java8 List 或map 根据多个字段拼接key 去重，map遍历

//先去重
Map<String, DigitalProductsUser> checkRelationMap = digitalProductsUserList.stream().filter(entity->ObjectUtil.isNotNull(entity.getProductsId())&&ObjectUtil.isNotNull(entity.getUserId())).collect(Collectors.toMap(arr -> String.valueOf(arr.getProductsId())+"_"+String.valueOf(arr.getUserId()), v -> v, (a, b) -> a));

if(ObjectUtil.isNotNull(checkRelationMap)){
            //Lamubda表达式遍历
            checkRelationMap.forEach((t,u)-> {
//                    System.out.println(t+"->>>"+u);
             
            });
         }

list转list

List<Contract> ls = null;
List<String> num_list = ls.stream().map(a -> a.getNum().split("_")[a.getNum().split("_").length-1]).collect(Collectors.toList());