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概述
什么是函数式接口？简略来说便是只有一个笼统函数的接口。为了使得函数式接口的界说愈加规范，java8 供给了@FunctionalInterface 注解告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犯错时给出提示。为了愈加规范界说函数接口，给出如下函数式接口界说规则：
- 有且仅有一个笼统函数
- 有必要要有@FunctionalInterface 注解
- 能够有默许办法
能够看出函数式接口的编写界说十分简略，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，它便是一个函数式接口：
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();
}
曩昔我们会运用匿名内部类来完成线程的履行体：
COPYnew Thread(new Runnable() {
@Override
public void run() {
System.out.println("Hello FunctionalInterface");
}
}).start();
现在我们运用Lambda 表达式，这儿函数式接口的运用没有体现函数式编程思维，这儿输出字符到规范输出流中，产生了副效果，起到了简化代码的效果，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 功用接口
- Operator 操作接口
- Predicate 断语接口
- Supplier 生产接口
其他接口都是在此根底上变形定制化罢了。
函数式接口详细介绍
这儿只介绍最根底的函数式接口，至于它的变体只需明白了根底天然就能够明白
Consumer
顾客接口，便是用来消费数据的。
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 笼统办法，accept承受一个变量，也便是说你在运用这个函数式接口的时候，给你供给了数据，你只需接纳运用就能够了；andThen 是一个默许办法，承受一个Consumer 类型，当你对一个数据运用一次还不行爽的时候，你还能再运用一次，当然你其实能够爽无数次，只需一向运用andThan办法。
Function
何为Function呢？比如电视机，给你带来精神上的愉悦，可是它需要用电啊，电视它把电转化成了你荷尔蒙，这便是Function，简略电说，Function 供给一种转化功用。
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 最主要的便是apply 函数，apply 承受T类型数据并回来R类型数据，便是将T类型的数据转化成R类型的数据，它还供给了compose、andThen、identity 三个默许办法，compose 承受一个Function，andThen也同样承受一个Function，这儿的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详细处理内容一般是不同的），identity 起到了类似海关的效果，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，当然了想不想收税仍是你说了算的。
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，由于它只界说了运算这个界说，但至于运算成什么样你说了算。由于没有最根底的Operator，这儿将通过 BinaryOperator、IntBinaryOperator来了解Operator 函数式接口，先从简略的IntBinaryOperator开始。
IntBinaryOperator
从姓名能够知道，这是一个二元操作，而且是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，还能够能够完成平方（两个相同int 数操作起来不便是平方吗），仍是先看看它的界说吧：
@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，其接纳两个int 类型的参数，并回来一个int 类型的成果，其实这个跟Function 接口的apply 有点像，可是这儿限定了，只能是int类型。
BinaryOperator
BinaryOperator 二元操作，看起来它和IntBinaryOperator 是父子联系，实际上这两者没有半点联系，但他们在功用上仍是有相似之处的：
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 本身界说了minBy、maxBy默许办法，而且参数都是Comparator，便是依据传入的比较器的比较规则找出最小最大的数据。
Predicate
断语、判别，对输入的数据依据某种规范进行评判，最终回来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 接纳T类型的数据，回来 boolean 类型，即对数据进行某种规则的评判，假如契合则回来true，否则回来false；Predicate接口还供给了 and、negate、or，与 取反 或等，isEqual 判别两个参数是否相等等默许函数。
Supplier
生产、供给数据：
COPY@FunctionalInterface
public interface Supplier<T> {
/**
* Gets a result.
*
* @return a result
*/
T get();
}
十分easy，get办法回来一个T类数据，能够供给重复的数据，或许随机种子都能够，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)); //输出1，2，3，4，5，6
}
}
这儿运用规范输出，仍是产生了副效果，可是这种程度是能够允许的
Function
转化，将字符串转生长度
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; //界说平方运算
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)) //减再三平方
.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);
}
}
成果如下
COPY1
16
81
256
------
0
1
4
9
------
0
4
16
64
Operator
BinaryOperator
这儿完成找最大值：
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
这儿完成累加功用：
COPYpublic class BinaryOperatorTest {
public static void main(String[] args) {
IntBinaryOperator intBinaryOperator = (e1, e2)->e1+e2; //界说求和二元操作
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() //天然次序排序
.limit(2) //取前两个
.forEach(System.out::println);
}
}
Supplier
这儿一向生产2这个数字，为了能停下来，运用limit
COPYpublic class Main {
public static void main(String[] args) {
Stream.generate(()->2)
.limit(10)
.forEach(System.out::println);
}
}
输出成果
COPY2
2
2
2
2
2
2
2
2
2
总结
Java8的Stream 基本上都是运用util.function包下的函数式接口来完成函数式编程的，而函数式接口也就只分为 Function、Operator、Consumer、Predicate、Supplier 这五大类，只需能了解把握最根底的五大类用法，其他变种也能触类旁通。
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