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

Tanisha Suyal — Sat, 22 Aug 2026 16:42:25 +0000

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.

Project Explanation

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 Endpoint

/cart/addToCart
Request body
application/json
Example Value
Schema
{
"userId": 9007199254740991,
"productId": 9007199254740991,
"quantity": 1073741824
}
Response
{
"success": boolean,
"message": "string",
"data": "string"
}

Error resolution

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 :

Fix Mapping Annotations

Use @RequestBody for JSON data sent in a POST or PUT body.
Use @RequestParam for query parameters in the URL (like ?name=test).
Use @PathVariable for variables built directly into the URL path (like /users/{id}).

Check Data Binding and Names

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.

Let me explain you these by example

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;~~

}

What I learned