学习之：JDK1.8新特性: Collectors原理以及用法

以你之姓@ 2022-03-20 13:42 590阅读 0赞

借鉴至：[https://blog.csdn.net/vbirdbest/article/details/80216713 ][https_blog.csdn.net_vbirdbest_article_details_80216713] 作者：vbirdbest

借鉴至：[https://blog.csdn.net/qq\_39629277/article/details/83108585][https_blog.csdn.net_qq_39629277_article_details_83108585]

--------------------

**目录**

一：简介

二、Collectors

1、将数据收集进一个列表

1.1 Collectors toList()

1.2 Collectors  toSet()

2、将数据收集进一个集合

2.1 Collectors  toMap()

3、用自定义的实现Collection的数据结构收集

3.1 Collectors  toCollection()

4、集合元素拼接

4.1Collectors  Joining()

4.2 collectingAndThen() 收集之后继续做一些处理。

5、元素聚合

5.1 maxBy,minBy,summingInt,averagingDouble

5.2 mapping

6、分组

7、累计操作

总结：

--------------------

# 一：简介 #

**Stream**中有两个个方法**collect**和**collectingAndThen**用于对流中的数据进行处理，可以对流中的数据进行聚合操作，如：

将流中的数据转成集合类型: **toList、toSet、toMap、toCollection**  
将流中的数据(字符串)使用分隔符拼接在一起：**joining**  
对流中的数据求最大值**maxBy**、最小值**minBy**、求和**summingInt**、求平均值**averagingDouble**  
对流中的数据进行映射处理 **mapping**  
对流中的数据分组：**groupingBy、partitioningBy**  
对流中的数据累计计算：**reducing**

--------------------

#     二、Collectors #

## 1、将数据收集进一个列表 ##

public final class Collectors {
    
    	// 转换成集合
    	public static <T> Collector<T, ?, List<T>> toList();
    	public static <T> Collector<T, ?, Set<T>> toSet();
    	public static <T, K, U> Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                        Function<? super T, ? extends U> valueMapper);
        public static <T, C extends Collection<T>> Collector<T, ?, C> toCollection(Supplier<C> collectionFactory);
                                         
       // 拼接字符串，有多个重载方法                                  
       public static Collector<CharSequence, ?, String> joining(CharSequence delimiter);   
       public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                                 CharSequence prefix,
                                                                 CharSequence suffix);      
        // 最大值、最小值、求和、平均值                                                         
        public static <T> Collector<T, ?, Optional<T>> maxBy(Comparator<? super T> comparator);
        public static <T> Collector<T, ?, Optional<T>> minBy(Comparator<? super T> comparator);
        public static <T> Collector<T, ?, Integer> summingInt(ToIntFunction<? super T> mapper);      
        public static <T> Collector<T, ?, Double> averagingDouble(ToDoubleFunction<? super T> mapper);                   
        
        // 分组：可以分成true和false两组，也可以根据字段分成多组                                 
    	public static <T, K> Collector<T, ?, Map<K, List<T>>> groupingBy(Function<? super T, ? extends K> classifier);
    	// 只能分成true和false两组
    	public static <T> Collector<T, ?, Map<Boolean, List<T>>> partitioningBy(Predicate<? super T> predicate);
    	
    	 // 映射
    	 public static <T, U, A, R> Collector<T, ?, R> mapping(Function<? super T, ? extends U> mapper,
                                   Collector<? super U, A, R> downstream);
                                   
    	 public static <T, U> Collector<T, ?, U> reducing(U identity,
                                    Function<? super T, ? extends U> mapper,
                                    BinaryOperator<U> op);
    }

### 1.1 Collectors toList() ###

**提示：Java 8 中我们可以通过 \`::\` 关键字来访问类的构造方法，对象方法，静态方法。**

**参考文章：[https://blog.csdn.net/kegaofei/article/details/80582356][https_blog.csdn.net_kegaofei_article_details_80582356]**

**->是lambda表达式,就是匿名函数。**

最简单的例子是 Collections.sort(list, (x, y) -> y - x); 其中

(x, y) -> y - x  是一个lambda表达式，输入两个参数x, y，返回值 x - y。“->”起分隔作用。，Java会自动翻译

**参考文章：[https://blog.csdn.net/kegaofei/article/details/80582356][https_blog.csdn.net_kegaofei_article_details_80582356]**

源码：

/**
         * Returns a {@code Collector} that accumulates the input elements into a
         * new {@code List}. There are no guarantees on the type, mutability,
         * serializability, or thread-safety of the {@code List} returned; if more
         * control over the returned {@code List} is required, use {@link #toCollection(Supplier)}.
         *
         * @param <T> the type of the input elements
         * @return a {@code Collector} which collects all the input elements into a
         * {@code List}, in encounter order
         */
        public static <T>
        Collector<T, ?, List<T>> toList() {
            return new CollectorImpl<>((Supplier<List<T>>) ArrayList::new, List::add,
                                       (left, right) -> { left.addAll(right); return left; },
                                       CH_ID);
        }

示例：

@Test
        public void testCollectToList(){
            List<Integer> list = Arrays.asList(1, 2, 3);
            // 要求：将list中每个数值扩大10倍 结果为： [10, 20, 30]
            
            //先将list转化为stream类型，map内 等于遍历map方法使所有数据乘以10
            Stream stream=list.stream().map(i -> i * 10);//
            List<Integer> collectOne=(List<Integer>)stream.collect(Collectors.toList());
            
            //简写可如下，一般使用简写方便快捷
            List<Integer> collectTwo = list.stream().map(i -> i * 10).collect(Collectors.toList());
    
            System.out.println(collectOne.toString());
            System.out.println(collectTwo.toString());
        }

![20190213120909457.jpg][]

此写法的好处，省去了复杂冗余的遍历操作。一行即可完成对list内数据的简单操作。

### 1.2 Collectors  toSet() ###

源码：

/**
         * Returns a {@code Collector} that accumulates the input elements into a
         * new {@code Set}. There are no guarantees on the type, mutability,
         * serializability, or thread-safety of the {@code Set} returned; if more
         * control over the returned {@code Set} is required, use
         * {@link #toCollection(Supplier)}.
         *
         * <p>This is an {@link Collector.Characteristics#UNORDERED unordered}
         * Collector.
         *
         * @param <T> the type of the input elements
         * @return a {@code Collector} which collects all the input elements into a
         * {@code Set}
         */
        public static <T>
        Collector<T, ?, Set<T>> toSet() {
            return new CollectorImpl<>((Supplier<Set<T>>) HashSet::new, Set::add,
                                       (left, right) -> { left.addAll(right); return left; },
                                       CH_UNORDERED_ID);
        }

示例：

@Test
        public void testCollectToSet(){
            List<Integer> list = Arrays.asList(1, 2, 3);
            // 要求：将list中每个数值扩大10倍  
    
            //先将list转化为stream类型，map内 等于遍历map方法使所有数据乘以10
            Stream stream=list.stream().map(i -> i * 10);//
            Set<Integer> collect1=(Set<Integer>)stream.collect(Collectors.toSet());
    
            //简写可如下，一般使用简写方便快捷
            Set<Integer> collect2 = list.stream().map(i -> i * 10).collect(Collectors.toSet());
    
            System.out.println(collect1.toString());
            System.out.println(collect2.toString());
        }

![20190213121644952.jpg][]

因为set是无序的所以结果为\[20,10,30\]

--------------------

## 2、将数据收集进一个集合 ##

### 2.1 Collectors  toMap() ###

源码：

/**
         * students to their grade point average:
         * <pre>{@code
         *     Map<Student, Double> studentToGPA
         *         students.stream().collect(toMap(Functions.identity(),
         *                                         student -> computeGPA(student)));
         * }</pre>
         * And the following produces a {@code Map} mapping a unique identifier to
         * students:
         * <pre>{@code
         *     Map<String, Student> studentIdToStudent
         *         students.stream().collect(toMap(Student::getId,
         *                                         Functions.identity());
         * }</pre>
    */
    public static <T, K, U>
        Collector<T, ?, Map<K,U>> toMap(Function<? super T, ? extends K> keyMapper,
                                        Function<? super T, ? extends U> valueMapper) {
            return toMap(keyMapper, valueMapper, throwingMerger(), HashMap::new);
        }

示例：

@Test
        public void testCollectToMap(){
            List<Integer> list = Arrays.asList(1, 2, 3);
            // {key1=value:10, key2=value:20, key3=value:30}  {key1=value:1 , key2=value:2 , key3=value:3 }
            // toMap 可用于将List转为Map，便于通过key快速查找到某个value
            Map<String, String> collect1 = list.stream().map(i -> i * 10).collect(Collectors.toMap(key -> "key" + key/10, value -> "value:" + value));
            Map<String, String> collect2 = list.stream().map(i -> i).collect(Collectors.toMap(key -> "key" + key, value -> "value:" + value));
    
            //实体list转化map id作为主键，对象作为value
            List<DatabusTask> databusTaskArrayList=new ArrayList<DatabusTask>();
            DatabusTask databusTask=new DatabusTask();
            databusTask.setId(1);
            databusTask.setFile_name("测试");
            databusTaskArrayList.add(databusTask);
    
            Map<Integer,DatabusTask> taskMap = databusTaskArrayList.stream().collect(Collectors.toMap(DatabusTask::getId, entity -> entity));
            System.out.println(collect1.toString());
            System.out.println(collect2.toString());
            System.out.println(taskMap.toString());
        }

![20190213141628711.jpg][]

**此写法的好处可以快速将实体内容转化自定义map集合，可以将实体list 转化为以id为key以对象为value的map集合方便在某些情况下直接取id得到相应对象信息**

--------------------

## 3、用自定义的实现Collection的数据结构收集 ##

### 3.1 Collectors  toCollection() ###

源码：

/**
         * Returns a {@code Collector} that accumulates the input elements into a
         * new {@code Collection}, in encounter order.  The {@code Collection} is
         * created by the provided factory.
         *
         * @param <T> the type of the input elements
         * @param <C> the type of the resulting {@code Collection}
         * @param collectionFactory a {@code Supplier} which returns a new, empty
         * {@code Collection} of the appropriate type
         * @return a {@code Collector} which collects all the input elements into a
         * {@code Collection}, in encounter order
         */
        public static <T, C extends Collection<T>>
        Collector<T, ?, C> toCollection(Supplier<C> collectionFactory) {
            return new CollectorImpl<>(collectionFactory, Collection<T>::add,
                                       (r1, r2) -> { r1.addAll(r2); return r1; },
                                       CH_ID);
        }

示例

@Test
        public void testCollectToCollection(){
            //直接创建treeSet数据 [1, 3, 4]
            TreeSet<Integer> collect3= Stream.of(1, 3, 4).collect(Collectors.toCollection(TreeSet::new));
            System.out.println(collect3.toString());
            List<String> list = Arrays.asList("java", "python", "C++","php","java");
            //用LinkedList收集
            List<String> linkedListResult = list.stream().collect(Collectors.toCollection(LinkedList::new));
            linkedListResult.forEach(System.out::println);
            System.out.println("--------------");
    
            //用CopyOnWriteArrayList收集
            List<String> copyOnWriteArrayListResult = list.stream().collect(Collectors.toCollection(CopyOnWriteArrayList::new));
            copyOnWriteArrayListResult.forEach(System.out::println);
            System.out.println("--------------");
    
            //用TreeSet收集
            TreeSet<String> treeSetResult = list.stream().collect(Collectors.toCollection(TreeSet::new));
            treeSetResult.forEach(System.out::println);
    
        }

![watermark_type_ZmFuZ3poZW5naGVpdGk_shadow_10_text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L0FsaWNlX3FpeGlu_size_16_color_FFFFFF_t_70][]

--------------------

## 4、集合元素拼接 ##

### 4.1Collectors  Joining() ###

源码 joining三种参数类型

/**
         * Returns a {@code Collector} that concatenates the input elements into a
         * {@code String}, in encounter order.
         *
         * @return a {@code Collector} that concatenates the input elements into a
         * {@code String}, in encounter order
         */
        public static Collector<CharSequence, ?, String> joining() {
            return new CollectorImpl<CharSequence, StringBuilder, String>(
                    StringBuilder::new, StringBuilder::append,
                    (r1, r2) -> { r1.append(r2); return r1; },
                    StringBuilder::toString, CH_NOID);
        }
    
        /**
         * Returns a {@code Collector} that concatenates the input elements,
         * separated by the specified delimiter, in encounter order.
         *
         * @param delimiter the delimiter to be used between each element
         * @return A {@code Collector} which concatenates CharSequence elements,
         * separated by the specified delimiter, in encounter order
         */
        public static Collector<CharSequence, ?, String> joining(CharSequence delimiter) {
            return joining(delimiter, "", "");
        }
    
        /**
         * Returns a {@code Collector} that concatenates the input elements,
         * separated by the specified delimiter, with the specified prefix and
         * suffix, in encounter order.
         *
         * @param delimiter the delimiter to be used between each element
         * @param  prefix the sequence of characters to be used at the beginning
         *                of the joined result
         * @param  suffix the sequence of characters to be used at the end
         *                of the joined result
         * @return A {@code Collector} which concatenates CharSequence elements,
         * separated by the specified delimiter, in encounter order
         */
        public static Collector<CharSequence, ?, String> joining(CharSequence delimiter,
                                                                 CharSequence prefix,
                                                                 CharSequence suffix) {
            return new CollectorImpl<>(
                    () -> new StringJoiner(delimiter, prefix, suffix),
                    StringJoiner::add, StringJoiner::merge,
                    StringJoiner::toString, CH_NOID);
        }

示例：

@Test
        public void testCollectJoining(){
    
            List<String> list = Arrays.asList("java", "python", "C++","php","java");
            //直接将输出结果拼接
            System.out.println(list.stream().collect(Collectors.joining()));
            //每个输出结果之间加拼接符号“|”
            System.out.println(list.stream().collect(Collectors.joining(" | ")));
            //输出结果开始头为Start--，结尾为--End，中间用拼接符号“||”
            System.out.println(list.stream().collect(Collectors.joining(" || ", "Start--", "--End")));
            
            // a,b,c
            List<String> list2 = Arrays.asList("a", "b", "c");
            //此方法等同于直接 String result = Joiner.on(",").join(list2); 将数组转化逗号分隔字符串
            String result = list2.stream().collect(Collectors.joining(","));
            System.out.println(result);
          
        }

![20190213150520649.jpg][]

### 4.2 collectingAndThen() 收集之后继续做一些处理。  ###

源码：

/**
         * Adapts a {@code Collector} to perform an additional finishing
         * transformation.  For example, one could adapt the {@link #toList()}
         * collector to always produce an immutable list with:
         * <pre>{@code
         *     List<String> people
         *         = people.stream().collect(collectingAndThen(toList(), Collections::unmodifiableList));
         * }</pre>
         *
         * @param <T> the type of the input elements
         * @param <A> intermediate accumulation type of the downstream collector
         * @param <R> result type of the downstream collector
         * @param <RR> result type of the resulting collector
         * @param downstream a collector
         * @param finisher a function to be applied to the final result of the downstream collector
         * @return a collector which performs the action of the downstream collector,
         * followed by an additional finishing step
         */
        public static<T,A,R,RR> Collector<T,A,RR> collectingAndThen(Collector<T,A,R> downstream,
                                                                    Function<R,RR> finisher) {
            Set<Collector.Characteristics> characteristics = downstream.characteristics();
            if (characteristics.contains(Collector.Characteristics.IDENTITY_FINISH)) {
                if (characteristics.size() == 1)
                    characteristics = Collectors.CH_NOID;
                else {
                    characteristics = EnumSet.copyOf(characteristics);
                    characteristics.remove(Collector.Characteristics.IDENTITY_FINISH);
                    characteristics = Collections.unmodifiableSet(characteristics);
                }
            }
            return new CollectorImpl<>(downstream.supplier(),
                                       downstream.accumulator(),
                                       downstream.combiner(),
                                       downstream.finisher().andThen(finisher),
                                       characteristics);
        }

示例：

@Test
        public void testCollectingAndThan(){
            List<String> list2 = Arrays.asList("a", "b", "c");
            // Collectors.joining(",")的结果是：a,b,c  然后再将结果 x + "d"操作, 最终返回a,b,cd
            String str= Stream.of("a", "b", "c").collect(Collectors.collectingAndThen(Collectors.joining(","), x -> x + "d"));
            System.out.println(str);
    
    
        }

## ![20190213151054886.jpg][] ##

--------------------

## 5、元素聚合 ##

### 5.1 maxBy,minBy,summingInt,averagingDouble ###

@Test
    public void test(){
        // 求最值 3
        List<Integer> list = Arrays.asList(1, 2, 3);
        Integer maxValue = list.stream().collect(Collectors.collectingAndThen(Collectors.maxBy((a, b) -> a - b), Optional::get));
    
    
        // 最小值 1
        Integer minValue = list.stream().collect(Collectors.collectingAndThen(Collectors.minBy((a, b) -> a - b), Optional::get));
    
        // 求和 6
        Integer sumValue = list.stream().collect(Collectors.summingInt(item -> item));
    
        // 平均值 2.0
        Double avg = list.stream().collect(Collectors.averagingDouble(x -> x));
    }

### 5.2 mapping ###

## ##

@Test
    public void test(){
        // 映射：先对集合中的元素进行映射，然后再对映射的结果使用Collectors操作
        // A,B,C
        Stream.of("a", "b", "c").collect(Collectors.mapping(x -> x.toUpperCase(), Collectors.joining(",")));
    }

--------------------

##     6、分组 ##

public class User {
        private Long id;
        private String username;
        private Integer type;
    
        // Getter & Setter & toString
    }
    
    @Test
    public void testGroupBy(){
        List<Integer> list = Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10);
        // 奇偶数分组：奇数分一组，偶数分一组
        // groupingBy(Function<? super T, ? extends K> classifier) 参数是Function类型，Function返回值可以是要分组的条件，也可以是要分组的字段
        // 返回的结果是Map，其中key的数据类型为Function体中计算类型，value是List<T>类型，为分组的结果
        Map<Boolean, List<Integer>> result = list.stream().collect(Collectors.groupingBy(item -> item % 2 == 0));
        // {false=[1, 3, 5, 7, 9], true=[2, 4, 6, 8, 10]}
        System.out.println(result);
    
    
    	 // partitioningBy 用于分成两组的情况
        Map<Boolean, List<Integer>> twoPartiton = list.stream().collect(Collectors.partitioningBy(item -> item % 2 == 0));
        System.out.println(twoPartiton);
        
        
        User user = new User(1L, "zhangsan", 1);
        User user2 = new User(2L, "lisi", 2);
        User user3 = new User(3L, "wangwu", 3);
        User user4 = new User(4L, "fengliu", 1);
        List<User> users = Arrays.asList(user, user2, user3, user4);
        // 根据某个字段进行分组
        Map<Integer, List<User>> userGroup = users.stream().collect(Collectors.groupingBy(item -> item.type));
    
        /**
         * key 为要分组的字段
         * value 分组的结果
         * {
         *  1=[User{id=1, username='zhangsan', type=1}, User{id=4, username='fengliu', type=1}],
         *  2=[User{id=2, username='lisi', type=2}],
         *  3=[User{id=3, username='wangwu', type=3}]
         * }
         */
        System.out.println(userGroup);
    }

## 7、累计操作 ##

@Test
    public void testReducing(){
    		
        // sum: 是每次累计计算的结果，b是Function的结果
        System.out.println(Stream.of(1, 3, 4).collect(Collectors.reducing(0, x -> x + 1, (sum, b) -> {
            System.out.println(sum + "-" + b);
            return sum + b;
        })));
    
    	
    	 // 下面代码是对reducing函数功能实现的描述，用于理解reducing的功能
        int sum = 0;
        List<Integer> list3 = Arrays.asList(1, 3, 4);
        for (Integer item : list3) {
            int b = item + 1;
            System.out.println(sum + "-" + b);
            sum = sum + b;
        }
        System.out.println(sum);
    
    		
        // 注意reducing可以用于更复杂的累计计算，加减乘除或者更复杂的操作
        // result = 2 * 4 * 5 = 40
        System.out.println(Stream.of(1, 3, 4).collect(Collectors.reducing(1, x -> x + 1, (result, b) -> {
            System.out.println(result + "-" + b);
            return result * b;
        })));
    }

小示例：要求将"a,bb\#b\#c|\#d" \#替换逗号，并加上单引号

@Test
        public void test(){
            String param="a,bb#b#c|#d";
            //第一种写法
            Optional<String> optional=  Stream.of(param.split("#")).reduce((s1, s2)->s1+"','"+s2);
            String value= "'"+optional.get()+"'";
            //第二种写法
            List<String> lis = Arrays.asList(param.split("#"));
            String value2=lis.stream().collect(Collectors.joining("','", "'", "'"));
            
            System.out.println(value);
            System.out.println(value2);
            //结果一致
        }

![20190213154142206.jpg][]

利于jdk1.8新特性 collectors会发现很多东西有了多种解法。都能快速解决某些常用的遍历问题，数据类型转换问题，提供性能，减少冗余代码，唯一的缺点可能就是对不了解的人来说可读性很差。 利用了很多1.8的新的特性，关键字符号等等。

--------------------

# 总结： #

![70][]

[https_blog.csdn.net_vbirdbest_article_details_80216713]: https://blog.csdn.net/vbirdbest/article/details/80216713%C2%A0
[https_blog.csdn.net_qq_39629277_article_details_83108585]: https://blog.csdn.net/qq_39629277/article/details/83108585
[https_blog.csdn.net_kegaofei_article_details_80582356]: https://blog.csdn.net/kegaofei/article/details/80582356
[20190213120909457.jpg]: /images/20220320/4f870c30319346628b84405f3b8b3f79.png
[20190213121644952.jpg]: /images/20220320/27ffb4f6175f4af9ba4676faec33cdd8.png
[20190213141628711.jpg]: /images/20220320/45c0b49bd7c349d596ec7b200865e955.png
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[20190213150520649.jpg]: /images/20220320/8246b5aef94f4e40baab1349aa2b793a.png
[20190213151054886.jpg]: /images/20220320/8d5e03efa3bc47c48c194b66384ba57f.png
[20190213154142206.jpg]: /images/20220320/903a9c8b21d443fab24f1246d0032a63.png
[70]: /images/20220320/ac3b3dcd7c3f4fb9a788d042a745738c.png