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    <p>There are basically two useful libraries that can help with this; <a href="http://guava-libraries.googlecode.com/" rel="nofollow noreferrer">Google Guava</a> and <a href="http://commons.apache.org/collections/" rel="nofollow noreferrer">Apache Commons Collections</a>.</p> <p>What you are trying to do is basically two operations, first mapping, then reduction. I've never used Commons Collections to any extent myself so I can't tell you more about that, but I know there is no support for reduction (or folding) in Google Guava at least (see <a href="http://code.google.com/p/guava-libraries/issues/detail?id=218" rel="nofollow noreferrer">Issue 218</a>). This is not too hard to add yourself though (not tested):</p> <pre><code>interface Function2&lt;A, B&gt; { B apply(B b, A a); } public class Iterables2 { public static &lt;A, B&gt; B reduce(Iterable&lt;A&gt; iterable, B initial, Function2&lt;A, B&gt; fun) { B b = initial; for (A item : iterable) b = fun.apply(b, item); return b; } } </code></pre> <p>That way you can combine it with Guavas Iterables.transform() like so:</p> <pre><code>class Summer implements Function2&lt;Integer, Integer&gt; { Integer apply(Integer b, Integer a) { return b + a; } } class MyMapper&lt;T&gt; implements Function&lt;T, Integer&gt; { Integer apply(T t) { // Do stuff } } </code></pre> <p>And then (provided you've import static'ed the relevant classes):</p> <pre><code>reduce(transform(iterable, new MyMapper()), 0, new Summer()); </code></pre> <p>Also see <a href="https://stackoverflow.com/questions/950751/how-to-implement-a-list-fold-in-java">this question</a>.</p>
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