文章内容整理自 博学谷狂野架构师
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@FunctionalInterfacepublic interface Runnable { /** * When an object implementing interface Runnable is used * to create a thread, starting the thread causes the object's * run method to be called in that separately executing * thread. * * The general contract of the method run 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@FunctionalInterfacepublic interface Consumer { /** * 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 andThen(Consumer 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@FunctionalInterfacepublic interface Function { /** * 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 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 Function compose(Function 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 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 Function andThen(Function after) { Objects.requireNonNull(after); return (T t) -> after.apply(apply(t)); } /** * Returns a function that always returns its input argument. * * @param the type of the input and output objects to the function * @return a function that always returns its input argument */ static Function 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：
@FunctionalInterfacepublic 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@FunctionalInterfacepublic interface BinaryOperator extends BiFunction { /** * Returns a {@link BinaryOperator} which returns the lesser of two elements * according to the specified {@code Comparator}. * * @param 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 BinaryOperator minBy(Comparator 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 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 BinaryOperator maxBy(Comparator 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@FunctionalInterfacepublic interface Predicate { /** * 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. * * 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 and(Predicate 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 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. * *
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 or(Predicate 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 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 Predicate 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@FunctionalInterfacepublic interface Supplier { /** * 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 square = integer -> integer * integer; //定义平方运算 List 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
COPY11681256------0149------041664OperatorBinaryOperator
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
COPY2222222222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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