Append relative URL to java.net.URL

This one does not need any extra libs or code and gives the desired result:

//import java.net.URL;
URL url1 = new URL("http://petstore.swagger.wordnik.com/api/api-docs?foo=1&bar=baz");
URL url2 = new URL(url1.getProtocol(), url1.getHost(), url1.getPort(), url1.getPath() + "/pet" + "?" + url1.getQuery(), null);
System.out.println(url1);
System.out.println(url2);

This prints:

http://petstore.swagger.wordnik.com/api/api-docs?foo=1&bar=baz
http://petstore.swagger.wordnik.com/api/api-docs/pet?foo=1&bar=baz

The accepted answer only works if there is no path after the host (IMHO the accepted answer is wrong)

URL has a constructor that takes a base URL and a String spec.

Alternatively, java.net.URI adheres more closely to the standards, and has a resolve method to do the same thing. Create a URI from your URL using URL.toURI.