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概述

什么是函数式接口？简单来说就是只有一个抽象函数的接口。为了使得函数式接口的定义更加规范，java8 提供了@FunctionalInterface 注解告诉编译器在编译器去检查函数式接口的合法性，以便在编译器在编译出错时给出提示。为了更加规范定义函数接口，给出如下函数式接口定义规则：

• 有且仅有一个抽象函数
• 必须要有@FunctionalInterface 注解
• 可以有默认方法

可以看出函数式接口的编写定义非常简单，不知道大家有没有注意到，其实我们经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();
}

过去我们会使用匿名内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();

现在我们使用Lambda 表达式，这里函数式接口的使用没有体现函数式编程思想，这里输出字符到标准输出流中，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 消费接口
• Function 功能接口
• Operator 操作接口
• Predicate 断言接口
• Supplier 生产接口

其他接口都是在此基础上变形定制化罢了。

函数式接口详细介绍

这里只介绍最基础的函数式接口，至于它的变体只要明白了基础自然就能够明白

Consumer

消费者接口，就是用来消费数据的。

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 抽象方法，accept接受一个变量，也就是说你在使用这个函数式接口的时候，给你提供了数据，你只要接收使用就可以了；andThen 是一个默认方法，接受一个Consumer 类型，当你对一个数据使用一次还不够爽的时候，你还能再使用一次，当然你其实可以爽无数次，只要一直使用andThan方法。

Function

何为Function呢？比如电视机，给你带来精神上的愉悦，但是它需要用电啊，电视它把电转换成了你荷尔蒙，这就是Function，简单电说，Function 提供一种转换功能。

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 最主要的就是apply 函数，apply 接受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 三个默认方法，compose 接受一个Function，andThen也同样接受一个Function，这里的andThen 与Consumer 的andThen 类似，在apply之后在apply一遍，compose 则与之相反，在apply之前先apply（这两个apply具体处理内容一般是不同的），identity 起到了类似海关的作用，外国人想要运货进来，总得交点税吧，然后货物才能安全进入中国市场，当然了想不想收税还是你说了算的。

Operator

可以简单理解成算术中的各种运算操作，当然不仅仅是运算这么简单，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。由于没有最基础的Operator，这里将通过 BinaryOperator、IntBinaryOperator来理解Operator 函数式接口，先从简单的IntBinaryOperator开始。

IntBinaryOperator

从名字可以知道，这是一个二元操作，并且是Int 类型的二元操作，那么这个接口可以做什么呢，除了加减乘除，还可以可以实现平方（两个相同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 方法，其接收两个int 类型的参数，并返回一个int 类型的结果，其实这个跟Function 接口的apply 有点像，但是这里限定了，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，但他们在功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 自身定义了minBy、maxBy默认方法，并且参数都是Comparator，就是根据传入的比较器的比较规则找出最小最大的数据。

Predicate

断言、判断，对输入的数据根据某种标准进行评判，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 接收T类型的数据，返回 boolean 类型，即对数据进行某种规则的评判，如果符合则返回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();
}

非常easy，get方法返回一个T类数据，可以提供重复的数据，或者随机种子都可以，就这么简单。

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)); //输出1，2，3，4，5，6
    }
}

这里使用标准输出，还是产生了副作用，但是这种程度是可以允许的

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))) //先平方然后除2再平方
                .forEach(System.out::println);

    }
}

结果如下

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

这里实现累加功能：

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() //自然顺序排序
                .limit(2) //取前两个
                .forEach(System.out::println);
    }
}

Supplier

这里一直生产2这个数字，为了能停下来，使用limit

COPYpublic class Main {

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

输出结果

COPY2
2
2
2
2
2
2
2
2
2

总结

Java8的Stream 基本上都是使用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，只要能理解掌握最基础的五大类用法，其他变种也能触类旁通。

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