code: public Map<Integer, String> getSignatures() {
          ensureRunning();
          // Make a copy so callers get an immutable snapshot of the current state.
          return Collections.unmodifiableMap(new HashMap<Integer, String>(signatures));
      }
docs: Get the signatures that have been computed for all tracks currently loaded in any player for which we have
  been able to obtain all necessary metadata.
  
  @return the signatures that uniquely identify the tracks loaded in each player
  
  @throws IllegalStateException if the SignatureFinder is not running