code: public int computeIfAbsent(final int key, final IntUnaryOperator mappingFunction)
      {
          int value = get(key);
          if (value == missingValue)
          {
              value = mappingFunction.applyAsInt(key);
              if (value != missingValue)
              {
                  put(key, value);
              }
          }
  
          return value;
      }
docs: Primitive specialised version of {@link #computeIfAbsent(Object, Function)}
  
  @param key             to search on.
  @param mappingFunction to provide a value if the get returns null.
  @return the value if found otherwise the missing value.