Implementing custom pagination in Spring Boot

Question

I need to have a custom pagination with request and response objects, in request I need page, size, and totalCountOfApplicants, which I am searching using MongoDB. I have created an endpoint that filters the applicants for various fields, I just need to add the count of totalApplicants.

public class PaginationRequest implements Serializable {

    private static final long serialVersionUID = 1L;
    private int pageNumber = 0;
    private int pageSize=20;
    private int totalApplicants;
}

@Data
@ToString(callSuper = true)
@EqualsAndHashCode(callSuper = true)
public class PaginationResponse extends Response {
    
    private static final long serialVersionUID = 1L;
    
    private Pageable pageable;
    
}

Mongo code to find the total number of applicants

request.getPaginationRequest().setTotalApplicants((int) mongoTemplate.count(query, Applicant.class));

I am getting a NullPointerException in this line when I try to find the number of pages during validation.

int page = request.getPaginationRequest().getPageNumber(), size = request.getPaginationRequest().getPageSize();

I am just a beginner, so please provide any relevant info you have.