4-函數式編程 · guava

# 4-函數式編程 # 4-函數式編程 [原文鏈接](http://code.google.com/p/guava-libraries/wiki/FunctionalExplained) [譯文鏈接](http://ifeve.com/google-guava-functional) 譯者：沈義揚，校對：丁一 ## 注意事項截至JDK7，Java中也只能通過笨拙冗長的匿名類來達到近似函數式編程的效果。預計JDK8中會有所改變，但Guava現在就想給JDK5以上用戶提供這類支持。過度使用Guava函數式編程會導致冗長、混亂、可讀性差而且低效的代碼。這是迄今為止最容易（也是最經常）被濫用的部分，如果你想通過函數式風格達成一行代碼，致使這行代碼長到荒唐，Guava團隊會淚流滿面。比較如下代碼： ``` <pre class="calibre11">``` Function<String, Integer> lengthFunction = new Function<String, Integer>() { public Integer apply(String string) { return string.length(); } }; Predicate<String> allCaps = new Predicate<String>() { public boolean apply(String string) { return CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string); } }; Multiset<Integer> lengths = HashMultiset.create( Iterables.transform(Iterables.filter(strings, allCaps), lengthFunction)); ``` ``` 或FluentIterable的版本 ``` <pre class="calibre11">``` Multiset<Integer> lengths = HashMultiset.create( FluentIterable.from(strings) .filter(new Predicate<String>() { public boolean apply(String string) { return CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string); } }) .transform(new Function<String, Integer>() { public Integer apply(String string) { return string.length(); } })); ``` ``` 還有 ``` <pre class="calibre11">``` Multiset<Integer> lengths = HashMultiset.create(); for (String string : strings) { if (CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string)) { lengths.add(string.length()); } } ``` ``` 即使用了靜態導入，甚至把Function和Predicate的聲明放到別的文件，第一種代碼實現仍然不簡潔，可讀性差并且效率較低。截至JDK7，命令式代碼仍應是默認和第一選擇。不應該隨便使用函數式風格，除非你絕對確定以下兩點之一： - 使用函數式風格以后，整個工程的代碼行會凈減少。在上面的例子中，函數式版本用了11行，命令式代碼用了6行，把函數的定義放到另一個文件或常量中，并不能幫助減少總代碼行。 - 為了提高效率，轉換集合的結果需要懶視圖，而不是明確計算過的集合。此外，確保你已經閱讀和重讀了Effective Java的第55條，并且除了閱讀本章后面的說明，你還真正做了性能測試并且有測試數據來證明函數式版本更快。請務必確保，當使用Guava函數式的時候，用傳統的命令式做同樣的事情不會更具可讀性。嘗試把代碼寫下來，看看它是不是真的那么糟糕？會不會比你想嘗試的極其笨拙的函數式更具可讀性。 ## Functions\[函數\]和Predicates\[斷言\] 本節只討論直接與Function和Predicate打交道的Guava功能。一些其他工具類也和”函數式風格”相關，例如[`Iterables.concat(Iterable<Iterable>)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#concat%28java.lang.Iterable%29)，和其他用常量時間返回視圖的方法。嘗試看看[2.3節的集合工具類](http://ifeve.com/google-guava-collectionutilities)。 Guava提供兩個基本的函數式接口： - Function<A, B>，它聲明了單個方法B apply(A input)。Function對象通常被預期為引用透明的——沒有副作用——并且引用透明性中的”相等”語義與equals一致，如a.equals(b)意味著function.apply(a).equals(function.apply(b))。 - Predicate<T>，它聲明了單個方法boolean apply(T input)。Predicate對象通常也被預期為無副作用函數，并且”相等”語義與equals一致。 ### 特殊的斷言字符類型有自己特定版本的Predicate——[`CharMatcher`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/CharMatcher.html)，它通常更高效，并且在某些需求方面更有用。CharMatcher實現了Predicate<Character>，可以當作Predicate一樣使用，要把Predicate轉成CharMatcher，可以使用[`CharMatcher.forPredicate`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6common/base/CharMatcher.html#forPredicate%28com.google.common.base.Predicate%29)。更多細節請參考第6章-字符串處理。此外，對可比較類型和基于比較邏輯的Predicate，Range類可以滿足大多數需求——它表示一個不可變區間。Range類實現了Predicate，用以判斷值是否在區間內。例如，Range.atMost(2)就是個完全合法的Predicate<Integer>。更多使用Range的細節請參照第8章。 ### 操作Functions和Predicates [Functions](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Functions.html)提供簡便的Function構造和操作方法，包括： [`forMap(Map<A, B>)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Functions.html#forMap%28java.util.Map%29)[`compose(Function<B, C>, Function<A, B>)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Functions.html#compose%28com.google.common.base.Function,%20com.google.common.base.Function%29)[`constant(T)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Functions.html#constant%28E%29)[`identity()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Functions.html#identity%28%29)[`toStringFunction()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Functions.html#toStringFunction%28%29)細節請參考Javadoc。相應地，[Predicates](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Predicates.html)提供了更多構造和處理Predicate的方法，下面是一些例子： [`instanceOf(Class)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#instanceOf%28java.lang.Class%29)[`assignableFrom(Class)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#assignableFrom%28java.lang.Class%29)[`contains(Pattern)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#contains%28java.util.regex.Pattern%29)[`in(Collection)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#in%28java.util.Collection%29)[`isNull()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#isNull%28%29)[`alwaysFalse()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#alwaysFalse%28%29)[`alwaysTrue()`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#alwaysTrue%28%29)[`equalTo(Object)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#equalTo%28T%29)[`compose(Predicate, Function)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#compose%28com.google.common.base.Predicate,%20com.google.common.base.Function%29)[`and(Predicate...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#and%28com.google.common.base.Predicate...%29)[`or(Predicate...)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#or%28com.google.common.base.Predicate...%29)[`not(Predicate)`](http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Predicates.html#not%28com.google.common.base.Predicate%29)細節請參考Javadoc。 ## 使用函數式編程 Guava提供了很多工具方法，以便用Function或Predicate操作集合。這些方法通常可以在集合工具類找到，如Iterables，Lists，Sets，Maps，Multimaps等。 ### 斷言斷言的最基本應用就是過濾集合。所有Guava過濾方法都返回”視圖”——*譯者注：即并非用一個新的集合表示過濾，而只是基于原集合的視圖*。 **集合類型****過濾方法**Iterable[`Iterables.filter(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#filter%28java.lang.Iterable,%20com.google.common.base.Predicate%29)[`FluentIterable.filter(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co%E2%80%A6common/collect/FluentIterable.html#filter%28com.google.common.base.Predicate%29)Iterator[`Iterators.filter(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#filter%28java.util.Iterator,%20com.google.common.base.Predicate%29)Collection[`Collections2.filter(Collection, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Collections2.html#filter%28java.util.Collection,%20com.google.common.base.Predicate%29)Set[`Sets.filter(Set, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Sets.html#filter%28java.util.Set,%20com.google.common.base.Predicate%29)SortedSet[`Sets.filter(SortedSet, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Sets.html#filter%28java.util.SortedSet,%20com.google.common.base.Predicate%29)Map[`Maps.filterKeys(Map, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterKeys%28java.util.Map,%20com.google.common.base.Predicate%29)[`Maps.filterValues(Map, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterValues%28java.util.Map,%20com.google.common.base.Predicate%29)[`Maps.filterEntries(Map, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterEntries%28java.util.Map,%20com.google.common.base.Predicate%29)SortedMap[`Maps.filterKeys(SortedMap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterKeys%28java.util.SortedMap,%20com.google.common.base.Predicate%29)[`Maps.filterValues(SortedMap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterValues%28java.util.SortedMap,%20com.google.common.base.Predicate%29)[`Maps.filterEntries(SortedMap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#filterEntries%28java.util.SortedMap,%20com.google.common.base.Predicate%29)Multimap[`Multimaps.filterKeys(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6ommon/collect/Multimaps.html#filterKeys%28com.google.common.collect.Multimap,%20com.google.common.base.Predicate%29)[`Multimaps.filterValues(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6mon/collect/Multimaps.html#filterValues%28com.google.common.collect.Multimap,%20com.google.common.base.Predicate%29)[`Multimaps.filterEntries(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6on/collect/Multimaps.html#filterEntries%28com.google.common.collect.Multimap,%20com.google.common.base.Predicate%29)\*List的過濾視圖被省略了，因為不能有效地支持類似get(int)的操作。請改用Lists.newArrayList(Collections2.filter(list, predicate))做拷貝過濾。除了簡單過濾，Guava另外提供了若干用Predicate處理Iterable的工具——通常在[`Iterables`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/com/google/common/collect/Iterables.html)工具類中，或者是[`FluentIterable`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/com/google/common/collect/FluentIterable.html)的”fluent”（鏈式調用）方法。 **Iterables\*\***方法簽名\*\***說明****另請參見**[`boolean all(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#all%28java.lang.Iterable,%20com.google.common.base.Predicate%29)是否所有元素滿足斷言？懶實現：如果發現有元素不滿足，不會繼續迭代[`Iterators.all(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#all%28java.util.Iterator,%20com.google.common.base.Predicate%29)[`FluentIterable.allMatch(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co%E2%80%A6mmon/collect/FluentIterable.html#allMatch%28com.google.common.base.Predicate%29)[`boolean any(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#any%28java.lang.Iterable,%20com.google.common.base.Predicate%29)是否有任意元素滿足元素滿足斷言？懶實現：只會迭代到發現滿足的元素[`Iterators.any(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#any%28java.util.Iterator,%20com.google.common.base.Predicate%29)[`FluentIterable.anyMatch(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co%E2%80%A6mmon/collect/FluentIterable.html#anyMatch%28com.google.common.base.Predicate%29)[`T find(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#find%28java.lang.Iterable,%20com.google.common.base.Predicate%29)循環并返回**一個**滿足元素滿足斷言的元素，如果沒有則拋出NoSuchElementException[`Iterators.find(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#find%28java.util.Iterator,%20com.google.common.base.Predicate%29) [`Iterables.find(Iterable, Predicate, T default)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#find%28java.lang.Iterable,%20com.google.common.base.Predicate,%20T%29) [`Iterators.find(Iterator, Predicate, T default)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#find%28java.util.Iterator,%20com.google.common.base.Predicate,%20T%29)[`Optional<T> tryFind(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#tryFind%28java.lang.Iterable,%20com.google.common.base.Predicate%29)返回**一個**滿足元素滿足斷言的元素，若沒有則返回`O`ptional.absent()[`Iterators.find(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#find%28java.util.Iterator,%20com.google.common.base.Predicate%29) [`Iterables.find(Iterable, Predicate, T default)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#find%28java.lang.Iterable,%20com.google.common.base.Predicate,%20T%29) [`Iterators.find(Iterator, Predicate, T default)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#find%28java.util.Iterator,%20com.google.common.base.Predicate,%20T%29)[`indexOf(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#indexOf%28java.lang.Iterable,%20com.google.common.base.Predicate%29)返回第一個滿足元素滿足斷言的元素索引值，若沒有返回-1[`Iterators.indexOf(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#indexOf%28java.util.Iterator,%20com.google.common.base.Predicate%29)[`removeIf(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#removeIf%28java.lang.Iterable,%20com.google.common.base.Predicate%29)移除所有滿足元素滿足斷言的元素，實際調用Iterator.remove()方法[`Iterators.removeIf(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#removeIf%28java.util.Iterator,%20com.google.common.base.Predicate%29)### 函數到目前為止，函數最常見的用途為轉換集合。同樣，所有的Guava轉換方法也返回原集合的視圖。 **集合類型****轉換\*\***方法\*\*Iterable[`Iterables.transform(Iterable, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#transform%28java.lang.Iterable,%20com.google.common.base.Function%29)[`FluentIterable.transform(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co%E2%80%A6mmon/collect/FluentIterable.html#transform%28com.google.common.base.Function%29)Iterator[`Iterators.transform(Iterator, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#transform%28java.util.Iterator,%20com.google.common.base.Function%29)Collection[`Collections2.transform(Collection, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Collections2.html#transform%28java.util.Collection,%20com.google.common.base.Function%29)List[`Lists.transform(List, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Lists.html#transform%28java.util.List,%20com.google.common.base.Function%29)Map\*[`Maps.transformValues(Map, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#transformValues%28java.util.Map,%20com.google.common.base.Function%29)[`Maps.transformEntries(Map, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#transformEntries%28java.util.Map,%20com.google.common.collect.Maps.EntryTransformer%29)SortedMap\*[`Maps.transformValues(SortedMap, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#transformValues%28java.util.SortedMap,%20com.google.common.base.Function%29)[`Maps.transformEntries(SortedMap, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.html#transformEntries%28java.util.SortedMap,%20com.google.common.collect.Maps.EntryTransformer%29)Multimap\*[`Multimaps.transformValues(Multimap, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/collect/Multimaps.html#transformValues%28com.google.common.collect.Multimap,%20com.google.common.base.Function%29)[`Multimaps.transformEntries(Multimap, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6collect/Multimaps.html#transformEntries%28com.google.common.collect.Multimap,%20com.google.common.collect.Maps.EntryTransformer%29)ListMultimap\*[`Multimaps.transformValues(ListMultimap, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6lect/Multimaps.html#transformValues%28com.google.common.collect.ListMultimap,%20com.google.common.base.Function%29)[`Multimaps.transformEntries(ListMultimap, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6ect/Multimaps.html#transformEntries%28com.google.common.collect.ListMultimap,%20com.google.common.collect.Maps.EntryTransformer%29)Table[`Tables.transformValues(Table, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6common/collect/Tables.html#transformValues%28com.google.common.collect.Table,%20com.google.common.base.Function%29)\*Map和[`Multimap`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Multimap.html)有特殊的方法，其中有個[`EntryTransformer<K, V1, V2>`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.EntryTransformer.html)參數，它可以使用舊的鍵值來計算，并且用計算結果替換舊值。 \*對Set的轉換操作被省略了，因為不能有效支持contains(Object)操作——*譯者注：懶視圖實際上不會全部計算轉換后的**Set**元素，因此不能高效地支持**contains(Object)**。*請改用Sets.newHashSet(Collections2.transform(set, function))進行拷貝轉換。 ``` <pre class="calibre11">``` List<String> names; Map<String, Person> personWithName; List<Person> people = Lists.transform(names, Functions.forMap(personWithName)); ListMultimap<String, String> firstNameToLastNames; // maps first names to all last names of people with that first name ListMultimap<String, String> firstNameToName = Multimaps.transformEntries(firstNameToLastNames, new EntryTransformer<String, String, String> () { public String transformEntry(String firstName, String lastName) { return firstName + " " + lastName; } }); ``` ``` 可以組合Function使用的類包括： Ordering[`Ordering.onResultOf(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6le/common/collect/Ordering.html#onResultOf%28com.google.common.base.Function%29)Predicate[`Predicates.compose(Predicate, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Predicates.html#compose%28com.google.common.base.Predicate,%20com.google.common.base.Function%29)Equivalence[`Equivalence.onResultOf(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6le/common/base/Equivalence.html#onResultOf%28com.google.common.base.Function%29)Supplier[`Suppliers.compose(Function, Supplier)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Suppliers.html#compose%28com.google.common.base.Function,%20com.google.common.base.Supplier%29)Function[`Functions.compose(Function, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Functions.html#compose%28com.google.common.base.Function,%20com.google.common.base.Function%29)此外，[ListenableFuture](http://code.google.com/p/guava-libraries/wiki/ListenableFutureExplained) API支持轉換ListenableFuture。Futures也提供了接受[`AsyncFunction`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/util/concurrent/AsyncFunction.html)參數的方法。AsyncFunction是Function的變種，它允許異步計算值。 [`Futures.transform(ListenableFuture, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function%29)[`Futures.transform(ListenableFuture, Function, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.base.Function,%20java.util.concurrent.Executor%29)[`Futures.transform(ListenableFuture, AsyncFunction)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.AsyncFunction%29)[`Futures.transform(ListenableFuture, AsyncFunction, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/%E2%80%A6/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture,%20com.google.common.util.concurrent.AsyncFunction,%20java.util.concurrent.Executor%29)