Lots of Threads a Handler and little experience

Question

Im trying to make a list of local machines connected to a wifi. The easy way to do this, is to ping every IP from abc1 to abc255, but we all know that would take "forever".

Therefore I'd like to use Threads!

What I have is a MainActivity that contains this:

list = (ListView) findViewById(R.id.list);
ipList.add("Nothing yet");

adapter = new ArrayAdapter<String>(this, R.layout.simplerow, ipList);
list.setAdapter(adapter);

networkList = new NetworkList(); //This is the shared list between the Threads

for(int i = 1; i<255; i++){  //Tries to ping the range of IP's

    Ping ping = new Ping(localIP + i, networkList)
    new Thread(ping).start();
}

The class Ping implements Runnable:

public void run() {
  try {
    InetAddress address = InetAddress.getByName(IP);

    if(address.isReachable(2000)){
        synchronized(networkList){
            networkList.addIP(address.getHostAddress());
            MainActivity.handler.sendEmptyMessage(0); //probably wrong
        }
    }
  } catch (Exception e) {}
}

Im not sure if the usage of synchronize is correct, but please hang on! The class NetworkList is where the synchronized stuff happends:

public synchronized void addIP( String IP ){
    foundIPs.add(IP);
}

public synchronized ArrayList<String> getIPList(){
    return foundIPs;
}

OK. Back to the MainActivity :

This is probably very wrong, but its what Ive managed to do:

    handler = new Handler() {
        public void sendEmptyMessage(android.os.Message msg) {
            adapter.clear();
            synchronized(networkList){
                ipList = networkList.getIPList();
            }
            adapter.notifyDataSetChanged();
        }
    };

What is wrong here? I want the list to be updated immediately as a thread gets a respond on the ping. In other words, I want to see the ListView expand as new IPs are found, not just retrieve the complete list when the ping process is done.

Answer 1

what i think is the problem is that the adapter itself doesn't get a reference to the new list of ip addresses , so it probably looks at the oldest one which has no entries.

i can see it by looking at the code:

adapter = new ArrayAdapter<String>(this, R.layout.simplerow, ipList);
...
ipList = networkList.getIPList();

Answer 2

With ipList = networkList.getIPList(); you're creating a new list, one that the array adapter doesn't know anything about. You should make calls directly on the adapter, such as ArrayAdapter.add or ArrayAdapter.addAll().