[开发工具]java收集器Collector

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)