java收集器Collector怎么使用

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一、收集器Collector

//T：表示流中每個(gè)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(gè)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(gè)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(gè)接口，它是一個(gè)可變的匯聚操作，將輸入元素累計(jì)到一個(gè)可變的結(jié)果容器中；它會(huì)在所有元素都處理完畢后，將累積的結(jié)果轉(zhuǎn)換為一個(gè)最終的表示（這是一個(gè)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(gè)
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(shí)，會(huì)拋出異常：java.lang.IllegalStateException: Duplicate key 
//2、當(dāng)value為null時(shí)，會(huì)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(gè)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(gè)參數(shù)就會(huì)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(jì)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(jì)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(jì)數(shù)據(jù)（如計(jì)數(shù)，最小值，最大值，總和和平均值）的狀態(tài)對(duì)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?yàn)椴⑿袑?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(gè)值
void combine(IntSummaryStatistics other)//將另一個(gè)的狀態(tài)合并IntSummaryStatistics到這個(gè)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(jì)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(duì)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(chǔ)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(jì)數(shù)

//返回Collector類型的接受元素，T用于計(jì)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(duì)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(gè)輸入Collector元素，使類型的接受元素適應(yīng)一個(gè)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)

到此，相信大家對(duì)“java收集器Collector怎么使用”有了更深的了解，不妨來實(shí)際操作一番吧！這里是億速云網(wǎng)站，更多相關(guān)內(nèi)容可以進(jìn)入相關(guān)頻道進(jìn)行查詢，關(guān)注我們，繼續(xù)學(xué)習(xí)！