http://blog.cgdecker.com/2010/06/property-interfaces-and-guava.html

FilterByName.java

List<Person> people = ...
Iterable<Person> bobs = filter(people, compose(equalTo("Bob"),
  Names.GET));

HasId.java

interface HasId {
   Long getId();
}

Ids.java

class Ids {
  private Ids() {}

  public static final Function<HasId, Long> GET = 
    new Function<HasId, Long>() {
      public Long apply(HasId hasId) {
        return hasId.getId();
      }
  };

  public static Iterable<Long> of(
      Iterable<? extends HasId> items) {
    return Iterables.transform(items, GET);
  }
}

IterateIds.java

List<Person> people = ...
for(Long id : Ids.of(people)) { ... }