JDK1.8新特性(四): Stream流

一：简介

java.util.Stream 表示能应用在一组元素上一次执行的操作序列。Stream 操作分为中间操作或者最终操作两种，最终操作返回一特定类型的计算结果，而中间操作返回Stream本身，这样就可以将多个操作依次串起来。Stream 的创建需要指定一个数据源，比如 java.util.Collection的子类，List或者Set， Map不支持。Stream的操作可以串行stream()执行或者并行parallelStream()执行。

二：Stream

package java.util.stream;

public interface Stream<T> extends BaseStream<T, Stream<T>> {
    Stream<T> filter(Predicate<? super T> predicate);

    IntStream mapToInt(ToIntFunction<? super T> mapper);

    LongStream mapToLong(ToLongFunction<? super T> mapper);

    DoubleStream mapToDouble(ToDoubleFunction<? super T> mapper);

    <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper);

    IntStream flatMapToInt(Function<? super T, ? extends IntStream> mapper);

    LongStream flatMapToLong(Function<? super T, ? extends LongStream> mapper);

    DoubleStream flatMapToDouble(Function<? super T, ? extends DoubleStream> mapper);

    Stream<T> distinct();

    Stream<T> sorted();

    Stream<T> sorted(Comparator<? super T> comparator);

    Stream<T> peek(Consumer<? super T> action);

    Stream<T> limit(long maxSize);

    Stream<T> skip(long n);

    void forEach(Consumer<? super T> action);

    void forEachOrdered(Consumer<? super T> action);

    Object[] toArray();

    <A> A[] toArray(IntFunction<A[]> generator);

    T reduce(T identity, BinaryOperator<T> accumulator);

    Optional<T> reduce(BinaryOperator<T> accumulator);

    <U> U reduce(U identity, BiFunction<U, ? super T, U> accumulator,
                 BinaryOperator<U> combiner);

    <R> R collect(Supplier<R> supplier, BiConsumer<R, ? super T> accumulator,
                  BiConsumer<R, R> combiner);

    <R, A> R collect(Collector<? super T, A, R> collector);

    Optional<T> min(Comparator<? super T> comparator);

    Optional<T> max(Comparator<? super T> comparator);

    long count();

    boolean anyMatch(Predicate<? super T> predicate);

    boolean allMatch(Predicate<? super T> predicate);

    boolean noneMatch(Predicate<? super T> predicate);

    Optional<T> findFirst();

    Optional<T> findAny();

    public static<T> Builder<T> builder() {
        return new Streams.StreamBuilderImpl<>();
    }

    public static<T> Stream<T> empty() {
        return StreamSupport.stream(Spliterators.<T>emptySpliterator(), false);
    }

    public static<T> Stream<T> of(T t) {
        return StreamSupport.stream(new Streams.StreamBuilderImpl<>(t), false);
    }


    @SafeVarargs
    @SuppressWarnings("varargs") // Creating a stream from an array is safe
    public static<T> Stream<T> of(T... values) {
        return Arrays.stream(values);
    }

    public static<T> Stream<T> iterate(final T seed, final UnaryOperator<T> f) {
        Objects.requireNonNull(f);
        final Iterator<T> iterator = new Iterator<T>() {
            @SuppressWarnings("unchecked")
            T t = (T) Streams.NONE;

            @Override
            public boolean hasNext() {
                return true;
            }

            @Override
            public T next() {
                return t = (t == Streams.NONE) ? seed : f.apply(t);
            }
        };
        return StreamSupport.stream(Spliterators.spliteratorUnknownSize(
                iterator,
                Spliterator.ORDERED | Spliterator.IMMUTABLE), false);
    }


    public static<T> Stream<T> generate(Supplier<T> s) {
        Objects.requireNonNull(s);
        return StreamSupport.stream(
                new StreamSpliterators.InfiniteSupplyingSpliterator.OfRef<>(Long.MAX_VALUE, s), false);
    }


    public static <T> Stream<T> concat(Stream<? extends T> a, Stream<? extends T> b) {
        Objects.requireNonNull(a);
        Objects.requireNonNull(b);

        @SuppressWarnings("unchecked")
        Spliterator<T> split = new Streams.ConcatSpliterator.OfRef<>(
                (Spliterator<T>) a.spliterator(), (Spliterator<T>) b.spliterator());
        Stream<T> stream = StreamSupport.stream(split, a.isParallel() || b.isParallel());
        return stream.onClose(Streams.composedClose(a, b));
    }


    public interface Builder<T> extends Consumer<T> {
        @Override
        void accept(T t);

        default Builder<T> add(T t) {
            accept(t);
            return this;
        }

        Stream<T> build();
    }
}

三：示例

1. forEach 循环

@Test
public void forEach(){
    // 你不鸟我,我也不鸟你
    List<String> list = Arrays.asList("you", "don't", "bird", "me", ",", 
                                       "I", "don't", "bird", "you");

    // 方式一：JDK1.8之前的循环方式
    for (String item: list) {
        System.out.println(item);
    }

    // 方式二：使用Stream的forEach方法
    // void forEach(Consumer<? super T> action)
    list.stream().forEach(item -> System.out.println(item));

    // 方式三：方式二的简化方式
    // 由于方法引用也属于函数式接口，所以方式二Lambda表达式也可以使用方法引用来代替
    // 此种方式就是方式一、方式二的简写形式
    list.stream().forEach(System.out::println);
}

2. filter 过滤

public class User {
    private Long id;
    private String phone;
    private Integer age;

    public User(){}
    public User(Long id, String username, Integer age) {
        this.id = id;
        this.username = username;
        this.age = age;
    }
    // Getter & Setter & toString
}


@Test
public void filter(){
    List<User> users = Arrays.asList(
            new User(1L, "mengday", 28),
            new User(2L, "guoguo", 18),
            new User(3L, "liangliang", 17)
    );

    // Stream<T> filter(Predicate<? super T> predicate);
    users.stream().filter(user -> user.getAge() > 18).forEach(System.out::println);
}

3. map 映射

@Test
public void map(){
    List<String> list = Arrays.asList("how", "are", "you", "how", "old", "are", "you", "?");
    // <R> Stream<R> map(Function<? super T, ? extends R> mapper);
    list.stream().map(item -> item.toUpperCase()).forEach(System.out::println);
}

4. flatMap

@Test
public void flatMap(){
    List<Integer> a = Arrays.asList(1, 2, 3);
    List<Integer> b = Arrays.asList(4, 5, 6);

    // <R> Stream<R> flatMap(Function<? super T, ? extends Stream<? extends R>> mapper)
    List<List<Integer>> collect = Stream.of(a, b).collect(Collectors.toList());
    // [[1, 2, 3], [4, 5, 6]] 
    System.out.println(collect);

    // 将多个集合中的元素合并成一个集合
    List<Integer> mergeList = Stream.of(a, b).flatMap(list -> list.stream()).collect(Collectors.toList());
    // [1, 2, 3, 4, 5, 6]
    System.out.println(mergeList);

    // 通过Builder模式来构建
    Stream<Object> stream = Stream.builder().add("hello").add("hi").add("byebye").build();
}

5. sorted 排序

@Test
public void sort(){
    List<String> list = Arrays.asList("c", "e", "a", "d", "b");
    // Stream<T> sorted(Comparator<? super T> comparator);
    // int compare(T o1, T o2);
    list.stream().sorted((s1, s2) -> s1.compareTo(s2)).forEach(System.out::println);
}

6. distinct 去重复

@Test
public void distinct(){
    // 知之为知之,不知为不知
    Stream<String> stream = Stream.of("know", "is", "know", "noknow", "is", "noknow");
    stream.distinct().forEach(System.out::println); // know is noknow
}

7. count 总数量

@Test
public void count(){
    Stream<String> stream = Stream.of("know", "is", "know", "noknow", "is", "noknow");
    long count = stream.count();
    System.out.println(count);
}

8. min、max

@Test
public void min(){
    List<String> list = Arrays.asList("1", "2", "3", "4", "5");
    // Optional<T> min(Comparator<? super T> comparator);
    Optional<String> optional = list.stream().min((a, b) -> a.compareTo(b));
    String value = optional.get();
    System.out.println(value);
}

9. skip、limit

@Test
public void skip(){
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    // Stream<T> skip(long n)
    list.stream().skip(2).forEach(System.out::println);  // c、d、e
}

@Test
public void limit(){
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    list.stream().skip(2).limit(2).forEach(System.out::println);    // c、d
}

10. collect

@Test
public void collect(){
    List<String> list = Arrays.asList("a", "b", "c", "d", "e");
    // Stream -> Collection
    List<String> collect = list.stream().collect(Collectors.toList());

    // Stream -> Object[]
    Object[] objects = list.stream().toArray();
}

11. concat

@Test
public void concat(){
    List<String> list = Arrays.asList("a", "b");
    List<String> list2 = Arrays.asList("c", "d");
    Stream<String> concatStream = Stream.concat(list.stream(), list2.stream());
    concatStream.forEach(System.out::println);
}

12. anyMatch、allMatch

@Test
public void match(){
    // 你给我站住
    List<String> list = Arrays.asList("you", "give", "me", "stop");
    // boolean anyMatch(Predicate<? super T> predicate);
    // parallelStream可以并行计算，速度比stream更快
    boolean result = list.parallelStream().anyMatch(item -> item.equals("me"));
    System.out.println(result);
}

13. reduce 归纳

@Test
public void reduce(){
    Stream<String> stream = Stream.of("you", "give", "me", "stop");
    // Optional<T> reduce(BinaryOperator<T> accumulator);
    Optional<String> optional = stream.reduce((before, after) -> before + "," + after);
    optional.ifPresent(System.out::println);    // you,give,me,stop
}

14. findFirst、findAny

@Test
public void findFirst(){
    Stream<String> stream = Stream.of("you", "give", "me", "stop");
    String value = stream.findFirst().get();
    System.out.println(value);
}

@Test
public void findAny(){
    Stream<String> stream = Stream.of("you", "give", "me", "stop");
    String value2 = stream.findAny().get();
    System.out.println(value2);
}

---

Builder 构建模式：https://blog.csdn.net/lms1719/article/details/70741691

---

public class User {
    private Long id;
    private String phone;
    private String username;
    private String nickname;
    private Integer age;

    // 提供一个接收当前对象的构造函数
    private User(Builder builder) {
        this.id = builder.id;
        this.phone = builder.phone;
        this.username = builder.username;
        this.nickname = builder.nickname;
        this.age = builder.age;
    }

    // Getter & Setter
    public Long getId() {
        return id;
    }
    public void setId(Long id) {
        this.id = id;
    }

    public String getPhone() {
        return phone;
    }
    public void setPhone(String phone) {
        this.phone = phone;
    }

    public String getUsername() {
        return username;
    }
    public void setUsername(String username) {
        this.username = username;
    }

    public String getNickname() {
        return nickname;
    }
    public void setNickname(String nickname) {
        this.nickname = nickname;
    }

    public Integer getAge() {
        return age;
    }
    public void setAge(Integer age) {
        this.age = age;
    }

    // 静态内部类 Builder
    // User的创建完全依靠User.Builder，使用一种方法链的方式来创建
    public static class Builder {
        private Long id;
        private String phone;
        private String username;
        private String nickname;
        private Integer age;

        // Setter方法，方法名为属性名，方法返回Builder对象
        public Builder id(Long id) {
            this.id = id;
            return this;
        }

        public Builder phone(String phone) {
            this.phone = phone;
            return this;
        }

        public Builder username(String username) {
            this.username = username;
            return this;
        }

        public Builder nickname(String nickname) {
            this.nickname = nickname;
            return this;
        }

        public Builder age(Integer age) {
            this.age = age;
            return this;
        }

        // build 方法
        public User build(){
            return new User(this);
        }
    }
}

public class Order {
    private Long id;
    private String orderNo;
    private Date createTime;
    private BigDecimal amout;

    Order(Long id, String orderNo, Date createTime, BigDecimal amout) {
        this.id = id;
        this.orderNo = orderNo;
        this.createTime = createTime;
        this.amout = amout;
    }

    public Long getId() {
        return id;
    }

    public String getOrderNo() {
        return orderNo;
    }

    public Date getCreateTime() {
        return createTime;
    }

    public BigDecimal getAmout() {
        return amout;
    }

    public static Order.OrderBuilder builder() {
        return new Order.OrderBuilder();
    }

    public static class OrderBuilder {
        private Long id;
        private String orderNo;
        private Date createTime;
        private BigDecimal amout;

        OrderBuilder() {
        }

        public Order.OrderBuilder id(Long id) {
            this.id = id;
            return this;
        }

        public Order.OrderBuilder orderNo(String orderNo) {
            this.orderNo = orderNo;
            return this;
        }

        public Order.OrderBuilder createTime(Date createTime) {
            this.createTime = createTime;
            return this;
        }

        public Order.OrderBuilder amout(BigDecimal amout) {
            this.amout = amout;
            return this;
        }

        public Order build() {
            return new Order(this.id, this.orderNo, this.createTime, this.amout);
        }

        public String toString() {
            return "Order.OrderBuilder(id=" + this.id + ", orderNo=" + this.orderNo + ", createTime=" + this.createTime + ", amout=" + this.amout + ")";
        }
    }
}

public class Main {
    public static void main(String[] args) {
        User user = new User.Builder().id(1L).phone("666").nickname("mengday").age(18).build();

        Order order = Order.builder().id(1L).orderNo("1111").build();
        order = new Order.OrderBuilder().id(1L).orderNo("1111").build();
    }
}