public static <K, V> Map<K, V> parseListToMap(Collection<V> list, Function<V, K> mapper) {
if (list == null || list.size() == 0) {
return null;
}
Map<K, V> map = new HashMap<>();
for (V v : list) {
map.put(mapper.apply(v), v);
}
return map;
}
public static <K, V> Map<K, List<V>> parseListToListMap(List<V> list, Function<V, K> mapper) {
if (list == null || list.size() == 0) {
return null;
}
Map<K, List<V>> map = new HashMap<>();
for (V v : list) {
if (map.containsKey(mapper.apply(v))) {
map.get(mapper.apply(v)).add(v);
}
else {
List<V> values = new ArrayList<>();
values.add(v);
map.put(mapper.apply(v), values);
}
}
return map;
}
public static <K, V> Map<K, Integer> frequencyOfListElements(Collection<V> list, Function<V, K> mapper) {
if (null == list || list.size() == 0) {
return null;
}
Map<K, Integer> map = new HashMap<>();
for (V v : list) {
Integer count = map.get(mapper.apply(v));
map.put(mapper.apply(v), (null == count) ? 1 : (count + 1));
}
return map;
}
