Question

I’ve got an existing instance of HashMap simply called sale (it is Map<String, Set<String>>) I use it to log customers and items history.

Is there a way to create a new instance of HashMap, that effectively reverses this usage? i.e will show each item purchased as a unique key and the corresponding value as a String set of the customers that have purchased that product. I suspect there is a simple process using keySet() in some way to iterate over the sales map but I just can’t see how to do this.
Any help would be much appreciated.

Answer 1

do you mean you want some thing like Map, String> !!

Thn you can iterate over existing map and put reversed key values in a new map

Example:

Map<Set<String>, String> somemap = new HashMap<Set<String>, String>();
foreach(Map.entry entry : existingMap.entrySet()) {
  Set<String> value = entry.getValue();
  String key = entry.getKey();
  somemap.put(value,key);

}