// Mongo client with credentials on a non existing user
// we expect an auth failure error
MongoClient mongo = ...;
try {
    // this will raise an auth failure exception
    mongo.getDatabaseNames().contains( databaseName );
}
catch (MongoException me) {
    // auth failure exception, that's cool
}

try {
    // we try again to connect with the same credentials
    // we should get an auth failure
    db.collectionExists( "WeDoNotCareWhatItIsWeNeedToConnect" );
}
catch (MongoException me) {
    if ( me.getCode() == 18 ) { //AUTHENTICATION_FAILED_CODE
        System.out.println("Auth failure");
    }
    if ( me instanceof MongoTimeoutException ) {
        System.out.println("Timeout failure");
    }
}