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。
Java
Java
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
Stream操作.png
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。
作者：fengzhizi715
链接：https://www.imooc.com/article/23473?block_id=tuijian_wz
来源：慕课网
