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Lists or Iterables can be filtered easily using guavas filter(Iterable<?> unfiltered, Class<T> type) . This operation performs two tasks: the list is filtered and transformed into a sequence of the given type T. Quite often however I end up with Iterables<Something<?>> and I want to get a subsequence of Iterables<Something<T>> for some specialized T. It is clear, that Guava can't solve this problem out of the box due to type erasure: Something<T> does not provide any direct information about its T. Lets say I have something like S<? extends Number> . If I am able to define some predicate which tells me if S<?> may be casted to S<Double> I may use it as a filer: <T extends Number> Predicate<S<?>> isOfType(Class<N> type) {...} with: Iterable<S<?>> numbers; Iterable<S<?>> filtered = Iterable.filter(numbers, isOfType(Double.class)); This performs the task of filtering but it misses the transformation step. If I think my Predicate works well I may even think of casting: Iterable<S<Double>> doubles = (Iterable<S<Double>>) filtered; But this exposes some ugly cast operation. As an alternative I may provide a Function<S<?>, S<Double>> to perform the cast. In constrast to Class.cast() however it should not throw a ClassCastException b

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