How to pass variable in HTTPRequest Header?

I want to add header in my HTTPRequest with variable but it will give error not taking it but when i pass direct string value in inverted commas it work well.

Hers is the code in 3rd header i am passing variable checksum which contains String value but it is not working and if i pass direct String value it's work without problem.

HttpRequest request1 = (HttpRequest) HttpRequest.newBuilder()
                    .uri(URI.create("https://api-preprod.phonepe.com/apis/pg-sandbox/pg/v1/pay"))
                    .header("accept", "application/json")
                    .header("Content-Type", "application/json")
                    .header("X-VERIFY",checksum)
                    .method("POST", HttpRequest.BodyPublishers.ofString("{\"request\":\"\"}"))
                    .build();

The most probable explanation is that you haven't overridden toString() method in the class of your checksum. Try to override the method returning String value of checksum:

class Checksum {
 @Override
 public String toString() {
   return value;
 }
}

HttpRequestBuilder converts an object passed into header() method according to certain rules and for object of custom class it just calls toString() method.