Better way to construct REST request params?

RestROFactory.java

package com.dealer.modules.cms.promotions.factory;

import java.util.AbstractMap;
import java.util.Arrays;
import java.util.List;
import java.util.Locale;
import java.util.Map;
import static java.util.stream.Collectors.toList;
import static java.util.stream.Collectors.toMap;

public class RestROFactory {

	// the enum represent the query string to filter down the promotions.
	// enum give a nice way to see all the params instead of a map you have to know all the keys
	public enum QueryRequestEnum {
		ACCOUNT_ID("accountId", ""), //required
		IS_ACTIVE("isActive",true), //optional
		TYPE("type", Arrays.asList(PromotionType.VEHICLE)),//optional, w/o type get all types result back
		SORT("sort", "-dateCreated"),//optional
		INCLUDE_AUTO_GENERATED("includeAutoGenerated", false),//optional
		LOCALE("locale", Locale.US),
		PROMO_IDS("promotionId", "");
		private final String key;
		private Object value;

		QueryRequestEnum(String k, Object v){
			this.key = k;
			this.value = v;
		}

		public String key(){
			return key;
		}

		public Object getValue() {
			return value;
		}

		public QueryRequestEnum setValue(Object o) {
			this.value = o;
			return this;
		}

		//consumers(banner,listing,slideshow) create enum request list this method convert it into rest query string
		public static Map<String, List<String>> buildResourceQueryParams(List<RestROFactory.QueryRequestEnum> requestEnums) {
			Map<String, List<String>> queryParams = requestEnums.stream()
					.collect(toMap(RestROFactory.QueryRequestEnum::key, RestROFactory.QueryRequestEnum::getValue))
					.entrySet().stream()
					.map(e -> {
						if(e.getValue() instanceof List<?>){
							return new AbstractMap.SimpleEntry<>(e.getKey(), ((List<?>)e.getValue()).stream().map(i -> i.toString()).collect(toList()));
						}else{
							return new AbstractMap.SimpleEntry<>(e.getKey(), Arrays.asList(e.getValue().toString()));
						}

					}).collect(toMap(AbstractMap.SimpleEntry::getKey, AbstractMap.SimpleEntry::getValue));

			return queryParams;
		}
	}

	public static PromoRO createPromoRO(Placement placement, List<QueryRequestEnum> requestEnums){
		return new PromoRO(placement, requestEnums);
	}

	public static class PromoRO {
		private String accountId;
		private Locale locale;
		private Placement placement;
		private List<QueryRequestEnum> requestEnums;

		private PromoRO(Placement placement, List<QueryRequestEnum> requestEnums) {
			if(requestEnums.contains(QueryRequestEnum.ACCOUNT_ID)){
				this.accountId = requestEnums.get(requestEnums.indexOf(QueryRequestEnum.ACCOUNT_ID)).getValue().toString();
			}else {
				throw new IllegalArgumentException(QueryRequestEnum.ACCOUNT_ID + " is required.");
			}
			if(requestEnums.contains(QueryRequestEnum.LOCALE)){
				this.locale = (Locale)requestEnums.get(requestEnums.indexOf(QueryRequestEnum.LOCALE)).getValue();
			}else{
				this.locale = Locale.US;
			}
			this.placement = placement;
			this.requestEnums = requestEnums;
		}

		public String getAccountId() {
			return accountId;
		}

		public Locale getLocale() { return locale; }
		public Placement getPlacement() {
			return placement;
		}

		public Map<String, List<String>> getQueryStrings(){
			return QueryRequestEnum.buildResourceQueryParams(requestEnums);
		}
	}
}