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    <p>The monadic approach to this problem is to define an operation that transforms an iterable into an iterable of iterables, by defining a transformation function that for an object of type <code>T</code>, returns an object of type <code>Iterable&lt;T&gt;</code>. You can then concatenate each iterable to form a single one again. This combination of a mapping followed by a concatenation is called <code>concatMap</code> in Haskell and <code>flatMap</code> in Scala, and I'm sure it has other names elsewhere.</p> <p>To implement this, we first create a function that transforms your <code>S&lt;? extends Number&gt;</code> into <code>Iterable&lt;S&lt;Double&gt;&gt;</code>. This is very similar to your existing function, but our success case is an iterable of one, containing our <code>S</code>, and the failure case (our null state) is an empty iterable.</p> <pre><code>&lt;T extends Number&gt; Function&lt;S&lt;?&gt;, Iterable&lt;S&lt;T&gt;&gt;&gt; castOrNull(Class&lt;T&gt; type) { return new Function&lt;S&lt;?&gt;, Iterable&lt;S&lt;T&gt;&gt;&gt; { @Override public Iterable&lt;S&lt;T&gt;&gt; apply(S&lt;?&gt; s) { Object contained = s.get(); if (!(contained instanceof T)) { return ImmutableSet.of(); } return ImmutableSet.of(new S&lt;T&gt;(contained)); } }; } </code></pre> <p>We then apply this to the original iterable as you specify above.</p> <pre><code>Iterable&lt;Iterable&lt;S&lt;Double&gt;&gt;&gt; doubleIterables = Iterables.map(numbers, castOrNull(Double.class)); </code></pre> <p>We can then concatenate all these together to produce one iterable again, which has all of the desired values and none of those we want to remove.</p> <pre><code>Iterable&lt;S&lt;Double&gt;&gt; doubles = Iterables.concat(doubleIterables); </code></pre> <p><em>Disclaimer: I haven't tried compiling this. You may have to play around with generics to get it to work.</em></p>
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    1. USSamir Talwar
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    1. COHaving the function return an Iterable containing zero or one element is a very interesting approach. I hadn't thought of this, and I think it's great that it lets your function return null if needed. Since Guava adds the Optional type in r10 ( http://docs.guava-libraries.googlecode.com/git/javadoc/com/google/common/base/Optional.html ), I wonder if we could use a Function<S<?>, Optional<S<T>>> instead, filtering the resulting Iterable for "present" values.
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    2. COI just read luckyjaca's answer, and it seems that's the approach they use in Scala, using the "Option" type, with the added benefit of the flatMap function. See this SO answer for more info: http://stackoverflow.com/questions/1059776/scala-iterablemap-vs-iterableflatmap/1060400#1060400 "`flatMap` turns a `List[Option[A]]` into `List[A]`, with any `Option` that drills down to `None`, removed". Pretty cool.
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    3. CO@eneveu: That would be great if `Optional` implemented `Iterable`, but as far as I'm aware there are no plans to make it so. Nat Pryce's implementation of Maybe in Java (https://github.com/npryce/maybe-java) does, however. A guy at the company I work for forked it and improved it somewhat—you can see that version at https://github.com/youdevise/maybe-java .
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