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作者：litesky
www.jianshu.com/p/2338cabc59e1

函數(shù)式接口是伴隨著Stream的誕生而出現(xiàn)的，Java8Stream 作為函數(shù)式編程的一種具體實(shí)現(xiàn)，開(kāi)發(fā)者無(wú)需關(guān)注怎么做，只需知道要做什么，各種操作符配合簡(jiǎn)潔明了的函數(shù)式接口給開(kāi)發(fā)者帶來(lái)了簡(jiǎn)單快速處理數(shù)據(jù)的體驗(yàn)。

函數(shù)式接口

什么是函數(shù)式接口？簡(jiǎn)單來(lái)說(shuō)就是只有一個(gè)抽象函數(shù)的接口。為了使得函數(shù)式接口的定義更加規(guī)范，java8 提供了@FunctionalInterface 注解告訴編譯器在編譯器去檢查函數(shù)式接口的合法性，以便在編譯器在編譯出錯(cuò)時(shí)給出提示。為了更加規(guī)范定義函數(shù)接口，給出如下函數(shù)式接口定義規(guī)則：

• 有且僅有一個(gè)抽象函數(shù)

• 必須要有@FunctionalInterface 注解

• 可以有默認(rèn)方法

可以看出函數(shù)式接口的編寫(xiě)定義非常簡(jiǎn)單，不知道大家有沒(méi)有注意到，其實(shí)我們經(jīng)常會(huì)用到函數(shù)式接口，如Runnable 接口，它就是一個(gè)函數(shù)式接口：

@FunctionalInterface
public?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();
}

過(guò)去我們會(huì)使用匿名內(nèi)部類來(lái)實(shí)現(xiàn)線程的執(zhí)行體：

new?Thread(new?Runnable()?{
????????????@Overridepublic?void?run()?{
????????????????System.out.println("Hello?FunctionalInterface");
????????????}
????????}).start();

現(xiàn)在我們使用Lambda 表達(dá)式，這里函數(shù)式接口的使用沒(méi)有體現(xiàn)函數(shù)式編程思想，這里輸出字符到標(biāo)準(zhǔn)輸出流中，產(chǎn)生了副作用，起到了簡(jiǎn)化代碼的作用，當(dāng)然還有裝B。

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

Java8 util.function 包下自帶了43個(gè)函數(shù)式接口，大體分為以下幾類：

• Consumer 消費(fèi)接口

• Function 功能接口

• Operator 操作接口

• Predicate 斷言接口

• Supplier 生產(chǎn)接口

其他接口都是在此基礎(chǔ)上變形定制化罷了。

函數(shù)式接口詳細(xì)介紹

這里只介紹最基礎(chǔ)的函數(shù)式接口，至于它的變體只要明白了基礎(chǔ)自然就能夠明白。前篇：玩轉(zhuǎn)Java8中的 Stream 之從零認(rèn)識(shí) Stream

Consumer

消費(fèi)者接口，就是用來(lái)消費(fèi)數(shù)據(jù)的。

@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?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接受一個(gè)變量，也就是說(shuō)你在使用這個(gè)函數(shù)式接口的時(shí)候，給你提供了數(shù)據(jù)，你只要接收使用就可以了；andThen 是一個(gè)默認(rèn)方法，接受一個(gè)Consumer 類型，當(dāng)你對(duì)一個(gè)數(shù)據(jù)使用一次還不夠爽的時(shí)候，你還能再使用一次，當(dāng)然你其實(shí)可以爽無(wú)數(shù)次，只要一直使用andThan方法。

Function

何為Function呢？比如電視機(jī)，給你帶來(lái)精神上的愉悅，但是它需要用電啊，電視它把電轉(zhuǎn)換成了你荷爾蒙，這就是Function，簡(jiǎn)單電說(shuō)，Function 提供一種轉(zhuǎn)換功能。

@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??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<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??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<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??the?type?of?the?input?and?output?objects?to?the?function
?????*?@return?a?function?that?always?returns?its?input?argument
?????*/static??Function<T,?T>?identity()?{return?t?->?t;
????}
}

Function 接口 最主要的就是apply 函數(shù)，apply 接受T類型數(shù)據(jù)并返回R類型數(shù)據(jù)，就是將T類型的數(shù)據(jù)轉(zhuǎn)換成R類型的數(shù)據(jù)，它還提供了compose、andThen、identity 三個(gè)默認(rèn)方法，compose 接受一個(gè)Function，andThen也同樣接受一個(gè)Function，這里的andThen 與Consumer 的andThen 類似，在apply之后在apply一遍，compose 則與之相反，在apply之前先apply(這兩個(gè)apply具體處理內(nèi)容一般是不同的)，identity 起到了類似海關(guān)的作用，外國(guó)人想要運(yùn)貨進(jìn)來(lái)，總得交點(diǎn)稅吧，然后貨物才能安全進(jìn)入中國(guó)市場(chǎng)，當(dāng)然了想不想收稅還是你說(shuō)了算的:。

Operator

可以簡(jiǎn)單理解成算術(shù)中的各種運(yùn)算操作，當(dāng)然不僅僅是運(yùn)算這么簡(jiǎn)單，因?yàn)樗欢x了運(yùn)算這個(gè)定義，但至于運(yùn)算成什么樣你說(shuō)了算。由于沒(méi)有最基礎(chǔ)的Operator，這里將通過(guò) BinaryOperator、IntBinaryOperator來(lái)理解Operator 函數(shù)式接口，先從簡(jiǎn)單的IntBinaryOperator開(kāi)始。

IntBinaryOperator

從名字可以知道，這是一個(gè)二元操作，并且是Int 類型的二元操作，那么這個(gè)接口可以做什么呢，除了加減乘除，還可以可以實(shí)現(xiàn)平方(兩個(gè)相同int 數(shù)操作起來(lái)不就是平方嗎)，還是先看看它的定義吧：

@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 接口內(nèi)只有一個(gè)applyAsInt 方法，其接收兩個(gè)int 類型的參數(shù)，并返回一個(gè)int 類型的結(jié)果，其實(shí)這個(gè)跟Function 接口的apply 有點(diǎn)像，但是這里限定了，只能是int類型。

BinaryOperator

BinaryOperator 二元操作，看起來(lái)它和IntBinaryOperator 是父子關(guān)系，實(shí)際上這兩者沒(méi)有半點(diǎn)關(guān)系，但他們?cè)诠δ苌线€是有相似之處的：

@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??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?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??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?super?T>?comparator)?{
????????Objects.requireNonNull(comparator);return?(a,?b)?->?comparator.compare(a,?b)?>=?0???a?:?b;
????}
}

BinaryOperator 是 BiFunction 生的，而IntBinaryOperator 是從石頭里蹦出來(lái)的，BinaryOperator 自身定義了minBy、maxBy默認(rèn)方法，并且參數(shù)都是Comparator，就是根據(jù)傳入的比較器的比較規(guī)則找出最小最大的數(shù)據(jù)。

Predicate

斷言、判斷，對(duì)輸入的數(shù)據(jù)根據(jù)某種標(biāo)準(zhǔn)進(jìn)行評(píng)判，最終返回boolean值：

@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.
?????*
?????*?

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?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?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?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??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類型的數(shù)據(jù)，返回 boolean 類型，即對(duì)數(shù)據(jù)進(jìn)行某種規(guī)則的評(píng)判，如果符合則返回true，否則返回false；Predicate接口還提供了 and、negate、or，與 取反 或等，isEqual 判斷兩個(gè)參數(shù)是否相等等默認(rèn)函數(shù)。

Supplier

生產(chǎn)、提供數(shù)據(jù)：

@FunctionalInterface
public?interface?Supplier<T>?{

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

非常easy，get方法返回一個(gè)T類數(shù)據(jù)，可以提供重復(fù)的數(shù)據(jù)，或者隨機(jī)種子都可以，就這么簡(jiǎn)單。

函數(shù)式接口實(shí)戰(zhàn)

Consumer

Consumer 用的太多了，不想說(shuō)太多，如下：

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

這里使用標(biāo)準(zhǔn)輸出，還是產(chǎn)生了副作用，但是這種程度是可以允許的

Function

1.轉(zhuǎn)換，將字符串轉(zhuǎn)成長(zhǎng)度

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

2.運(yùn)算

public?class?FunctionTest?{

????public?static?void?main(String[]?args)?{

?????????public?static?void?main(String[]?args)?{

????????Function<Integer,?Integer>?square?=?integer?->?integer?*?integer;?//定義平方運(yùn)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))?//減一再平方
????????????????.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);
????}
}

結(jié)果如圖：

Operator

1.BinaryOperator

這里實(shí)現(xiàn)找最大值：

public?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);

????}
}

Comparator 后期會(huì)講到

2.IntOperator

這里實(shí)現(xiàn)累加功能：

public?class?BinaryOperatorTest?{

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

Predicate

篩選出大于0最小的兩個(gè)數(shù)

public?class?Main?{

????public?static?void?main(String[]?args)?{
????????IntStream.of(200,45,89,10,-200,78,94)
????????????????.filter(e->e>0)?//過(guò)濾小于0的數(shù)
????????????????.sorted()?//自然順序排序
????????????????.limit(2)?//取前兩個(gè)
????????????????.forEach(System.out::println);
????}
}

Supplier

這里一直生產(chǎn)2這個(gè)數(shù)字，為了能停下來(lái)，使用limit

public?class?Main?{

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

如圖：

總結(jié)

Java8的Stream 基本上都是使用util.function包下的函數(shù)式接口來(lái)實(shí)現(xiàn)函數(shù)式編程的，而函數(shù)式接口也就只分為 Function、Operator、Consumer、Predicate、Supplier 這五大類，只要能理解掌握最基礎(chǔ)的五大類用法，其他變種也能觸類旁通。

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