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    <p>Guava has <a href="http://docs.guava-libraries.googlecode.com/git-history/v10.0.1/javadoc/com/google/common/collect/Maps.html#uniqueIndex%28java.lang.Iterable,%20com.google.common.base.Function%29">Maps.uniqueIndex(Iterable values, Function keyFunction)</a> and <a href="http://docs.guava-libraries.googlecode.com/git-history/v10.0.1/javadoc/com/google/common/collect/Multimaps.html#index%28java.lang.Iterable,%20com.google.common.base.Function%29">Multimaps.index(Iterable values, Function keyFunction)</a>, but they don't transform the values. There are <a href="http://code.google.com/p/guava-libraries/issues/detail?id=679">some</a> <a href="http://code.google.com/p/guava-libraries/issues/detail?id=56">requests</a> to add utility methods that do what you want, but for now, you'll have to roll it yourself using Multimaps.index() and Multimaps.transformValues():</p> <pre><code>static class Person { private final Integer age; private final String name; public Person(Integer age, String name) { this.age = age; this.name = name; } public Integer getAge() { return age; } public String getName() { return name; } } private enum GetAgeFunction implements Function&lt;Person, Integer&gt; { INSTANCE; @Override public Integer apply(Person person) { return person.getAge(); } } private enum GetNameFunction implements Function&lt;Person, String&gt; { INSTANCE; @Override public String apply(Person person) { return person.getName(); } } public void example() { List&lt;Person&gt; persons = ImmutableList.of( new Person(100, "Alice"), new Person(200, "Bob"), new Person(100, "Charles"), new Person(300, "Dave") ); ListMultimap&lt;Integer, String&gt; ageToNames = getAgeToNamesMultimap(persons); System.out.println(ageToNames); // prints {100=[Alice, Charles], 200=[Bob], 300=[Dave]} } private ListMultimap&lt;Integer, String&gt; getAgeToNamesMultimap(List&lt;Person&gt; persons) { ImmutableListMultimap&lt;Integer, Person&gt; ageToPersons = Multimaps.index(persons, GetAgeFunction.INSTANCE); ListMultimap&lt;Integer, String&gt; ageToNames = Multimaps.transformValues(ageToPersons, GetNameFunction.INSTANCE); // Multimaps.transformValues() returns a *lazily* transformed view of "ageToPersons" // If we want to iterate multiple times over it, it's better to create a copy return ImmutableListMultimap.copyOf(ageToNames); } </code></pre> <hr> <p>A re-usable utility method could be:</p> <pre><code>public static &lt;E, K, V&gt; ImmutableListMultimap&lt;K, V&gt; keyToValuesMultimap(Iterable&lt;E&gt; elements, Function&lt;E, K&gt; keyFunction, Function&lt;E, V&gt; valueFunction) { ImmutableListMultimap&lt;K, E&gt; keysToElements = Multimaps.index(elements, keyFunction); ListMultimap&lt;K, V&gt; keysToValuesLazy = Multimaps.transformValues(keysToElements, valueFunction); return ImmutableListMultimap.copyOf(keysToValuesLazy); } </code></pre> <p>I guess we could improve the generics in the signature by using <code>Function&lt;? extends E, K&gt;</code> or something, but I don't have the time to delve further...</p>
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    1. COThank you, @eneveu, that does it nicely.
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