4-函数式编程 · Java 开源项目中文文档

# 4-函数式编程 [原文链接](http://code.google.com/p/guava-libraries/wiki/FunctionalExplained) [译文链接](http://ifeve.com/google-guava-functional) 译者：沈义扬，校对：丁一 ## 注意事项截至JDK7，Java中也只能通过笨拙冗长的匿名类来达到近似函数式编程的效果。预计JDK8中会有所改变，但Guava现在就想给JDK5以上用户提供这类支持。过度使用Guava函数式编程会导致冗长、混乱、可读性差而且低效的代码。这是迄今为止最容易（也是最经常）被滥用的部分，如果你想通过函数式风格达成一行代码，致使这行代码长到荒唐，Guava团队会泪流满面。比较如下代码： ``` 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 ``` 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(); } })); ``` 还有 ``` Multiset<Integer> lengths = HashMultiset.create(); for (String string : strings) { if (CharMatcher.JAVA_UPPER_CASE.matchesAllOf(string)) { lengths.add(string.length()); } } ``` 即使用了静态导入，甚至把Function和Predicate的声明放到别的文件，第一种代码实现仍然不简洁，可读性差并且效率较低。截至JDK7，命令式代码仍应是默认和第一选择。不应该随便使用函数式风格，除非你绝对确定以下两点之一： * 使用函数式风格以后，整个工程的代码行会净减少。在上面的例子中，函数式版本用了11行，命令式代码用了6行，把函数的定义放到另一个文件或常量中，并不能帮助减少总代码行。 * 为了提高效率，转换集合的结果需要懒视图，而不是明确计算过的集合。此外，确保你已经阅读和重读了Effective Java的第55条，并且除了阅读本章后面的说明，你还真正做了性能测试并且有测试数据来证明函数式版本更快。请务必确保，当使用Guava函数式的时候，用传统的命令式做同样的事情不会更具可读性。尝试把代码写下来，看看它是不是真的那么糟糕？会不会比你想尝试的极其笨拙的函数式更具可读性。 ## Functions[函数]和Predicates[断言] 本节只讨论直接与Function和Predicate打交道的Guava功能。一些其他工具类也和”函数式风格”相关，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)，和其他用常量时间返回视图的方法。尝试看看[2.3节的集合工具类](http://ifeve.com/google-guava-collectionutilities)。 Guava提供两个基本的函数式接口： * Function<A, B>，它声明了单个方法B apply(A input)。Function对象通常被预期为引用透明的——没有副作用——并且引用透明性中的”相等”语义与equals一致，如a.equals(b)意味着function.apply(a).equals(function.apply(b))。 * Predicate<T>，它声明了单个方法boolean apply(T input)。Predicate对象通常也被预期为无副作用函数，并且”相等”语义与equals一致。 ### 特殊的断言字符类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实现了Predicate<Character>，可以当作Predicate一样使用，要把Predicate转成CharMatcher，可以使用[`CharMatcher.forPredicate`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…common/base/CharMatcher.html#forPredicate%28com.google.common.base.Predicate%29)。更多细节请参考第6章-字符串处理。此外，对可比较类型和基于比较逻辑的Predicate，Range类可以满足大多数需求——它表示一个不可变区间。Range类实现了Predicate，用以判断值是否在区间内。例如，Range.atMost(2)就是个完全合法的Predicate<Integer>。更多使用Range的细节请参照第8章。 ### 操作Functions和Predicates [Functions](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Functions.html)提供简便的Function构造和操作方法，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, com.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) | 细节请参考Javadoc。相应地，[Predicates](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/base/Predicates.html)提供了更多构造和处理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, com.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) | 细节请参考Javadoc。 ## 使用函数式编程 Guava提供了很多工具方法，以便用Function或Predicate操作集合。这些方法通常可以在集合工具类找到，如Iterables，Lists，Sets，Maps，Multimaps等。 ### 断言断言的最基本应用就是过滤集合。所有Guava过滤方法都返回”视图”——_译者注：即并非用一个新的集合表示过滤，而只是基于原集合的视图_。 | **集合类型** | **过滤方法** | |:--- |:--- | | 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, com.google.common.base.Predicate%29)[`FluentIterable.filter(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co…common/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, com.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, com.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, com.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, com.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, com.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, com.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, com.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, com.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, com.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, com.google.common.base.Predicate%29) | | Multimap | [`Multimaps.filterKeys(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…ommon/collect/Multimaps.html#filterKeys%28com.google.common.collect.Multimap, com.google.common.base.Predicate%29)[`Multimaps.filterValues(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…mon/collect/Multimaps.html#filterValues%28com.google.common.collect.Multimap, com.google.common.base.Predicate%29)[`Multimaps.filterEntries(Multimap, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…on/collect/Multimaps.html#filterEntries%28com.google.common.collect.Multimap, com.google.common.base.Predicate%29) | *List的过滤视图被省略了，因为不能有效地支持类似get(int)的操作。请改用Lists.newArrayList(Collections2.filter(list, predicate))做拷贝过滤。除了简单过滤，Guava另外提供了若干用Predicate处理Iterable的工具——通常在[`Iterables`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/com/google/common/collect/Iterables.html)工具类中，或者是[`FluentIterable`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/com/google/common/collect/FluentIterable.html)的”fluent”（链式调用）方法。 | **Iterables****方法签名** | **说明** | **另请参见** | |:--- |:--- |:--- | | [`boolean all(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#all%28java.lang.Iterable, com.google.common.base.Predicate%29) | 是否所有元素满足断言？懒实现：如果发现有元素不满足，不会继续迭代 | [`Iterators.all(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#all%28java.util.Iterator, com.google.common.base.Predicate%29)[`FluentIterable.allMatch(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co…mmon/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, com.google.common.base.Predicate%29) | 是否有任意元素满足元素满足断言？懒实现：只会迭代到发现满足的元素 | [`Iterators.any(Iterator, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterators.html#any%28java.util.Iterator, com.google.common.base.Predicate%29)[`FluentIterable.anyMatch(Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co…mmon/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, com.google.common.base.Predicate%29) | 循环并返回**一个**满足元素满足断言的元素，如果没有则抛出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, com.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, com.google.common.base.Predicate, T%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, com.google.common.base.Predicate, T%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, com.google.common.base.Predicate%29) | 返回**一个**满足元素满足断言的元素，若没有则返回`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, com.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, com.google.common.base.Predicate, T%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, com.google.common.base.Predicate, T%29) | | [`indexOf(Iterable, Predicate)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Iterables.html#indexOf%28java.lang.Iterable, com.google.common.base.Predicate%29) | 返回第一个满足元素满足断言的元素索引值，若没有返回-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, com.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, com.google.common.base.Predicate%29) | 移除所有满足元素满足断言的元素，实际调用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, com.google.common.base.Predicate%29) | ### 函数到目前为止，函数最常见的用途为转换集合。同样，所有的Guava转换方法也返回原集合的视图。 | **集合类型** | **转换****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, com.google.common.base.Function%29)[`FluentIterable.transform(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release12/javadoc/co…mmon/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, com.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, com.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, com.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, com.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, com.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, com.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, com.google.common.collect.Maps.EntryTransformer%29) | | Multimap* | [`Multimaps.transformValues(Multimap, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/collect/Multimaps.html#transformValues%28com.google.common.collect.Multimap, com.google.common.base.Function%29)[`Multimaps.transformEntries(Multimap, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…collect/Multimaps.html#transformEntries%28com.google.common.collect.Multimap, com.google.common.collect.Maps.EntryTransformer%29) | | ListMultimap* | [`Multimaps.transformValues(ListMultimap, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…lect/Multimaps.html#transformValues%28com.google.common.collect.ListMultimap, com.google.common.base.Function%29)[`Multimaps.transformEntries(ListMultimap, EntryTransformer)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…ect/Multimaps.html#transformEntries%28com.google.common.collect.ListMultimap, com.google.common.collect.Maps.EntryTransformer%29) | | Table | [`Tables.transformValues(Table, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…common/collect/Tables.html#transformValues%28com.google.common.collect.Table, com.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，其中有个[`EntryTransformer<K, V1, V2>`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/google/common/collect/Maps.EntryTransformer.html)参数，它可以使用旧的键值来计算，并且用计算结果替换旧值。 *对Set的转换操作被省略了，因为不能有效支持contains(Object)操作——_译者注：懒视图实际上不会全部计算转换后的__Set__元素，因此不能高效地支持__contains(Object)__。_请改用Sets.newHashSet(Collections2.transform(set, function))进行拷贝转换。 ``` 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; } }); ``` 可以组合Function使用的类包括： | Ordering | [`Ordering.onResultOf(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…le/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, com.google.common.base.Function%29) | | Equivalence | [`Equivalence.onResultOf(Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…le/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, com.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, com.google.common.base.Function%29) | 此外，[ListenableFuture](http://code.google.com/p/guava-libraries/wiki/ListenableFutureExplained) API支持转换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)参数的方法。AsyncFunction是Function的变种，它允许异步计算值。 | [`Futures.transform(ListenableFuture, Function)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function%29) | |:--- |:--- | | [`Futures.transform(ListenableFuture, Function, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.base.Function, java.util.concurrent.Executor%29) | | [`Futures.transform(ListenableFuture, AsyncFunction)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.util.concurrent.AsyncFunction%29) | | [`Futures.transform(ListenableFuture, AsyncFunction, Executor)`](http://docs.guava-libraries.googlecode.com/git-history/release/javadoc/com/…/Futures.html#transform%28com.google.common.util.concurrent.ListenableFuture, com.google.common.util.concurrent.AsyncFunction, java.util.concurrent.Executor%29) |