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  1. import java.util.Map;
  2. import java.util.concurrent.ConcurrentHashMap;
  3.  
  4. class ListenerMap {
  5.  
  6.   final Map<Integer, Integer> listeners = new ConcurrentHashMap<>();
  7.  
  8.   void increaseListeners(final int id) {
  9.     listeners.merge(id, 1, Integer::sum);
  10.   }
  11.  
  12.   void decreaseListeners(final int id) {
  13.     listeners.computeIfPresent(id, ($, count) -> {
  14.       if (count == 1) {
  15.         /* no more listeners -- do something ? */
  16.         System.out.println("no more listeners for " + id);
  17.       }
  18.       return count - 1;
  19.     });
  20.   }
  21. }
  22.  
  23. class Ideone {
  24.  
  25.   public static void main(final String[] argv) {
  26.     final ListenerMap map = new ListenerMap();
  27.  
  28.     map.increaseListeners(1);
  29.     map.increaseListeners(2);
  30.     map.increaseListeners(2);
  31.  
  32.     for (int i = 0; i <= 2; ++i) {
  33.       map.decreaseListeners(i);
  34.     }
  35.   }	
  36. }
Success #stdin #stdout 0.08s 1372672KB
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Standard input is empty
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no more listeners for 1
https://ideone.com/PocgOQ
language:
Java (HotSpot 12)
created:
8 years ago
visibility:
public

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