JAVA8 Lambda表达式高级集合类&收集器

港控/mmm° 2024-02-19 12:41 27阅读 0赞

lambda表达式提供了很多的集合类和收集器来简化编程，使之更加方便和美观，所以这里介绍一些常用的集合类和收集器来处理繁杂的代码。

github的demo地址: [https://github.com/BradenLei/lambda][https_github.com_BradenLei_lambda]

1、方法引用：形如 User::getName，TreeSet::new 等价于user.getName(), new TreeSet<>();

可以引用静态方法、实例对象方法、构造方法等。

/**
         * 方法引用
         */
        @Test
        public void refTest() {
            List<String> collect = list.stream().map(u -> u.getName()).collect(Collectors.toList());
            System.out.println(collect);
            // 等价于 User::getName
            System.out.println(list.stream().map(User::getName).collect(Collectors.toList()));
    
            // TreeSet::new
            Stream<Integer> integerStream = Stream.of(1, 2, 3);
            TreeSet<Integer> integerTreeSet = integerStream.collect(Collectors.toCollection(TreeSet::new));
            System.out.println(integerTreeSet);
        }

2、流中元素的顺序，如果集合本身无序，则转换的流仍然无序，如HashSet(进入顺序)

3、maxBy(比较器),minBy(..), 找出最大最小值

/**
         * 转换成值
         */
        @Test
        public void biggestGroupTest() {
            Optional<Artist> optional = biggestGroup(artists.stream());
            System.out.println(optional.get().getName());  // -> ace
        }
        public Optional<Artist> biggestGroup(Stream<Artist> artists) {
            Function<Artist, Long> getCount = artist -> artist.getMembers().count();
            return artists.collect(maxBy(comparing(getCount))); // 找出最大成员数目
        }

4、averagingDouble()，求平均值

/**
         * 找出一组专辑上曲目的平均数
         */
        @Test
        public void test() {
            System.out.println(averageNumberOfTracks(albums));
        }
        public double averageNumberOfTracks(List<Album> albums) {
            return albums.stream().collect(averagingDouble(album -> album.getTrackList().size()));
        }

5、数据分块 partitioningBy，根据true, false分成两块数据

/**
         * 数据分块 partitioningBy, true or false值划分, 分成true or false 两部分
         */
        @Test
        public void partitioningByTest() {
            Map<Boolean, List<Artist>> booleanListMap = partitioningByNationality(artists.stream());
            List<Artist> trueList = booleanListMap.get(true);
            System.out.println(trueList);  // out -> each
            List<Artist> falseList = booleanListMap.get(false);
            System.out.println(falseList); // out -> ace
        }
        public Map<Boolean, List<Artist>> partitioningByNationality(Stream<Artist> artists) {
            // 将国籍分成中国和其他两组
            return artists.collect(Collectors.partitioningBy(artist -> "china".equals(artist.getNationality())));
        }

6、数据分组 groupingBy()，可以根据任意值进行分组

/**
         * 数据分组 groupingBy, 任意值划分, 分成true or false 两部分
         */
        @Test
        public void groupingByTest() {
            Map<String, List<Artist>> stringListMap = groupingByNationality(artists.stream());
            List<Artist> list = stringListMap.get("china");  // out -> each
            System.out.println(list);
            List<Artist> list1 = stringListMap.get("US");  // out -> ace
            System.out.println(list1);
    
            Map<Artist, List<Album>> streamListMap = groupingByArtist(albums.stream());
            System.out.println(streamListMap.get(artist1));  // 专辑1, 专辑2
        }
        public Map<String, List<Artist>> groupingByNationality(Stream<Artist> artists) {
            // 按照国籍分组
            return artists.collect(Collectors.groupingBy(artist -> artist.getNationality()));
        }
        /** 按照主唱分组 */
        public Map<Artist, List<Album>> groupingByArtist(Stream<Album> albums) {
            return albums.collect(groupingBy(album -> album.getMainMusician()));
        }

7、字符串拼装 Collectors.joining, 按照指定的格式拼装字符串

/**
         * 输出字符串
         */
        @Test
        public void joiningTest() {
            String joiningName = joiningName(artists.stream());
            System.out.println(joiningName);  // out -> [ace,each]
        }
        public String joiningName(Stream<Artist> artists) {
            return artists.map(Artist::getName).collect(Collectors.joining(",", "[", "]"));
        }

8、单一的收集器已经能实现很多基础的功能，组合方式更显强大

// 组合收集器
    
        /** ===== 下游收集器: 例子中用到的第二个收集器, 用来收集最终结果的一个子集 ===== */
    
        /**
         * 统计某个艺术家专辑数量
         */
        @Test
        public void combineTest() {
            long braden = numberOfAlbums(albums.stream(), "bradenlei");
            System.out.println(braden);
        }
        public long numberOfAlbums(Stream<Album> albums, String artistName) {
            return albums.map(album -> album.getMusicians())
                    .filter(musicians -> musicians.filter(musician -> artistName.equals(musician.getName())).count() > 0)
                    .count();
        }
    
        /**
         * 计算每个艺术家的专辑数(counting)
         */
        @Test
        public void combineTest2() {
            Map<Artist, Long> count = count(albums.stream());
            System.out.println(count.get(artist1)); // out -> 2
        }
        public Map<Artist, Long> count(Stream<Album> albums) {
            return albums.collect(groupingBy(album -> album.getMainMusician(), counting()));
        }
    
        /**
         * 收集每个艺术家的专辑名称 (mapping)
         */
        @Test
        public void combineTest3() {
            Map<Artist, List<String>> artistListMap = collectName(albums.stream());
            System.out.println(artistListMap.get(artist1)); // out -> [专辑1, 专辑2]
        }
        public Map<Artist, List<String>> collectName(Stream<Album> alubms) {
            return alubms.collect(groupingBy(Album::getMainMusician, mapping(Album::getName, toList())));
        }

9、有些时候现有的收集器不能满足我们的开发需求，当然也就可以重构和定制收集器了

/** =====重构和定制收集器===== */
    
        /**
         * 使用除joining之外的方法拼接字符串
         *
         * 获取艺术家姓名 (初步)
         */
        @Test
        public void jointTest() {
            StringBuilder reduced = artists.stream()
                    .map(Artist::getName)
                    .reduce(new StringBuilder(), (builder, name) -> {
                        if (builder.length() > 0) builder.append(", ");
    
                        return builder.append(name);
                    }, (left, right) -> left.append(right));
            reduced.insert(0, "[");
            reduced.append("]");
            String result = reduced.toString();
    
            System.out.println(result); // out -> [ace, each]
        }
        /**
         * 优化(自定义StringCombiner)
         */
        @Test
        public void jointTest2() {
            StringCombiner reduced = artists.stream()
                    .map(Artist::getName)
                    .reduce(new StringCombiner(", ", "[", "]"),
                            StringCombiner::add,
                            StringCombiner::merge); // 这里实际没有调用merge
            String string = reduced.toString();
            System.out.println(string);  // out -> [ace, each]
        }
    
        /***
         * 进一步优化(使用定制收集器StringCollector收集)
         */
         @Test
        public void jointTest3() {
             String collect = artists.stream()
                     .map(Artist::getName)
                     .collect(new StringCollector(", ", "[", "]"));
             System.out.println(collect); // out -> [ace, each]
         }
    
        /**
         * 用reducing代替自定义收集器
         */
        @Test
        public void jointtTest4() {
            // 这种方法的缺点是第二个参数每次都需要new一个对象
            StringCombiner collect = artists.stream()
                    .map(Artist::getName)
                    .collect(Collectors.reducing(
                            new StringCombiner(", ", "[", "]"),
                            name -> new StringCombiner(", ", "[", "]").add(name),
                            StringCombiner::merge));  // 这里会调用merge, 每次都是两个StringCombiner对象合并, 所以修改StringCombiner.add即可得到正确格式[ace, each]
            String s = collect.toString();
            System.out.println(s);  // out -> [ace[each]
        }

自定义类：

public class StringCombiner {
        // 前缀
        private String prefix;
        // 分隔符
        private String delim;
        // 后缀
        private String suffix;
    
        private StringBuilder builder;
    
        public StringCombiner(String delim, String prefix, String suffix) {
            this.delim = delim;
            this.prefix = prefix;
            this.suffix = suffix;
            builder = new StringBuilder();
        }
    
        public StringCombiner add(String element) {
            if(builder.length() > 0) {
                builder.append(delim);
            } else {
                builder.insert(0, prefix);
            }
                builder.append(element);
            return this;
        }
    
        public StringCombiner merge(StringCombiner combiner) {
            builder.append(combiner.builder);
            return this;
        }
    
        @Override
        public String toString() {
            if ("".equals(suffix)) {
                return builder.toString();
            } else {
                int tmp = builder.length();
                String result = builder.append(suffix).toString();
                builder.setLength(tmp);
                return result;
            }
        }
    }

其中：StringCombiner功能仿制java提供的StringJoiner

/**
     * String 待收集的元素字符串
     * StringCombiner 累加器类型
     * String 最终结果类型
     */
    public class StringCollector implements Collector<String, StringCombiner, String> {
        // 前缀
        private String prefix;
        // 分隔符
        private String delim;
        // 后缀
        private String suffix;
    
        public StringCollector(String delim, String prefix, String suffix) {
            this.delim = delim;
            this.prefix = prefix;
            this.suffix = suffix;
        }
        @Override
        public Supplier<StringCombiner> supplier() {
            return () -> new StringCombiner(delim, prefix, suffix);
        }
    
        @Override
        public BiConsumer<StringCombiner, String> accumulator() {
            return StringCombiner::add;
        }
    
        @Override
        public BinaryOperator<StringCombiner> combiner() {
            return StringCombiner::merge;
        }
    
        @Override
        public Function<StringCombiner, String> finisher() {
            return StringCombiner::toString;
        }
    
        @Override
        public Set<Characteristics> characteristics() {
            Set<Collector.Characteristics> emptySet = Collections.emptySet();
            return emptySet;
        }
    }

10、完整demo:

public class CollectorDemo {
        private static User user;
        private static List<User> list;
    
        @Before
        public void init() {
            if (user == null) {
                user = new User("张三", 11001);
            }
            if (list == null) {
                list = new ArrayList<>();
                list.add(new User("李四", 11002));
                list.add(new User("王五", 11003));
                list.add(new User("周庄", 11004));
            }
        }
    
        /**
         * 方法引用
         */
        @Test
        public void refTest() {
            List<String> collect = list.stream().map(u -> u.getName()).collect(Collectors.toList());
            System.out.println(collect);
            // 等价于 User::getName
            System.out.println(list.stream().map(User::getName).collect(Collectors.toList()));
    
            // TreeSet::new
            Stream<Integer> integerStream = Stream.of(1, 2, 3);
            TreeSet<Integer> integerTreeSet = integerStream.collect(Collectors.toCollection(TreeSet::new));
            System.out.println(integerTreeSet);
        }
    
        /**
         *  元素顺序, 如果集合本身无序, 则转换的流也是无序的如, HashSet
         */
        @Test
        public void orderTest() {
            // 进来的流无序， 则出去的流也是无序
            List<Integer> origin = Arrays.asList(1, 2, 3, 4);
            List<Integer> integerList = origin.stream().collect(Collectors.toList());
            assertEquals(origin, integerList); // ok
    
            Set<Integer> set = new HashSet<>(Arrays.asList(1, 2, 3, 4, 5, 6, 7, 8, 9, 10));
            Set<Integer> integerSet = set.stream().collect(Collectors.toSet());
            assertEquals(set, integerSet);  // 不能保证每次通过
    
            List<Integer> list = Arrays.asList(2, 1, 4, 3);
            List<Integer> collect = list.stream().sorted().collect(Collectors.toList());
            assertEquals(origin, collect); // ok
        }
    
        private static List<Artist> artists = null;
        private static List<Artist> artists2 = null;
        private static List<Track> tracks = null;
        private static List<Track> tracks2 = null;
        private static List<Album> albums = null;
        private static Artist artist1 = null;
        private static Artist artist2 = null;
        static {
            Artist artist = new Artist("braden", "china");
            artist1 = new Artist("ace", Arrays.asList(artist), "US");
            artist2 = new Artist("each", "china");
            artists = Arrays.asList(artist1, artist2);
    
            tracks = Arrays.asList(new Track("love you", 3), new Track("Shape of you", 5), new Track("夜上海", 4));
            tracks2 = Arrays.asList(new Track("喜欢你", 4), new Track("不分手的恋爱", 5));
    
            artists2 = Arrays.asList(new Artist("bradenlei","china"), new Artist("Mr.zhang","UK"));
            albums = Arrays.asList(new Album("专辑1", tracks, artists), new Album("专辑2", tracks2, artists), new Album("专辑3", tracks, artists2));
        }
        /**
         * 转换成值
         */
        @Test
        public void biggestGroupTest() {
            Optional<Artist> optional = biggestGroup(artists.stream());
            System.out.println(optional.get().getName());  // -> ace
        }
        public Optional<Artist> biggestGroup(Stream<Artist> artists) {
            Function<Artist, Long> getCount = artist -> artist.getMembers().count();
            return artists.collect(maxBy(comparing(getCount))); // 找出最大成员数目
        }
    
        /**
         * 找出一组专辑上曲目的平均数
         */
        @Test
        public void test() {
            System.out.println(averageNumberOfTracks(albums));
        }
        public double averageNumberOfTracks(List<Album> albums) {
            return albums.stream().collect(averagingDouble(album -> album.getTrackList().size()));
        }
    
        /**
         * 数据分块 partitioningBy, true or false值划分, 分成true or false 两部分
         */
        @Test
        public void partitioningByTest() {
            Map<Boolean, List<Artist>> booleanListMap = partitioningByNationality(artists.stream());
            List<Artist> trueList = booleanListMap.get(true);
            System.out.println(trueList);  // out -> each
            List<Artist> falseList = booleanListMap.get(false);
            System.out.println(falseList); // out -> ace
        }
        public Map<Boolean, List<Artist>> partitioningByNationality(Stream<Artist> artists) {
            // 将国籍分成中国和其他两组
            return artists.collect(Collectors.partitioningBy(artist -> "china".equals(artist.getNationality())));
        }
    
        /**
         * 数据分组 groupingBy, 任意值划分, 分成true or false 两部分
         */
        @Test
        public void groupingByTest() {
            Map<String, List<Artist>> stringListMap = groupingByNationality(artists.stream());
            List<Artist> list = stringListMap.get("china");  // out -> each
            System.out.println(list);
            List<Artist> list1 = stringListMap.get("US");  // out -> ace
            System.out.println(list1);
    
            Map<Artist, List<Album>> streamListMap = groupingByArtist(albums.stream());
            System.out.println(streamListMap.get(artist1));  // 专辑1, 专辑2
        }
        public Map<String, List<Artist>> groupingByNationality(Stream<Artist> artists) {
            // 按照国籍分组
            return artists.collect(Collectors.groupingBy(artist -> artist.getNationality()));
        }
        /** 按照主唱分组 */
        public Map<Artist, List<Album>> groupingByArtist(Stream<Album> albums) {
            return albums.collect(groupingBy(album -> album.getMainMusician()));
        }
    
        /**
         * 输出字符串
         */
        @Test
        public void joiningTest() {
            String joiningName = joiningName(artists.stream());
            System.out.println(joiningName);  // out -> [ace,each]
        }
        public String joiningName(Stream<Artist> artists) {
            return artists.map(Artist::getName).collect(Collectors.joining(",", "[", "]"));
        }
    
        // 组合收集器
    
        /** ===== 下游收集器: 例子中用到的第二个收集器, 用来收集最终结果的一个子集 ===== */
    
        /**
         * 统计某个艺术家专辑数量
         */
        @Test
        public void combineTest() {
            long braden = numberOfAlbums(albums.stream(), "bradenlei");
            System.out.println(braden);
        }
        public long numberOfAlbums(Stream<Album> albums, String artistName) {
            return albums.map(album -> album.getMusicians())
                    .filter(musicians -> musicians.filter(musician -> artistName.equals(musician.getName())).count() > 0)
                    .count();
        }
    
        /**
         * 计算每个艺术家的专辑数(counting)
         */
        @Test
        public void combineTest2() {
            Map<Artist, Long> count = count(albums.stream());
            System.out.println(count.get(artist1)); // out -> 2
        }
        public Map<Artist, Long> count(Stream<Album> albums) {
            return albums.collect(groupingBy(album -> album.getMainMusician(), counting()));
        }
    
        /**
         * 收集每个艺术家的专辑名称 (mapping)
         */
        @Test
        public void combineTest3() {
            Map<Artist, List<String>> artistListMap = collectName(albums.stream());
            System.out.println(artistListMap.get(artist1)); // out -> [专辑1, 专辑2]
        }
        public Map<Artist, List<String>> collectName(Stream<Album> alubms) {
            return alubms.collect(groupingBy(Album::getMainMusician, mapping(Album::getName, toList())));
        }
    
        /** =====重构和定制收集器===== */
    
        /**
         * 使用除joining之外的方法拼接字符串
         *
         * 获取艺术家姓名 (初步)
         */
        @Test
        public void jointTest() {
            StringBuilder reduced = artists.stream()
                    .map(Artist::getName)
                    .reduce(new StringBuilder(), (builder, name) -> {
                        if (builder.length() > 0) builder.append(", ");
    
                        return builder.append(name);
                    }, (left, right) -> left.append(right));
            reduced.insert(0, "[");
            reduced.append("]");
            String result = reduced.toString();
    
            System.out.println(result); // out -> [ace, each]
        }
        /**
         * 优化(自定义StringCombiner)
         */
        @Test
        public void jointTest2() {
            StringCombiner reduced = artists.stream()
                    .map(Artist::getName)
                    .reduce(new StringCombiner(", ", "[", "]"),
                            StringCombiner::add,
                            StringCombiner::merge); // 这里实际没有调用merge
            String string = reduced.toString();
            System.out.println(string);  // out -> [ace, each]
        }
    
        /***
         * 进一步优化(使用定制收集器StringCollector收集)
         */
         @Test
        public void jointTest3() {
             String collect = artists.stream()
                     .map(Artist::getName)
                     .collect(new StringCollector(", ", "[", "]"));
             System.out.println(collect); // out -> [ace, each]
         }
    
        /**
         * 用reducing代替自定义收集器
         */
        @Test
        public void jointtTest4() {
            // 这种方法的缺点是第二个参数每次都需要new一个对象
            StringCombiner collect = artists.stream()
                    .map(Artist::getName)
                    .collect(Collectors.reducing(
                            new StringCombiner(", ", "[", "]"),
                            name -> new StringCombiner(", ", "[", "]").add(name),
                            StringCombiner::merge));  // 这里会调用merge, 每次都是两个StringCombiner对象合并, 所以修改StringCombiner.add即可得到正确格式[ace, each]
            String s = collect.toString();
            System.out.println(s);  // out -> [ace[each]
        }
    }

[https_github.com_BradenLei_lambda]: https://github.com/BradenLei/lambda