private void validateClass(Class<?> source, ValidationProblemCollector problems) {
    int modifiers = source.getModifiers();

    if (Modifier.isInterface(modifiers)) {
      problems.add("Must be a class, not an interface");
    }

    if (source.getEnclosingClass() != null) {
      if (Modifier.isStatic(modifiers)) {
        if (Modifier.isPrivate(modifiers)) {
          problems.add("Class cannot be private");
        }
      } else {
        problems.add("Enclosed classes must be static and non private");
      }
    }

    Constructor<?>[] constructors = source.getDeclaredConstructors();
    for (Constructor<?> constructor : constructors) {
      if (constructor.getParameterTypes().length > 0) {
        problems.add("Cannot declare a constructor that takes arguments");
        break;
      }
    }

    Field[] fields = source.getDeclaredFields();
    for (Field field : fields) {
      int fieldModifiers = field.getModifiers();
      if (!field.isSynthetic()
          && !(Modifier.isStatic(fieldModifiers) && Modifier.isFinal(fieldModifiers))) {
        problems.add(field, "Fields must be static final.");
      }
    }
  }