Stream是Java 8新增的接口,Stream能夠認爲是一個高級版本的 Iterator。它表明着數據流,流中的數據元素的數量能夠是有限的,也能夠是無限的。java
Stream跟Iterator的差異是編程
Stream的操做是創建在函數式接口的組合之上的。Java8中新增的函數式接口都在java.util.function包下。這些函數式接口能夠有多種分類方式。app
Function是從T到R的一元映射函數。將參數T傳遞給一個函數,返回R。即R = Function(T)less
@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;
}
}
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Function默認實現了3個default方法,分別是compose、andThen和identity。dom
方法名 | 對應函數 | 描述 |
---|---|---|
compose | V=Function(ParamFunction(T)) | 它體現了嵌套關係 |
andThen | V= ParamFunction(Function(T)) | 轉換了嵌套的順序 |
identity | Function(T)=T | 傳遞自身的函數調用 |
compose和andThen對於兩個函數f和g來講,f.compose(g)等價於g.andThen(f)。ide
Predicate是一個謂詞函數,主要做爲一個謂詞演算推導真假值存在,返回布爾值的函數。Predicate等價於一個Function的boolean型返回值的子集。函數式編程
@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);
}
}
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Predicate的默認方法是and、negate、or。函數
Consumer是從T到void的一元函數,接受一個入參但不返回任何結果的操做。ui
@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); };
}
}
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Consumer的默認方法是andThen。this
Supplier是表示結果的供應者。
@FunctionalInterface
public interface Supplier<T> {
/** * Gets a result. * * @return a result */
T get();
}
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Supplier的用法:
Supplier<String> supplier = new Supplier<String>() {
@Override
public String get() {
return "hello suppiler";
}
};
System.out.println(supplier.get());
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或者:
Supplier<User> userSupplier = User::new;
userSupplier.get(); // new User
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Java 8新增了CompletableFuture,它的不少方法的入參都用到了Supplier。
Java 8有多種方式來建立Stream:
其實最終都是依賴底層的StreamSupport類來完成Stream建立。
中間操做又能夠分爲無狀態的(Stateless)和有狀態的(Stateful),無狀態中間操做是指元素的處理不受前面元素的影響,而有狀態的中間操做必須等到全部元素處理以後才知道最終結果。
Stream的中間操做只是一種標記,只有執行告終束操做纔會觸發實際計算。 熟悉RxJava、Scala的同窗能夠看到,Stream中間操做的各個方法在RxJava、Scala中均可以找到熟悉的身影。
短路操做是指不用處理所有元素就能夠返回結果。短路操做必須一個元素處理一次。
非短路操做能夠批量處理數據,可是須要處理徹底部元素纔會返回結果。
在建立Stream時,默認是建立串行流。可是可使用parallelStream()來建立並行流或者parallel()將串行流轉換成並行流。並行流也能夠經過sequential()轉換成串行流。
Java 8 Stream的並行流,本質上仍是使用Fork/Join模型。
在Java開發中,若是使用了Java 8,那麼強烈建議使用Stream。由於Stream的每一個操做均可以依賴Lambda表達式,它是一種聲明式的數據處理方式,而且Stream提升了數據處理效率和開發效率。