# 'u' Replaced By 'U' Is A Big Error,Insight From My On Going Spring Boot Project

> Source: <https://dev.to/tanisha_suyal_8efb4a5b201/u-replaced-by-u-is-a-big-errorinsight-from-my-on-going-spring-boot-project-eo>
> Published: 2026-08-22 16:42:25+00:00

For Three days I was struck on running my Cart endpoints.Everything seems correct.

But the Error on running the endpoint was :

```
{
  "success": false,
  "message": "The given id must not be null",
  "data": null
}
```

Somewhere the ID was becoming null before it reaches the repository even when i was passing the Id.

Let me first clear the endpoints and scenario to you.

I am Building an Full Fledged AI-aided E-com project using spring boot,OpenAI Models and BLIP-CLIP models for the Image-to-Text and Text-to-Image processing.

Right now,I am creating the Entities,Repository,Services and Controllers for basic working of the website.

I am also using DTO's for the structured data transfer.

The problem started with Cart endpoints,my 'addToCart' service was not actually adding the products into the cart even when I was passing the values I was getting the id to be null.

`/cart/addToCart`

Request body

application/json

Example Value

Schema

{

"userId": 9007199254740991,

"productId": 9007199254740991,

"quantity": 1073741824

}

Response

{

"success": boolean,

"message": "string",

"data": "string"

}

I queried all the sources and I was getting more confused,Nothing Clearly was error there in my code yet the endpoints were not responsing.

checklist :

`@RequestBody`

for JSON data sent in a POST or PUT body.`@RequestParam`

for query parameters in the URL (like ?name=test).`@PathVariable`

for variables built directly into the URL path (like /users/{id}).Ensure your JSON property names match your Java class variable names exactly.

Verify that your DTO class has getter and setter methods (or use Lombok `@Data`

or `@Getter`

/`@Setter`

), which Jackson needs to read the data.

Check that the Content-Type header in your client request is set to application/json when sending a JSON body.

Lets take one example,we are creating a cart service in an e-com project and we have this controller for add to cart service.

```
ResponseEntity<ApiResponse<String>> addToCart(@RequestBody CartRequest request){
        String status = service.addToCart(request);

        ApiResponse<String> response =
                new ApiResponse<>(
                        true,
                        "Added",
                        status
                );

        return ResponseEntity.ok(response);
    }
```

With the response DTO as cartRequest as follows:

```
public class CartRequest {

    @NotNull
    @Positive
    private Long UserId;

    @NotNull
    @Positive
    private Long ProductId;

    @NotNull
    @Positive
    @Min(1)
    private Integer Quantity;

}
```

Everything is in sync right?

Congrats! you have hit the error.

Remember?

Ensure your JSON property names match your Java class variable names exactly.

This is the error I was struck into, this the 'u' replaced with 'U'.Notice the Capital letter at the beginning of the the DTO variables.

```
public class CartRequest {

    @NotNull
    @Positive
    private Long ~~UserId;~~

    @NotNull
    @Positive
    private Long ~~ProductId;~~

    @NotNull
    @Positive
    @Min(1)
    private Integer ~~Quantity;~~

}
```

Conventions Matters and not every advice and error handling will be provided to you as such.

keep Things in mind and follow conventions for devlopment.