A program that repeatedly lists the IP addresses of all devices on a LAN at one second intervals

IPScanner.java

import java.net.*;
import java.lang.*;
import java.util.*;

public class IPScanner implements Runnable {
	// Create a list in which to store live host IP addresses
	static List<String> ips = Collections.synchronizedList(new ArrayList<String>());

	private String ip;

	public IPScanner(String ip) {
		this.ip = ip;
	}

	/**
	 * Checks if the IP address #ip is reachable with a timeout of one second.
	 */
	public void run() {
		try {
			if(InetAddress.getByName(this.ip).isReachable(1000))
				IPScanner.ips.add(this.ip);
		} catch(Exception e) {
			// TODO: Handle properly
			System.err.println(e);
		}
	}

	public static void main(String[] args) {
		Thread[] threads = new Thread[256];


		while (true) {
			IPScanner.ips.clear();

			// Create 256 threads for every possible private IP address
			//  and check reachability of hosts at all IPs concurrently
			for (int i = 0; i < 256; ++i) {
				// Depending on your network, this could be 192.168.*.* or 10.*.*.* or whatever
				threads[i] = new Thread(new IPScanner("192.168.0." + i));
				threads[i].start();
			}

			// Wait for all threads to finish
			for (Thread thread : threads)
				try { thread.join(); } catch (InterruptedException e) {}

			// Sort list of IPs for temporal consistency
			//  (just alphabetical; numerical might look better)
			Collections.sort(IPScanner.ips);

			// Print out results
			System.out.println("IPs:");
			for (String ip : IPScanner.ips)
				System.out.println(ip);
		}
	}
}