code: public static <T> void throwIllegalStateExceptionIfAlreadyInstantiated(Class<T> clazz) {
          if (isInstantiated(clazz)) {
              throw new IllegalStateException(clazz.getSimpleName() + " has already been instantiated. Call SingletonMap.getInstance(" + clazz.getSimpleName() + ".class) to get the initialized instance.");
          }
      }
docs: Throws an IllegalStateException if {@link #isInstantiated(Class)} returns {@code true}, does nothing otherwise
  
  @param clazz The class to check if it is already instantiated
  @param <T>   The type of the class to check