Question

Is there any easy way to get the keys for which same value exist?
Or more importantly, how can i get the number of same-value-more-than-once occurrences?

Consider the hashmap:

1->A
2->A
3->A
4->B
5->C
6->D
7->D

here same-value-more-than-once occurred 3 times(A two times, D one time).That(3) is what i want in return.

I could iterate over the hashmap by the keyset/map.values() list, but it seems quite cumbersome to do that way. Any suggestions or solutions?

EDIT :
My context is, i’m working on a timetable generator. The data-structure for a time-slot is

{String day-hour, HashMap<String,Event> Rooms}

For a day-hour, some Event-s are assigned on Rooms map. While checking the fitness of the solution, i need to know if one staff is assigned multiple events on same hour. Hence i want to check how many violations are there in Rooms map by the values Event.getStaff() .

EDIT :
Values are objects here, I don’t want to count the occurrences of the same objects, rather a field of the object. The EVENT object has a field staff and i need to count the multiple occurrences of staffs.

Answer 1

This is nice way I think:

int freq = Collections.frequency(map.values(), "A");

which returns “3” for your example. Cheers!

EDIT: sorry I misunderstood the question in my first attempt, this should do the trick:

int k = 0;
Set<String> set = new HashSet<String>(map.values());
for (String s : set) {
   int i = Collections.frequency(map.values(), s);
   k += i > 1 ? i - 1 : 0;
}

You will still not be able to retreive the actual keys though. But that was not the most important thing, right?