Duvida sobre herança

pq sou obrigado a implementar o metodo de uma classe que eu extends sendo que o metodo nao é abstrato?

@Component
public class SecurityFIlter extends OncePerRequestFilter {

   @Override
   protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {

   }

public abstract class OncePerRequestFilter extends GenericFilterBean {
   public static final String ALREADY_FILTERED_SUFFIX = ".FILTERED";


   public final void doFilter(ServletRequest request, ServletResponse response, FilterChain filterChain) throws ServletException, IOException {
      if (request instanceof HttpServletRequest httpRequest) {
         if (response instanceof HttpServletResponse httpResponse) {
            String alreadyFilteredAttributeName = this.getAlreadyFilteredAttributeName();
            boolean hasAlreadyFilteredAttribute = request.getAttribute(alreadyFilteredAttributeName) != null;
            if (!this.skipDispatch(httpRequest) && !this.shouldNotFilter(httpRequest)) {
               if (hasAlreadyFilteredAttribute) {
                  if (DispatcherType.ERROR.equals(request.getDispatcherType())) {
                     this.doFilterNestedErrorDispatch(httpRequest, httpResponse, filterChain);
                     return;
                  }

                  filterChain.doFilter(request, response);
               } else {
                  request.setAttribute(alreadyFilteredAttributeName, Boolean.TRUE);

                  try {
                     this.doFilterInternal(httpRequest, httpResponse, filterChain);
                  } finally {
                     request.removeAttribute(alreadyFilteredAttributeName);
                  }
               }
            } else {
               filterChain.doFilter(request, response);
            }

            return;
         }
      }

      throw new ServletException("OncePerRequestFilter only supports HTTP requests");
   }