There are plenty of libraries that can help you with URI building (don't reinvent the wheel). Here are three to get you started:
Java EE 7
import javax.ws.rs.core.UriBuilder;
...
return UriBuilder.fromUri(url).queryParam(key, value).build();
org.apache.httpcomponents:httpclient:4.5.2
import org.apache.http.client.utils.URIBuilder;
...
return new URIBuilder(url).addParameter(key, value).build();
org.springframework:spring-web:4.2.5.RELEASE
import org.springframework.web.util.UriComponentsBuilder;
...
return UriComponentsBuilder.fromUriString(url).queryParam(key, value).build().toUri();