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POCross-product calculator in Java
 

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    <p>I am working my way through <a href="http://rads.stackoverflow.com/amzn/click/1558601910" rel="nofollow">Norvig's book on AIP</a>. There is an exercise in it on writing a cross-product function -</p> <pre><code>(defun cross-product (fn list-1 list-2) (mappend #'(lambda (y) (mapcar #'(lambda (x) (funcall fn y x)) list-2)) list-1)) (defun mappend (fn the-list) (if (null the-list) nil (append (funcall fn (first the-list)) (mappend fn (rest the-list))))) </code></pre> <p>I am trying to write an implementation in Java - </p> <pre><code>interface Function&lt;T1, T2, T3&gt; { public T3 function(T1 t1, T2 t2); } public class CrossProduct&lt;T1, T2&gt; { private List&lt;T1&gt; list1; private List&lt;T2&gt; list2; public CrossProduct(List&lt;T1&gt; t1, List&lt;T2&gt; t2) { this.list1 = t1; this.list2 = t2; } public &lt;T3&gt; List&lt;T3&gt; calculate(Function&lt;T1, T2, T3&gt; fn) { List product = new ArrayList(); for (int i = 0; i &lt; list1.size(); i++) for (int j = 0; j &lt; list2.size(); j++) product.add(fn.function(list1.get(i), list2.get(j))); return product; } </code></pre> <p>}</p> <p>Usage -</p> <pre><code>@Test public void testWithStrings() { List&lt;String&gt; list1 = new ArrayList&lt;String&gt;(); list1.add("6"); list1.add("8"); List&lt;String&gt; list2 = new ArrayList&lt;String&gt;(); list2.add("2"); list2.add("3"); List&lt;String&gt; product = new CrossProduct&lt;String, String&gt;(list1, list2) .&lt;String&gt; calculate(new Function&lt;String, String, String&gt;() { public String function(String x, String y) { return (String) x + (String) y; } }); Assert.assertEquals("62", product.get(0)); Assert.assertEquals("63", product.get(1)); Assert.assertEquals("82", product.get(2)); Assert.assertEquals("83", product.get(3)); } </code></pre> <p>Is there a better way of doing this?</p>
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