Rule Definition
the Implementation of equals() and hashcode() calls a naming service(DNS) to resolve the IP address. This may take a lot of time and causes a dependency on Network Connection. It is inconsistent and may give different results with or without a network connection and takes long time to execute. The implementation is known to be incompatible with virtual hosting and should not be used.

Remediation
use java.net.URI instead of java.net.URL

Violation Code Sample

__________________________________________________________________________
(A)
public class Main {
  public static void main(String[] argv) throws Exception {
    URL relativeURL, baseURL;
    baseURL = new URL("http://www.yourserver.com/");
    relativeURL = new URL(baseURL, "./a.htm");
    System.out.println(relativeURL.equals(baseURL)); //non-compliant

  }
}
___________________________________________________________________________
(B)
public void checkUrl(URL url) {
  Set<URL> sites = new HashSet<URL>();  // Noncompliant

  URL homepage = new URL("http://castsoftware.com");  // Compliant
  if (homepage.equals(url)) { // Noncompliant
    // ...
  }
}

____________________________________________________________________________
(C)
import java.io.IOException;
import java.net.URL;
public class Main {

  public static void main(String[] args) {
    try {
      URL url = new URL("http://www.castsoftware.com");
      System.out.println("URL is " + url.hashCode());
    } catch (IOException e) {
      e.printStackTrace();
    }
  }

}

Fixed Code Sample

____________________________________________________________
(A)
public class Main {
  public static void main(String[] argv) throws Exception {
    URL relativeURL, baseURL;
    baseURL = new URL("http://www.yourserver.com/");
    relativeURL = new URL(baseURL, "./a.htm");
    URI.uri1 = relativeURL.toURI();
    URI.uri2 = relativeURL.toURI();
    System.out.println(uri1.equals(uri2));//compliant

  }
}
_______________________________________________________________
(B)
public void checkUrl(URL url) {
  Set<URI> sites = new HashSet<URI>();  // Compliant

  URI homepage = new URI("http://sonarsource.com");  // Compliant
  URI uri = url.toURI();
  if (homepage.equals(uri)) {  // Compliant
    // ...
  }
}______________________________________________________________

(C)
import java.net.URI;
import java.net.URISyntaxException;
public class Main {
  public static void main(String[] args) throws NullPointerException, URISyntaxException {
    URI uri = new URI("http://www.castsoftware.com");
    System.out.println("URI      : " + uri);
    System.out.println(uri.hashCode());
  }
}

https://javaantipatterns.wordpress.com/2007/11/24/comparing-urls-with-urlequals/ http://michaelscharf.blogspot.com/2006/11/javaneturlequals-and-hashcode-make.html