set<Collector.Characteristics>?? ?characteristics()//返回Set的Collector.Characteristics指示此收集器的特征。 ?? ?//返回一個新的Collector由給定的描述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) ?? ?//返回一個新的Collector由給定的描述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、收集器工廠Collectors
public final class Collectors extends Object
Collectors作為Stream的collect方法的參數(shù),Collector是一個接口,它是一個可變的匯聚操作,將輸入元素累計到一個可變的結(jié)果容器中;它會在所有元素都處理完畢后,將累積的結(jié)果轉(zhuǎn)換為一個最終的表示(這是一個可選操作);
Collectors本身提供了關(guān)于Collector的常見匯聚實(shí)現(xiàn),Collectors的內(nèi)部類CollectorImpl實(shí)現(xiàn)了Collector接口,Collectors本身實(shí)際上是一個
工廠。
2.1 變成ConcurrentMap
//返回將Collector元素累積到其中 ConcurrentMap的并發(fā)函數(shù),其鍵和值是將提供的映射函數(shù)應(yīng)用于輸入元素的結(jié)果。 static <T,K,U> Collector<T,?,ConcurrentMap<K,U>>?? ?toConcurrentMap(Function<? super T,? extends K> keyMapper,? ?? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ??? ? ?Function<? super T,? extends U> valueMapper) //返回將Collector元素累積到其中 ConcurrentMap的并發(fā)函數(shù),其鍵和值是將提供的映射函數(shù)應(yīng)用于輸入元素的結(jié)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) //返回將Collector元素累積到其中 ConcurrentMap的并發(fā)函數(shù),其鍵和值是將提供的映射函數(shù)應(yīng)用于輸入元素的結(jié)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 變成Map
static <T,K,U> Collector<T,?,Map<K,U>> toMap(Function<? super T,? extends K> keyMapper,? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ?Function<? super T,? extends U> valueMapper) //1、當(dāng)key重復(fù)時,會拋出異常:java.lang.IllegalStateException: Duplicate key? //2、當(dāng)value為null時,會拋出異常: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}
//第三個參數(shù)用在key值沖突的情況下:如果新元素產(chǎn)生的key在Map中已經(jīng)出現(xiàn)過了,第三個參數(shù)就會定義解決的辦法。 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}
//返回將Collector元素累積到 Map其鍵中的值,其值是將提供的映射函數(shù)應(yīng)用于輸入元素的結(jié)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 變成Collection
static <T> Collector<T,?,List<T>> toList() static <T> Collector<T,?,Set<T>> ?toSet() ? //自定義? 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 變成String
static Collector<CharSequence,?,String>?? ?joining() //delimiter分隔符連接 static Collector<CharSequence,?,String>?? ?joining(CharSequence delimiter)? //prefix前綴 //suffix后綴 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 計算最值
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 統(tǒng)計數(shù)據(jù)
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 用于收集統(tǒng)計數(shù)據(jù)(如計數(shù),最小值,最大值,總和和平均值)的狀態(tài)對象。
此實(shí)現(xiàn)不是線程安全的。但是,Collectors.toXXXStatistics()在并行流上使用是安全的 ,因為并行實(shí)現(xiàn)Stream.collect() 提供了必要的分區(qū),隔離和合并結(jié)果,以實(shí)現(xiàn)安全有效的并行執(zhí)行。
他們的方法如下:
void accept(int value)//添加一個值 void combine(IntSummaryStatistics other)//將另一個的狀態(tài)合并IntSummaryStatistics到這個狀態(tài)中。 double getAverage()//算術(shù)平均值,如果沒有記錄值,則返回零。 long getCount()//返回記錄的值的計數(shù)。 int getMax()//返回記錄的最大值,或者Integer.MIN_VALUE沒有記錄值。 int getMin()//返回記錄的最小值,或者Integer.MAX_VALUE沒有記錄值。 long getSum()//返回記錄的值的總和,如果沒有記錄值,則返回零。 String toString()//返回對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函數(shù)
//op 縮減的函數(shù) static <T> Collector<T,?,Optional<T>> reducing(BinaryOperator<T> op) ?? ? //identity儲存器初始值 static <T> Collector<T,?,T> reducing(T identity, BinaryOperator<T> op) //mapper作用的數(shù)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 計數(shù)
//返回Collector類型的接受元素,T用于計算輸入元素的數(shù)量。 static <T> Collector<T,?,Long>?? ?counting()
2.11 分組-變成map
//classifier分組依據(jù)函數(shù) 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將小組內(nèi)對象進(jìn)行處理 static <T,K,A,D> Collector<T,?,Map<K,D>>?? ?groupingBy(Function<? super T,? extends K> classifier,? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? ? Collector<? super T,A,D> downstream) //mapFactory中間操作 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 分組-變成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分區(qū)的依據(jù) 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
通過在累積之前將映射函數(shù)應(yīng)用于每個輸入Collector元素,使類型的接受元素適應(yīng)一個接受類型的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 收集之后繼續(xù)做一些處理
static <T,A,R,RR> Collector<T,A,RR>?? ?collectingAndThen(Collector<T,A,R> downstream, Function<R,RR> finisher)
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