import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;
import java.lang.reflect.Field;
import java.util.ArrayList;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface NotNull {}

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.FIELD)
@interface MinLength {
    int value() default 3;
}

public class Validator {
    public static List<String> validate(Object obj) {
        List<String> invalidFields = new ArrayList<>();
        Field[] fields = obj.getClass().getDeclaredFields();
        for (Field field : fields) {
            field.setAccessible(true);
            if (field.isAnnotationPresent(NotNull.class)) {
                try {
                    if (field.get(obj) == null) {
                        invalidFields.add(field.getName());
                    }
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
        }
        return invalidFields;
    }

    public static Map<String, List<String>> advancedValidate(Object obj) {
        Map<String, List<String>> invalidFieldsMap = new HashMap<>();
        Field[] fields = obj.getClass().getDeclaredFields();
        for (Field field : fields) {
            field.setAccessible(true);
            List<String> errorMessages = new ArrayList<>();
            if (field.isAnnotationPresent(NotNull.class)) {
                try {
                    if (field.get(obj) == null) {
                        errorMessages.add("Field cannot be null");
                    }
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
            if (field.isAnnotationPresent(MinLength.class)) {
                MinLength annotation = field.getAnnotation(MinLength.class);
                try {
                    String value = (String) field.get(obj);
                    if (value != null && value.length() < annotation.value()) {
                        errorMessages.add("Field length must be at least " + annotation.value());
                    }
                } catch (IllegalAccessException e) {
                    e.printStackTrace();
                }
            }
            if (!errorMessages.isEmpty()) {
                invalidFieldsMap.put(field.getName(), errorMessages);
            }
        }
        return invalidFieldsMap;
    }
}