@Override
    public ValidationResult validate(Object value) {
        if (value == null || value instanceof String) {
            return new ValidationResult.ValidationPassed();
        } else {
            return new ValidationResult.ValidationFailed("Value \"" + value + "\" is not a valid string!");
        }
    }
@Test
    public void validateNonString() {
        assertThat(validator.validate(new Object())).isInstanceOf(ValidationResult.ValidationFailed.class);
    }