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copied!<p>A quick driver of this showed it to work. Not responsible for all test cases. :-)</p> <pre><code>List<String> combineListsHorizontally(List<String> a, List<String> b) { assert a.size() == b.size(); // just avoids some checks List<String> result = new ArrayList<String>(a.size()); Iterator<String> itera = a.iterator(); Iterator<String> iterb = b.iterator(); for(int i = 0; i < a.size(); i++) { String combined = itera.next() + iterb.next(); result.add(combined); } return result; } </code></pre> <p>If you need something generic, you would need to know they ahve a way that they can be joined</p> <pre><code> List<E> combineListsHorizontally(List<E> a, List<E> b) { assert a.size() == b.size(); // just avoids some checks List<E> result = new ArrayList<E>(a.size()); Iterator<E> itera = a.iterator(); Iterator<E> iterb = b.iterator(); for(int i = 0; i < a.size(); i++) { E combined = new MagicCombiner<E>(a,b).get(); // define this line yourself result.add(combined); } return result; } </code></pre> <p>///////////////// EDIT - here's a working example based off @Brents (superior) example. Props to him for illustrating this pattern better than I did.</p> <pre><code>import java.util.*; /** * Compile: "javac BinaryListUtils" * Usage: "java BinaryListUtils" C:\Documents and Settings\user\My Documents>javac BinaryListUtils.java C:\Documents and Settings\user\My Documents>java BinaryListUtils APPLEBANANA PEARORANGE C:\Documents and Settings\user\My Documents> */ // general use transformer class BinaryListUtils { // tranformer static interface BinaryTransformer<X, Y, Z> { Z transform(X a, Y b); } // implementation for your problem static class ConcatTransformer implements BinaryTransformer<String, String, String> { public String transform(String a, String b) { return a + b; } } public static <X, Y, Z> List<Z> collect(List<X> aList, List<Y> bList, BinaryTransformer<X, Y, Z> t) { List<Z> ret = new ArrayList<Z>(aList.size()); Iterator<X> aIter = aList.iterator(); Iterator<Y> bIter = bList.iterator(); while(aIter.hasNext()) { ret.add(t.transform(aIter.next(), bIter.next())); } return ret; } public static void main(String[] args) { List<String> aList = new ArrayList<String>(); List<String> bList = new ArrayList<String>(); aList.add("APPLE"); aList.add("PEAR"); bList.add("BANANA"); bList.add("ORANGE"); ConcatTransformer ct = new ConcatTransformer(); List<String> cList = BinaryListUtils.collect(aList,bList,ct); for(String s : cList) System.out.println(s); } } </code></pre>

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