code: public static TestConstructor findSingle(Class<?> cls) {
      final TestConstructor[] constructors = findAll(cls);
  
      if (constructors.length == 0) {
        throw new IllegalStateException(cls.getName() + " requires at least 1 public constructor");
      } else if (constructors.length == 1) {
        return constructors[0];
      } else {
        throw new IllegalStateException("Class "
            + cls.getName()
            + " has too many parameterized constructors. "
            + "Should only be 1 (with enum variations).");
      }
    }
docs: Find the parameterized constructor of cls if there is one, or the default constructor if
  there isn't.
  
  @throws IllegalStateException if there are multiple parameterized constructors