关于java:这些JDK8-新特性我还是第一次听说

文章内容整顿自 博学谷狂野架构师

概述

什么是函数式接口？简略来说就是只有一个形象函数的接口。为了使得函数式接口的定义更加标准，java8 提供了 @FunctionalInterface 注解通知编译器在编译器去查看函数式接口的合法性，以便在编译器在编译出错时给出提醒。为了更加标准定义函数接口，给出如下函数式接口定义规定：

• 有且仅有一个形象函数
• 必须要有 @FunctionalInterface 注解
• 能够有默认办法

能够看出函数式接口的编写定义非常简单，不晓得大家有没有留神到，其实咱们常常会用到函数式接口，如 Runnable 接口，它就是一个函数式接口：

COPY@FunctionalInterface
public interface Runnable {
    /**
     * When an object implementing interface <code>Runnable</code> is used
     * to create a thread, starting the thread causes the object's
     * <code>run</code> method to be called in that separately executing
     * thread.
     * <p>
     * The general contract of the method <code>run</code> is that it may
     * take any action whatsoever.
     *
     * @see     java.lang.Thread#run()
     */
    public abstract void run();}

过来咱们会应用匿名外部类来实现线程的执行体：

COPYnew Thread(new Runnable() {
            @Override
            public void run() {System.out.println("Hello FunctionalInterface");
            }
        }).start();

当初咱们应用 Lambda 表达式，这里函数式接口的应用没有体现函数式编程思维，这里输入字符到规范输入流中，产生了副作用，起到了简化代码的作用，当然还有装 B。

COPYnew Thread(()->{System.out.println("Hello FunctionalInterface");
       }).start();

Java8 util.function 包下自带了 43 个函数式接口，大体分为以下几类：

• Consumer 生产接口
• Function 性能接口
• Operator 操作接口
• Predicate 断言接口
• Supplier 生产接口

其余接口都是在此基础上变形定制化罢了。

函数式接口具体介绍

这里只介绍最根底的函数式接口，至于它的变体只有明确了根底天然就可能明确

Consumer

消费者接口，就是用来生产数据的。

COPY@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 接口中有 accept 形象办法，accept 承受一个变量，也就是说你在应用这个函数式接口的时候，给你提供了数据，你只有接管应用就能够了；andThen 是一个默认办法，承受一个 Consumer 类型，当你对一个数据应用一次还不够爽的时候，你还能再应用一次，当然你其实能够爽无数次，只有始终应用 andThan 办法。

Function

何为 Function 呢？比方电视机，给你带来精力上的愉悦，然而它须要用电啊，电视它把电转换成了你荷尔蒙，这就是 Function，简略电说，Function 提供一种转换性能。

COPY@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 接口 最次要的就是 apply 函数，apply 承受 T 类型数据并返回 R 类型数据，就是将 T 类型的数据转换成 R 类型的数据，它还提供了 compose、andThen、identity 三个默认办法，compose 承受一个 Function，andThen 也同样承受一个 Function，这里的 andThen 与 Consumer 的 andThen 相似，在 apply 之后在 apply 一遍，compose 则与之相同，在 apply 之前先 apply（这两个 apply 具体解决内容个别是不同的），identity 起到了相似海关的作用，外国人想要运货进来，总得交点税吧，而后货物能力平安进入中国市场，当然了想不想收税还是你说了算的。

Operator

能够简略了解成算术中的各种运算操作，当然不仅仅是运算这么简略，因为它只定义了运算这个定义，但至于运算成什么样你说了算。因为没有最根底的 Operator，这里将通过 BinaryOperator、IntBinaryOperator 来了解 Operator 函数式接口，先从简略的 IntBinaryOperator 开始。

IntBinaryOperator

从名字能够晓得，这是一个二元操作，并且是 Int 类型的二元操作，那么这个接口能够做什么呢，除了加减乘除，还能够能够实现平方（两个雷同 int 数操作起来不就是平方吗），还是先看看它的定义吧：

@FunctionalInterface
public interface IntBinaryOperator {

    /**
     * Applies this operator to the given operands.
     *
     * @param left the first operand
     * @param right the second operand
     * @return the operator result
     */
    int applyAsInt(int left, int right);
}

IntBinaryOperator 接口内只有一个 applyAsInt 办法，其接管两个 int 类型的参数，并返回一个 int 类型的后果，其实这个跟 Function 接口的 apply 有点像，然而这里限定了，只能是 int 类型。

BinaryOperator

BinaryOperator 二元操作，看起来它和 IntBinaryOperator 是父子关系，实际上这两者没有半点关系，但他们在性能上还是有相似之处的：

COPY@FunctionalInterface
public interface BinaryOperator<T> extends BiFunction<T,T,T> {
    /**
     * Returns a {@link BinaryOperator} which returns the lesser of two elements
     * according to the specified {@code Comparator}.
     *
     * @param <T> the type of the input arguments of the comparator
     * @param comparator a {@code Comparator} for comparing the two values
     * @return a {@code BinaryOperator} which returns the lesser of its operands,
     *         according to the supplied {@code Comparator}
     * @throws NullPointerException if the argument is null
     */
    public static <T> BinaryOperator<T> minBy(Comparator<? super T> comparator) {Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) <= 0 ? a : b;
    }

    /**
     * Returns a {@link BinaryOperator} which returns the greater of two elements
     * according to the specified {@code Comparator}.
     *
     * @param <T> the type of the input arguments of the comparator
     * @param comparator a {@code Comparator} for comparing the two values
     * @return a {@code BinaryOperator} which returns the greater of its operands,
     *         according to the supplied {@code Comparator}
     * @throws NullPointerException if the argument is null
     */
    public static <T> BinaryOperator<T> maxBy(Comparator<? super T> comparator) {Objects.requireNonNull(comparator);
        return (a, b) -> comparator.compare(a, b) >= 0 ? a : b;
    }
}

BinaryOperator 是 BiFunction 生的，而 IntBinaryOperator 是从石头里蹦出来的，BinaryOperator 本身定义了 minBy、maxBy 默认办法，并且参数都是 Comparator，就是依据传入的比拟器的比拟规定找出最小最大的数据。

Predicate

断言、判断，对输出的数据依据某种规范进行评判，最终返回 boolean 值：

COPY@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 test 接管 T 类型的数据，返回 boolean 类型，即对数据进行某种规定的评判，如果合乎则返回 true，否则返回 false；Predicate 接口还提供了 and、negate、or，与 取反 或等，isEqual 判断两个参数是否相等等默认函数。

Supplier

生产、提供数据：

COPY@FunctionalInterface
public interface Supplier<T> {

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

十分 easy，get 办法返回一个 T 类数据，能够提供反复的数据，或者随机种子都能够，就这么简略。

函数式接口实战

Consumer

Consumer 用的太多了，不想说太多，如下：

COPYpublic class Main {public static void main(String[] args) {Stream.of(1,2,3,4,5,6)
                .forEach(integer -> System.out.println(integer)); // 输入 1，2，3，4，5，6
    }
}

这里应用规范输入，还是产生了副作用，然而这种水平是能够容许的

Function

转换，将字符串转成长度

COPYpublic class Main {public static void main(String[] args) {Stream.of("hello","FunctionalInterface")
                .map(e->e.length())
                .forEach(System.out::println);
    }
}

运算

COPYpublic class FunctionTest {public static void main(String[] args) {public static void main(String[] args) {

        Function<Integer, Integer> square = integer -> integer * integer; // 定义平方运算

        List<Integer> list = new ArrayList<>();
        list.add(1);
        list.add(2);
        list.add(3);
        list.add(4);


        list.stream()
                .map(square.andThen(square)) // 四次方
                .forEach(System.out::println);

        System.out.println("------");

        list.stream()
                .map(square.compose(e -> e - 1)) // 减一再平方
                .forEach(System.out::println);

        System.out.println("------");

        list.stream().map(square.andThen(square.compose(e->e/2))) // 先平方而后除 2 再平方
                .forEach(System.out::println);

    }
}

后果如下

COPY1
16
81
256
------
0
1
4
9
------
0
4
16
64

Operator

BinaryOperator

这里实现找最大值：

COPYpublic class BinaryOperatorTest {public static void main(String[] args) {Stream.of(2,4,5,6,7,1)
                .reduce(BinaryOperator.maxBy(Comparator.comparingInt(Integer::intValue))).ifPresent(System.out::println);

    }
}

IntOperator

这里实现累加性能：

COPYpublic class BinaryOperatorTest {public static void main(String[] args) {IntBinaryOperator intBinaryOperator = (e1, e2)->e1+e2; // 定义求和二元操作
        IntStream.of(2,4,5,6,7,1)
                .reduce(intBinaryOperator).ifPresent(System.out::println);
    }
}

Predicate

筛选出大于 0 最小的两个数

COPYpublic class Main {public static void main(String[] args) {IntStream.of(200,45,89,10,-200,78,94)
                .filter(e->e>0) // 过滤小于 0 的数
                .sorted() // 天然程序排序
                .limit(2) // 取前两个
                .forEach(System.out::println);
    }
}

Supplier

这里始终生产 2 这个数字，为了能停下来，应用 limit

COPYpublic class Main {public static void main(String[] args) {Stream.generate(()->2)
                .limit(10)
                .forEach(System.out::println);
    }
}

输入后果

COPY2
2
2
2
2
2
2
2
2
2

总结

Java8 的 Stream 基本上都是应用 util.function 包下的函数式接口来实现函数式编程的，而函数式接口也就只分为 Function、Operator、Consumer、Predicate、Supplier 这五大类，只有能了解把握最根底的五大类用法，其余变种也能举一反三。

本文由 传智教育博学谷狂野架构师 教研团队公布。

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