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
64Operator
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。
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