public static <K, V, V2> Map<K, Collection<V2>> groupBy(
Function<V, K> keyFn, Function<V, V2> valueFn, Collection<V> xs) {
checkNotNull(xs, "xs cannot be null");
checkNotNull(keyFn, "keyFn cannot be null");
checkNotNull(valueFn, "valueFn cannot be null");
Map<K, Collection<V2>> result = MapUtilities.newHashMap();
for (V v : xs) {
K key = keyFn.apply(v);
Collection<V2> bucket = result.computeIfAbsent(key, u -> newArrayList());
bucket.add(valueFn.apply(v));
result.put(key, bucket);
}
return result;
}
/**
* Similar to groupby, however the valueFn runs over the entire group after the initial grouping
* has been performed
*
* @param keyFn - extracts/derives the grouping key
* @param valueFn - function which transforms each group
* @param xs - initial values
* @param <K> - key type
* @param <V> - (initial value type)
* @param <V2> - resultant value type
* @return
*/
public static <K, V, V2> Map<K, V2> groupAndThen(
Function<V, K> keyFn, Function<Collection<V>, V2> valueFn, Collection<V> xs) {
checkNotNull(xs, "xs cannot be null");
checkNotNull(keyFn, "keyFn cannot be null");
checkNotNull(valueFn, "valueFn cannot be null");
Map<K, V2> result = MapUtilities.newHashMap();
Map<K, Collection<V>> step1 = groupBy(keyFn, xs);
for (Map.Entry<K, Collection<V>> entry : step1.entrySet()) {
K key = entry.getKey();
Collection<V> group = entry.getValue();
V2 transformedGroup = valueFn.apply(group);
result.put(key, transformedGroup);
}
return result;
}