一. Stream的特性

Stream是Java 8新增的接口，Stream可以认为是一个高级版本的 Iterator。它代表着数据流，流中的数据元素的数量可以是有限的，也可以是无限的。

Stream跟Iterator的差别是

无存储：Stream是基于数据源的对象，它本身不存储数据元素，而是通过管道将数据源的元素传递给操作。
函数式编程：对Stream的任何修改都不会修改背后的数据源，比如对Stream执行filter操作并不会删除被过滤的元素，而是会产生一个不包含被过滤元素的新的Stream。
延迟执行：Stream的操作由零个或多个中间操作（intermediate operation）和一个结束操作（terminal operation）两部分组成。只有执行了结束操作，Stream定义的中间操作才会依次执行，这就是Stream的延迟特性。
可消费性：Stream只能被“消费”一次，一旦遍历过就会失效。就像容器的迭代器那样，想要再次遍历必须重新生成一个新的Stream。

二. Java 8新增的函数式接口

Stream的操作是建立在函数式接口的组合之上的。Java8中新增的函数式接口都在java.util.function包下。这些函数式接口可以有多种分类方式。

2.1 Function

Function是从T到R的一元映射函数。将参数T传递给一个函数，返回R。即R = Function(T)

@FunctionalInterface
public interface Function<T, R> {

    /**
     * Applies this function to the given argument.
     *
     * @param t the function argument
     * @return the function result
     */
    R apply(T t);

    /**
     * Returns a composed function that first applies the {@code before}
     * function to its input, and then applies this function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of input to the {@code before} function, and to the
     *           composed function
     * @param before the function to apply before this function is applied
     * @return a composed function that first applies the {@code before}
     * function and then applies this function
     * @throws NullPointerException if before is null
     *
     * @see #andThen(Function)
     */
    default <V> Function<V, R> compose(Function<? super V, ? extends T> before) {
        Objects.requireNonNull(before);
        return (V v) -> apply(before.apply(v));
    }

    /**
     * Returns a composed function that first applies this function to
     * its input, and then applies the {@code after} function to the result.
     * If evaluation of either function throws an exception, it is relayed to
     * the caller of the composed function.
     *
     * @param <V> the type of output of the {@code after} function, and of the
     *           composed function
     * @param after the function to apply after this function is applied
     * @return a composed function that first applies this function and then
     * applies the {@code after} function
     * @throws NullPointerException if after is null
     *
     * @see #compose(Function)
     */
    default <V> Function<T, V> andThen(Function<? super R, ? extends V> after) {
        Objects.requireNonNull(after);
        return (T t) -> after.apply(apply(t));
    }

    /**
     * Returns a function that always returns its input argument.
     *
     * @param <T> the type of the input and output objects to the function
     * @return a function that always returns its input argument
     */
    static <T> Function<T, T> identity() {
        return t -> t;
    }
}

Function默认实现了3个default方法，分别是compose、andThen和identity。

方法名	对应函数	描述
compose	V=Function(ParamFunction(T))	它体现了嵌套关系
andThen	V= ParamFunction(Function(T))	转换了嵌套的顺序
identity	Function(T)=T	传递自身的函数调用

compose和andThen对于两个函数f和g来说，f.compose(g)等价于g.andThen(f)。

2.2 Predicate

Predicate是一个谓词函数，主要作为一个谓词演算推导真假值存在，返回布尔值的函数。Predicate等价于一个Function的boolean型返回值的子集。

@FunctionalInterface
public interface Predicate<T> {

    /**
     * Evaluates this predicate on the given argument.
     *
     * @param t the input argument
     * @return {@code true} if the input argument matches the predicate,
     * otherwise {@code false}
     */
    boolean test(T t);

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * AND of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code false}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ANDed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * AND of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> and(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) && other.test(t);
    }

    /**
     * Returns a predicate that represents the logical negation of this
     * predicate.
     *
     * @return a predicate that represents the logical negation of this
     * predicate
     */
    default Predicate<T> negate() {
        return (t) -> !test(t);
    }

    /**
     * Returns a composed predicate that represents a short-circuiting logical
     * OR of this predicate and another.  When evaluating the composed
     * predicate, if this predicate is {@code true}, then the {@code other}
     * predicate is not evaluated.
     *
     * <p>Any exceptions thrown during evaluation of either predicate are relayed
     * to the caller; if evaluation of this predicate throws an exception, the
     * {@code other} predicate will not be evaluated.
     *
     * @param other a predicate that will be logically-ORed with this
     *              predicate
     * @return a composed predicate that represents the short-circuiting logical
     * OR of this predicate and the {@code other} predicate
     * @throws NullPointerException if other is null
     */
    default Predicate<T> or(Predicate<? super T> other) {
        Objects.requireNonNull(other);
        return (t) -> test(t) || other.test(t);
    }

    /**
     * Returns a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}.
     *
     * @param <T> the type of arguments to the predicate
     * @param targetRef the object reference with which to compare for equality,
     *               which may be {@code null}
     * @return a predicate that tests if two arguments are equal according
     * to {@link Objects#equals(Object, Object)}
     */
    static <T> Predicate<T> isEqual(Object targetRef) {
        return (null == targetRef)
                ? Objects::isNull
                : object -> targetRef.equals(object);
    }
}

Predicate的默认方法是and、negate、or。

2.3 Consumer

Consumer是从T到void的一元函数，接受一个入参但不返回任何结果的操作。

@FunctionalInterface
public interface Consumer<T> {

    /**
     * Performs this operation on the given argument.
     *
     * @param t the input argument
     */
    void accept(T t);

    /**
     * Returns a composed {@code Consumer} that performs, in sequence, this
     * operation followed by the {@code after} operation. If performing either
     * operation throws an exception, it is relayed to the caller of the
     * composed operation.  If performing this operation throws an exception,
     * the {@code after} operation will not be performed.
     *
     * @param after the operation to perform after this operation
     * @return a composed {@code Consumer} that performs in sequence this
     * operation followed by the {@code after} operation
     * @throws NullPointerException if {@code after} is null
     */
    default Consumer<T> andThen(Consumer<? super T> after) {
        Objects.requireNonNull(after);
        return (T t) -> { accept(t); after.accept(t); };
    }
}

Consumer的默认方法是andThen。

2.4 Supplier

Supplier是表示结果的供应者。

@FunctionalInterface
public interface Supplier<T> {

    /**
     * Gets a result.
     *
     * @return a result
     */
    T get();
}

Supplier的用法：

        Supplier<String> supplier = new Supplier<String>() {
            @Override
            public String get() {
                return "hello suppiler";
            }
        };
        System.out.println(supplier.get());

或者：

Supplier<User> userSupplier = User::new;
userSupplier.get();   // new User

Java 8新增了CompletableFuture，它的很多方法的入参都用到了Supplier。

三. Stream用法

3.1 Stream的创建

Java 8有多种方式来创建Stream：

通过集合的stream()方法或者parallelStream()
使用流的静态方法，比如Stream.of(Object[]), IntStream.range(int, int) 或者 Stream.iterate(Object, UnaryOperator)。
通过Arrays.stream(Object[])方法。
BufferedReader.lines()从文件中获得行的流。
Files类的操作路径的方法，如list、find、walk等。
随机数流Random.ints()。
其它一些类提供了创建流的方法，如BitSet.stream(), Pattern.splitAsStream(java.lang.CharSequence), 和 JarFile.stream()。

其实最终都是依赖底层的StreamSupport类来完成Stream创建。

3.2 中间操作

中间操作又可以分为无状态的(Stateless)和有状态的(Stateful)，无状态中间操作是指元素的处理不受前面元素的影响，而有状态的中间操作必须等到所有元素处理之后才知道最终结果。

Stream的中间操作只是一种标记，只有执行了结束操作才会触发实际计算。熟悉RxJava、Scala的同学可以看到，Stream中间操作的各个方法在RxJava、Scala中都可以找到熟悉的身影。

3.3 结束操作

3.3.1 短路操作

短路操作是指不用处理全部元素就可以返回结果。短路操作必须一个元素处理一次。

3.3.1 非短路操作

非短路操作可以批量处理数据，但是需要处理完全部元素才会返回结果。

四. 并行流

在创建Stream时，默认是创建串行流。但是可以使用parallelStream()来创建并行流或者parallel()将串行流转换成并行流。并行流也可以通过sequential()转换成串行流。

Java 8 Stream的并行流，本质上还是使用Fork/Join模型。

五. 总结

在Java开发中，如果使用了Java 8，那么强烈建议使用Stream。因为Stream的每个操作都可以依赖Lambda表达式，它是一种声明式的数据处理方式，并且Stream提高了数据处理效率和开发效率。