with current data-structure you must loop over the map:
ArrayList<String> relatedKeys = new ArrayList<String>();
for(String k : departmentList.keySet()){
if(departmentList.get(k).equals(searchKey))
relatedKeys.add(k);
at the end, you have a list of all related keys and can use
for(String k : relatedKeys)
System.out.println(k);