Question

I seem to be getting the above error when trying to make a callable class. I have searched for the reason but can’t seem to find anything. NetBeans gives me a few options to make things abstract but I’m new to this and I’d rather find out why something is happening. Can anyone shed light on this?

public class doPing implements Callable<String>{

    public String call(String IPtoPing) throws Exception{

        String pingOutput = null;

        //gets IP address and places into new IP object
        InetAddress IPAddress = InetAddress.getByName(IPtoPing);
        //finds if IP is reachable or not. a timeout timer of 3000 milliseconds is set.
        //Results can vary depending on permissions so cmd method of doing this has also been added as backup
        boolean reachable = IPAddress.isReachable(1400);

        if (reachable){
              pingOutput = IPtoPing + " is reachable.\n";
        }else{
            //runs ping command once on the IP address in CMD
            Process ping = Runtime.getRuntime().exec("ping " + IPtoPing + " -n 1 -w 300");
            //reads input from command line
            BufferedReader in = new BufferedReader(new InputStreamReader(ping.getInputStream()));
            String line;
            int lineCount = 0;
            while ((line = in.readLine()) != null) {
                //increase line count to find part of command prompt output that we want
                lineCount++;
                //when line count is 3 print result
                if (lineCount == 3){
                    pingOutput = "Ping to " + IPtoPing + ": " + line + "\n";
                }
            }
        }
        return pingOutput;
    }
}

Answer 1

In your code, the call method has an argument: it does not override the call method of the Callable interface – It should look like:

public String call() throws Exception{ //not public String call(String IPtoPing)

}

If you are using Java 6+, it is good practice ot use the Override annotation, which can help spot wrong method signatures (in this case you already got a compilation error):

@Override
public String call() throws Exception{
}