一. Stream的特性
Stream是Java 8新增的接口，Stream可以認為是一個高級版本的 Iterator走诞。它代表著數(shù)據(jù)流僻孝，流中的數(shù)據(jù)元素的數(shù)量可以是有限的，也可以是無限的堡称。
Stream跟Iterator的差別是
無存儲：Stream是基于數(shù)據(jù)源的對象，它本身不存儲數(shù)據(jù)元素艺演，而是通過管道將數(shù)據(jù)源的元素傳遞給操作却紧。
函數(shù)式編程：對Stream的任何修改都不會修改背后的數(shù)據(jù)源，比如對Stream執(zhí)行filter操作并不會刪除被過濾的元素胎撤，而是會產(chǎn)生一個不包含被過濾元素的新的Stream晓殊。
延遲執(zhí)行：Stream的操作由零個或多個中間操作（intermediate operation）和一個結(jié)束操作（terminal operation）兩部分組成。只有執(zhí)行了結(jié)束操作伤提，Stream定義的中間操作才會依次執(zhí)行巫俺，這就是Stream的延遲特性。
可消費性：Stream只能被“消費”一次肿男，一旦遍歷過就會失效介汹。就像容器的迭代器那樣，想要再次遍歷必須重新生成一個新的Stream舶沛。
二. Java 8新增的函數(shù)式接口
Stream的操作是建立在函數(shù)式接口的組合之上的嘹承。Java8中新增的函數(shù)式接口都在java.util.function包下。這些函數(shù)式接口可以有多種分類方式如庭。
Java
Java
2.1 Function
Function是從T到R的一元映射函數(shù)叹卷。將參數(shù)T傳遞給一個函數(shù)，返回R坪它。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默認實現(xiàn)了3個default方法骤竹，分別是compose、andThen和identity往毡。
方法名
對應(yīng)函數(shù)
描述
compose V=Function(ParamFunction(T)) 它體現(xiàn)了嵌套關(guān)系
andThen V= ParamFunction(Function(T)) 轉(zhuǎn)換了嵌套的順序
identity Function(T)=T 傳遞自身的函數(shù)調(diào)用
compose和andThen對于兩個函數(shù)f和g來說蒙揣，f.compose(g)等價于g.andThen(f)。
2.2 Predicate
Predicate是一個謂詞函數(shù)卖擅，主要作為一個謂詞演算推導(dǎo)真假值存在鸣奔，返回布爾值的函數(shù)墨技。Predicate等價于一個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的默認方法是and挎狸、negate扣汪、or。
2.3 Consumer
Consumer是從T到void的一元函數(shù)锨匆，接受一個入?yún)⒌环祷厝魏谓Y(jié)果的操作崭别。
@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的默認方法是andThen。
2.4 Supplier
Supplier是表示結(jié)果的供應(yīng)者恐锣。
@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茅主，它的很多方法的入?yún)⒍加玫搅薙upplier。
三. Stream用法
Stream操作.png
3.1 Stream的創(chuàng)建
Java 8有多種方式來創(chuàng)建Stream：
通過集合的stream()方法或者parallelStream()
使用流的靜態(tài)方法土榴，比如Stream.of(Object[]), IntStream.range(int, int) 或者 Stream.iterate(Object, UnaryOperator)诀姚。
通過Arrays.stream(Object[])方法。
BufferedReader.lines()從文件中獲得行的流玷禽。
Files類的操作路徑的方法赫段，如list、find矢赁、walk等糯笙。
隨機數(shù)流Random.ints()。
其它一些類提供了創(chuàng)建流的方法撩银，如BitSet.stream(), Pattern.splitAsStream(java.lang.CharSequence), 和 JarFile.stream()给涕。
其實最終都是依賴底層的StreamSupport類來完成Stream創(chuàng)建。
3.2 中間操作
中間操作又可以分為無狀態(tài)的(Stateless)和有狀態(tài)的(Stateful)额获，無狀態(tài)中間操作是指元素的處理不受前面元素的影響够庙，而有狀態(tài)的中間操作必須等到所有元素處理之后才知道最終結(jié)果。
Stream的中間操作只是一種標記咪啡，只有執(zhí)行了結(jié)束操作才會觸發(fā)實際計算首启。
熟悉RxJava暮屡、Scala的同學可以看到撤摸，Stream中間操作的各個方法在RxJava、Scala中都可以找到熟悉的身影褒纲。
3.3 結(jié)束操作
3.3.1 短路操作
短路操作是指不用處理全部元素就可以返回結(jié)果准夷。短路操作必須一個元素處理一次。
3.3.1 非短路操作
非短路操作可以批量處理數(shù)據(jù)莺掠，但是需要處理完全部元素才會返回結(jié)果衫嵌。
四. 并行流
在創(chuàng)建Stream時，默認是創(chuàng)建串行流彻秆。但是可以使用parallelStream()來創(chuàng)建并行流或者parallel()將串行流轉(zhuǎn)換成并行流楔绞。并行流也可以通過sequential()轉(zhuǎn)換成串行流结闸。
Java 8 Stream的并行流，本質(zhì)上還是使用Fork/Join模型酒朵。
五. 總結(jié)
在Java開發(fā)中桦锄，如果使用了Java 8，那么強烈建議使用Stream蔫耽。因為Stream的每個操作都可以依賴Lambda表達式结耀，它是一種聲明式的數(shù)據(jù)處理方式，并且Stream提高了數(shù)據(jù)處理效率和開發(fā)效率匙铡。
作者：fengzhizi715
鏈接：https://www.imooc.com/article/23473?block_id=tuijian_wz
來源：慕課網(wǎng)
