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POFiltering lists of generic types
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2011-08-26T12:29:19.560
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2014-06-16T21:29:11.243
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Lists or Iterables can be filtered easily using guavas <a href="http://guava-libraries.googlecode.com/svn/tags/release09/javadoc/com/google/common/collect/Iterables.html#filter%28java.lang.Iterable,%20java.lang.Class%29" rel="nofollow noreferrer"><code>filter(Iterable<?> unfiltered, Class<T> type)</code></a>. 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 <code>Iterables<Something<?>></code> and I want to get a subsequence of <code>Iterables<Something<T>></code> for some specialized T. It is clear, that Guava can't solve this problem out of the box due to type erasure: <code>Something<T></code> does not provide any direct information about its T. Lets say I have something like <code>S<? extends Number></code>. If I am able to define some predicate which tells me if <code>S<?></code> may be casted to <code>S<Double></code> I may use it as a filer: <pre><code><T extends Number> Predicate<S<?>> isOfType(Class<N> type) {...} </code></pre> with: <pre><code>Iterable<S<?>> numbers; Iterable<S<?>> filtered = Iterable.filter(numbers, isOfType(Double.class)); </code></pre> 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: <pre><code>Iterable<S<Double>> doubles = (Iterable<S<Double>>) filtered; </code></pre> But this exposes some ugly cast operation. As an alternative I may provide a <code>Function<S<?>, S<Double>></code> to perform the cast. In constrast to <a href="http://download.oracle.com/javase/6/docs/api/java/lang/Class.html#cast%28java.lang.Object%29" rel="nofollow noreferrer"><code>Class.cast()</code></a> however it should not throw a <code>ClassCastException</code> but simply return <code>null</code> if the element can not be casted (or transformed). This way the sequence may be converted without any explicit cast: <pre><code><T extends Number> Function<S<?>, S<T>> castOrNull(Class<N> type) {...} Iterable<S<Double>> doubles = Iterable.filter(numbers, castOrNull(Double.class)); </code></pre> But the list is not really filtered: instead it still contains null objects for each element which could not converted or casted to <code>S<Double></code>. But this may solved easily by an additional filtering step like: <pre><code>Iterable<S<Double>> doubles = Iterables.filter(doubles, Predicates.notNull()); </code></pre> The second solution seems much smarter to me. The <code>Function</code> to be defined may either perform a cast (which hides the unchecked operation) or it may really create some new Object <code>S<T></code> if necessary. The remaining question is: Is there any smarter way to perform the necessary converting and filtering by a single step? I may simply define some utility function like: <pre><code><I,O> Iterables<O> convert( Iterables<O> input, Function<? super I, ? extends O> convert, Predicate<? super O> filter); <I,O> Iterables<O> convert( Iterables<O> input, Function<? super I, ? extends O> convert); </code></pre> Where the second function is a short cut of the first one with a <code>Predicates.notNull()</code>; But it's worth to have the first function, too, as the predicate is not necessary <code>Predicates.notNull()</code>. Imagine an <code>Iterable<Iterable<? extends Number>></code>. The converter function <code>Function<Iterable<? extends Number>, Iterable<Double>></code> may simply return a filtered sequence which may be empty instead of returning null. The additional filter may finally drop empty sequences using <code>Iterables.isEmpty()</code>. 
Tags
<java><generics><guava><iterable>
Title
Filtering lists of generic types
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1. COIt would be useful if `Iterable.filter(...)` returns an iterable with extended functionality so you can chain filters. `/* S extends Collection */ Iterable<S<Double>> doubles = Iterable.filter(numbers, castOrNull(Double.class)).filter(Predicates.notNull()).filter(Predicates.notEmpty());`
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2. COWhy do you want to do it in a single step? Transformation and filtering are distinct operations.
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