gistfile1.java

public interface Function<T, R> {
	R apply(T t);
}

public interface GroupBy<E> {
	Map<Object,List<E>> groupBy(Function<E,Object> f);
}


public class GroupByList<E> extends ArrayList<E> implements GroupBy<E> {
	
	private static final long serialVersionUID = 1L;

	public Map<Object,List<E>> groupBy(Function<E,Object> f){
		Map<Object,List<E>> result = new HashMap<Object,List<E>>();
		for(E e : this){
			Object key = f.apply(e);
			List<E> list = result.get(key);
			if(list == null){
				list = new ArrayList<E>();
			}
			list.add(e);
			result.put(key, list);
		}
		return result;
	}
}


public class GroupByListTest {
	
	public static List<Integer> oddList = new ArrayList<Integer>();
	
	{
		oddList.add(1);
		oddList.add(3);
		oddList.add(5);
	}
	
	public static List<Integer> evenList = new ArrayList<Integer>();
	
	{
		evenList.add(2);
		evenList.add(4);
		evenList.add(6);
	}

	@Test
	public void test() {
		GroupByList<Integer> intList = new GroupByList<Integer>();

		intList.add(1);
		intList.add(2);
		intList.add(3);
		intList.add(4);
		intList.add(5);
		intList.add(6);

		Map<Object, List<Integer>> groupBy = intList
				.groupBy(new Function<Integer, Object>() {
					@Override
					public Object apply(Integer t) {
						return t % 2 == 0;
					}
				});

		assertNotNull(groupBy);
		assertThat(groupBy.get(true), is(evenList));
		assertThat(groupBy.get(false), is(oddList));
		
	}

}