I need to invert an original map. which type is <Integer, String>
, like {1 = A, 2 = A, 3 = B....}
. I want to create a new map which is String
to ArrayList
because if 1 = A
, and 2 = A
, than I want to have something like this: A = [1, 2]
.
So how can I do that?
You can try this:
HashMap<Integer, String> original = new HashMap<>();
HashMap<String, ArrayList<Integer>> inverted = new HashMap<>();
original.put(1, "A");
original.put(2, "B");
original.put(3, "C");
original.put(4, "A");
for (Integer key: original.keySet()) {
String newKey = original.get(key);
inverted.computeIfAbsent(newKey, k -> new ArrayList<>());
inverted.get(newKey).add(key);
}
System.out.println(original);
System.out.println(inverted);
So, let's say HashMap<Integer, String> original
is {1=A, 2=B, 3=C, 4=A}
, then you will get {A=[1, 4], B=[2], C=[3]}
.
EDIT: If you want a more generic version, as @Mr.Polywhirl has suggested, you can use:
public static final <T, U> Map<U, List<T>> invertMap(Map<T, U> map) {
HashMap<U, List<T>> invertedMap = new HashMap<>();
for (T key : map.keySet()) {
U newKey = map.get(key);
invertedMap.computeIfAbsent(newKey, k -> new ArrayList<>());
invertedMap.get(newKey).add(key);
}
return invertedMap;
}
You can easily do it using Java 8's stream
API, below is an example:
public static void main(String[] args) throws FileNotFoundException {
Map<Integer, String> map = new HashMap<>();
map.put(1, "A");
map.put(2, "A");
map.put(3, "B");
Map<String, List<Integer>> invertedMap = map.entrySet()
.stream()
.collect(Collectors.groupingBy(Entry::getValue,
Collectors.mapping(Entry::getKey, Collectors.toList())));
System.out.println(invertedMap);
}