code: public <E extends Exception> int computeIfAbsent(T e, Try.Function<? super T, Integer, E> mappingFunction) throws E {
          N.checkArgNotNull(mappingFunction);
  
          final int oldValue = get(e);
  
          if (oldValue > 0) {
              return oldValue;
          }
  
          final int newValue = mappingFunction.apply(e);
  
          if (newValue > 0) {
              set(e, newValue);
          }
  
          return newValue;
      }
docs: The implementation is equivalent to performing the following steps for this Multiset:
  
  <pre>
  final int oldValue = get(e);
  
  if (oldValue > 0) {
  return oldValue;
  }
  
  final int newValue = mappingFunction.apply(e);
  
  if (newValue > 0) {
  set(e, newValue);
  }
  
  return newValue;
  </pre>
  
  @param e
  @param mappingFunction
  @return