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POImplementing simple functional-like paradigm in Java collections and type casting
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copied!<p>I've recently started working in Scala and that was my first real contact with a functional paradigm. Although I'm a big fan of Java and, I admit, it lacks functional paradigm sometimes. </p> <p>That's why I recently started a a mini pet-project to see if such thing, to a degree, could be achieved in Java. </p> <p>I started with a simple modification of an array list, and this is what I have so far:</p> <hr> <p>An interface any collection needs to implement in order to offer applying functions to it's elements:</p> <pre><code>public interface Functionalizable<E> { public Collection<E> apply(Function<E> f); } </code></pre> <p>An interface that defines a method for applying the function on a single element:</p> <pre><code>public interface Function<E> { public E apply(E e); } </code></pre> <p>A concrete class backed by an array list that allows applying functions on it's elements:</p> <pre><code>public class FunctionArrayList<E> implements List<E>, Functionalizable<E> { private List<E> list; //implemented methods from `List` interface and ctors @Override public List<E> apply(Function<E> f) { List<E> applied = new FunctionArrayList<>(this.list.size()); for (E e : this.list) { applied.add(f.apply(e)); } return applied; } } </code></pre> <p>I've written s small test method for Integer and it works OK: </p> <p>Code:</p> <pre><code> List<Integer> listOfIntegersBefore = new FunctionArrayList<>(); listOfIntegersBefore.add(-1); listOfIntegersBefore.add(0); listOfIntegersBefore.add(1); listOfIntegersBefore.add(2); listOfIntegersBefore.add(3); listOfIntegersBefore.add(4); System.out.println("Before<Integer>: " + listOfIntegersBefore.toString()); List<Integer> listOfIntegersAfter = ((FunctionArrayList<Integer>) listOfIntegersBefore).apply(new Function<Integer>() { @Override public Integer apply(Integer e) { return (e + 1); } }); System.out.println("After<Integer> : " + listOfIntegersAfter.toString()); </code></pre> <p>Output:</p> <pre><code>Before<Integer>: [-1, 0, 1, 2, 3, 4] After<Integer> : [0, 1, 2, 3, 4, 5] </code></pre> <hr> <p>However, when I try a tad more complex thing with List, I end up with much type-casting, which I don't like (and I'd like to avoid it as much as possible).<br> Code:</p> <pre><code> List<List<Integer>> listOfListOfIntegersBefore = new FunctionArrayList<>(); List<Integer> temp = new FunctionArrayList<>(); temp.add(1); listOfListOfIntegersBefore.add(temp); temp = new FunctionArrayList<>(); temp.add(1); temp.add(2); listOfListOfIntegersBefore.add(temp); temp = new FunctionArrayList<>(); temp.add(1); temp.add(2); temp.add(3); listOfListOfIntegersBefore.add(temp); temp = new FunctionArrayList<>(); temp.add(1); temp.add(2); temp.add(3); temp.add(4); listOfListOfIntegersBefore.add(temp); List<List<Integer>> listOfListOfIntegersAfter = (List<List<Integer>>) ((Functionalizable<List<Integer>>) listOfListOfIntegersBefore).apply(new Function<List<Integer>>() { @Override public List<Integer> apply(List<Integer> e) { List<Integer> list = new FunctionArrayList<>(e); return ((FunctionArrayList<Integer>) list).apply(new Function<Integer>() { @Override public Integer apply(Integer e) { return (e + 1); } }); } }); System.out.println("Before<List<Integer>>: " + listOfListOfIntegersBefore); System.out.println("After<List<Integer>> : " + listOfListOfIntegersAfter); </code></pre> <p>Output:</p> <pre><code>Before<List<Integer>>: [[1], [1, 2], [1, 2, 3], [1, 2, 3, 4]] After<List<Integer>> : [[2], [2, 3], [2, 3, 4], [2, 3, 4, 5]] </code></pre> <hr> <p>As I have already mentioned, I would like to avoid casting. Plus, Eclipse warns me about <code>Type safety: Unchecked cast from List<List<Integer>> to Functionalizable<List<Integer>></code> on this line:</p> <pre><code>List<List<Integer>> listOfListOfIntegersAfter = (List<List<Integer>>) ((Functionalizable<List<Integer>>) listOfListOfIntegersBefore).apply(new Function<List<Integer>>() { ... } </code></pre> <p>Is there an elegant way to accomplish that?</p>

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