groupby for java 6 & 7

groupby.java

/**
 * A generic interface for an one-element lambda
 */
private interface Predicate<P1, R> {
    R apply(P1 p1);
}

private static <T, C> List<List<T>> groupBy(Collection<T> collection,
                                            Predicate<? super T, C> byValue) {
    // PRECONDITION collection and byValue must not be null
    // PRECONDITION collection must be sorted by the value byValue returns
    // PRECONDITION byValue must never return null
    C currentItem = null;
    List<List<T>> result = new ArrayList<List<T>>();
    for (T item : collection) {
        C itemValue = byValue.apply(item);
        if (itemValue == null) {
            throw new RuntimeException("byValue must never return null");
        }

        if (currentItem != itemValue) {
            currentItem = itemValue;
            result.add(new ArrayList<T>());
        }

        result.get(result.size() - 1).add(item);
    }

    return result;
}