code: public static @FieldDescriptor String primitiveTypeNameToFieldDescriptor(String primitiveName) {
      String result = primitiveClassesJvm.get(primitiveName);
      if (result == null) {
        throw new IllegalArgumentException("Not the name of a primitive type: " + primitiveName);
      }
      return result;
    }
docs: Convert a primitive Java type name (e.g., "int", "double", etc.) to a field descriptor (e.g.,
  "I", "D", etc.).
  
  @param primitiveName name of the type, in Java format
  @return name of the type, in field descriptor format
  @throws IllegalArgumentException if primitiveName is not a valid primitive type name