Norm Radder

Thanks for the recommendations and observations.

I'be been looking at multicast, but as Tim suggests, that protocol could be locked down.
Here's some testing code I've used:

/* Code source - https://www.developer.com/java/data/how-to-multicast-using-java-sockets.html
*/
import java.io.IOException;
import java.net.*; //DatagramPacket;
import java.net.InetAddress;
import java.net.MulticastSocket;
import java.util.*;
public class UDPMulticastTest {
  static int Port = 4321;
  static String GroupIP = "230.0.0.0";
  public static void main(String[] args) throws Exception {
     UDPMulticastClient.main(args);
     UDPMulticastServer.main(args);
  }
static class UDPMulticastClient implements Runnable {
  public static void main(String[] args) {
     Thread t=new Thread(new UDPMulticastClient());
     t.start();
  }
  public void receiveUDPMessage(String ip, int port) throws IOException {
     byte[] buffer=new byte[1024];
     MulticastSocket socket=new MulticastSocket(Port);
     InetAddress group=InetAddress.getByName(GroupIP);
     socket.joinGroup(group);
     while(true){
        System.out.println("C Waiting for multicast message... at " + new Date());
        DatagramPacket packet=new DatagramPacket(buffer, buffer.length);
        socket.receive(packet);
        String msg=new String(packet.getData(), packet.getOffset(),packet.getLength());
        System.out.println("C [Multicast UDP message received]>> "+msg +"< at " + new Date());
        if("OK".equals(msg)) {
           System.out.println("C No more message. Exiting : "+msg);
           break;
        }
     }
     socket.leaveGroup(group);
     socket.close();
  }
  @Override
  public void run(){
     try {
        receiveUDPMessage(GroupIP, Port);
     }catch(IOException ex){
        ex.printStackTrace();
     }
  }
}
//=======================================================================================
static class UDPMulticastServer {
  public static void sendUDPMessage(String message, String ipAddress, int port) throws IOException {
     DatagramSocket socket = new DatagramSocket();
     InetAddress group = InetAddress.getByName(ipAddress);
     byte[] msg = message.getBytes();
     DatagramPacket packet = new DatagramPacket(msg, msg.length, group, port);
     System.out.println("S sending msg="+message +"< at " + new Date());
     socket.send(packet);
     socket.close();
  }
  public static void main(String[] args) throws Exception {
     Thread.sleep(5000);                              // wait before sending msg
     sendUDPMessage("This is a multicast messge", GroupIP, Port);
     Thread.sleep(5000);                              // wait before sending msg
     sendUDPMessage("This is the second multicast messge", GroupIP, Port);
     Thread.sleep(5000);                              // wait before sending msg
     sendUDPMessage("This is the third multicast messge",GroupIP, Port);
     Thread.sleep(5000);                              // wait before sending msg
     sendUDPMessage("OK", GroupIP, Port);
  }
}
}

The next idea is to use the IP addresses for the two tablets that are assigned on connecting to the LAN. They can be swapped at the start of the day and then with some scheduling could be used during the day to communicate.

3 days ago

The design has taken another turn. I found an app in Google Store that does most of what I want: Lan Messanger
However I'm having timeout/disconnect problems with it. Also I'd like it to make a sound (ring a bell) when it gets a special message.
So the new project is an app to listen and ring a bell when connected to. Otherwise it still is to run P2P on a LAN.
When the bell rings, the receiver would start the Lan Messenger program to have a chat session.

One problem is to find the other tablet. I found some code on SO and modified it to to the search:
https://stackoverflow.com/questions/39335835/how-to-get-ip-address-and-names-of-all-devices-in-local-network-on-android

package com.normsstuff.myhelloworld;
import java.lang.ref.WeakReference;
import java.net.InetAddress;
import java.util.Date;
import java.util.concurrent.CountDownLatch;
import android.app.ProgressDialog;
import android.content.Context;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.net.wifi.WifiInfo;
import android.net.wifi.WifiManager;
import android.os.AsyncTask;
import android.text.format.Formatter;
import android.widget.Toast;
public class NetworkSniffTask extends AsyncTask<Void, Void, String> {
 ProgressDialog progressDialog = null;
 private WeakReference<Context> mContextRef;
 public NetworkSniffTask(Context context) {
   mContextRef = new WeakReference<Context>(context);
 }
 
 //===================================================================
     @Override
     protected void onPreExecute() {
         progressDialog = ProgressDialog.show(mContextRef.get(), "Searching for IP addresses",
                 "Please wait, may take 30 seconds", true, true);
    }
 @Override
 protected String doInBackground(Void... voids) {
   System.out.println("Let's sniff the network");
       final StringBuffer retVal = new StringBuffer();
   try {
     Context context = mContextRef.get();
     if (context != null) {
       ConnectivityManager cm = (ConnectivityManager)context.getSystemService(Context.CONNECTIVITY_SERVICE);
       NetworkInfo activeNetwork = cm.getActiveNetworkInfo();
       WifiManager wm = (WifiManager)context.getSystemService(Context.WIFI_SERVICE);
       WifiInfo connectionInfo = wm.getConnectionInfo();
       int ipAddress = connectionInfo.getIpAddress();
       String ipString = Formatter.formatIpAddress(ipAddress);
       System.out.println("activeNetwork: " + String.valueOf(activeNetwork));
       System.out.println("ipString: " + String.valueOf(ipString));
       final String prefix = ipString.substring(0, ipString.lastIndexOf(".") + 1);
       System.out.println("prefix: " + prefix);
       
       // Start several threads to search for valid IP addresses
       int[][] values = {{0, 25}, {26, 50}, {51, 75}, {76, 100}, {101, 125},
                 {126, 150}, {151, 175}, {176, 200}, {201, 225}, {226, 255}};
                    //{{0, 32}, {33, 64}, {65, 96}, {97, 128}, {129, 160},
                //  {161, 192}, {193, 224}, {225, 255}};
                  //{0,50},{51,100},{101, 150},{151, 200},{200, 255}};
       final CountDownLatch doneSignal = new CountDownLatch(values.length);
       
   //==================================
       class SearchThread extends Thread {
           SearchThread(int s, int e)  {
           start = s;
           end = e;
           }
           int start, end;
           @Override
           public void run() {
              System.out.println("before loop for start= " + start + " " + new Date());
              for(int i=start; i <= end; i++) {
                 String testIp = prefix + String.valueOf(i);
                      try{
                 InetAddress address = InetAddress.getByName(testIp);
                 boolean reachable = address.isReachable(1000);  //  timeout (> 4min for 255)
                 String hostName = address.getCanonicalHostName();
                 if (reachable) {
                 String temp = "Host: " + String.valueOf(hostName)
                             + "(" + String.valueOf(testIp) + ") is reachable!";
                 retVal.append(temp + "\n");
                 System.out.println(temp);
                 }
                      }catch(Exception x) {
                    x.printStackTrace();
                    }                
              }
              System.out.println("after loop for start= " + start + " " + new Date());
              doneSignal.countDown();
           }
        } // end class
       
       for(int i=0; i < values.length; i++) {
           Thread t =  new SearchThread(values[i][0], values[i][1]);
           t.start();
       }
       doneSignal.await();
       System.out.println("after await() for all threads to end  at " + new Date());
       
     }
   } catch (Throwable t) {
    System.out.println("Well that's not good. " + t);
    t.printStackTrace();
   }
 return retVal.toString();
}
 
     @Override
     protected void onPostExecute(String result) {
     progressDialog.dismiss();
     System.out.println("result=\n"+result);
         if (result != null) {
             Toast.makeText(mContextRef.get(), "Found these IP addresses\n"+result,
                            Toast.LENGTH_LONG).show();
         }
     }
}

1 week ago

Create a simple store-and-forward webapp. An Android client app could easily invoke this as a web service.

I'll have to do some research. I have no idea what the specifics of that means.

My simple HTTP server handles GET and POST.
https://github.com/NormR/Android-Simple-Web-Server

1 week ago

Security isn't an issue.
The messages would be like this:
Will be late. Meet you on Stbd side at 1145
or
Free drinks at TiKi-bar until noon. See you there at 1130
Ok, will be there

1 week ago

Thanks for the responses.

My app was to allow my wife and I to communicate from anywhere in the ship using the ship's LAN.

require the devices to connect to an access point.

Yes, we use the ship's LAN which is free.
It is possible to connect to the ship's LAN without then going on to the internet. I have a simple HTTP server app that I use to transfer files using the app on one tablet and a browser on the other.

do you really need to communicate digital?

Yes. We have different events we attend at different locations and times. It would be nice to tell the other one if there is a delay or rescheduling of an event.

And this looks like an interesting project for my Android toolkit.

1 week ago

I'm looking at a project for a P2P program that would allow two computers (tablets) to communicate on a LAN without the internet.
My assumptions/ideas so far:
The computers can connect to the LAN.
For P2P to work both computers need to have ServeSockets and use a Socket to connect to the other computer.
There needs to be a discovery/search feature that allows the program to find the other program amoungst all the other computers that are connected to the LAN. If the computers always get the same IP address every time they connect, finding the other computer would be a lot simpler.
The computers would need to connect to the LAN and wait for the other computer to connect to its ServerSocket. No idea how much battery this would take.
The basic code should be the same on a PC as on the tablet. The differences would be in the GUI and perhaps the services available on he tablet.

Does anyone have thoughts, ideas or examples for this project?

Background: My wife and I travel on cruise ships that provide a LAN with several free services and pay-for internet access.
We both have Android tablets. We do not buy the internet access.
I wonder if it would be possible to send messages to each other using the LAN.

I just finished a WiFi-Direct program for our tablets (based on an example from a Google samples site). The basic discovery part is done by the OS. It defines an IP address and decides which tablet is the server and which the client. The program then opens a ServerSocket on the server and uses a Socket from the client to connect to the server. After that it is up to whatever protocol you want to write.

1 week ago

cannot convert from element type object to ArrayList

What is the contents of the variable: results that is passed to the method?

2 weeks ago

NullPointerException - don't know why

Also posted at: http://www.javaprogrammingforums.com/whats-wrong-my-code/42240-nullpointerexception-dont-know-why.html

2 weeks ago

Java in General

Prevent portrait images from overflowing screen

I'm trying to write a javascript slideshow (yes another one) to show images listed in a .js file. The images are a mix of landscape and portrait layouts. I am having a problem keeping the portrait images from overflowing the browser's display area and making it necessary to scroll to see the full image. Is there a way in javascript and css to keep the images within the browser's viewing area?
Here's one of my many attempts:

<!DOCTYPE html>

<html>
<meta name="viewport" content="width=device-width, initial-scale=1">
<style>
body {
 font-family: Arial;
 margin: 0;
}
* {
 box-sizing: border-box;
}
img {
 vertical-align: middle;
 margin: auto;             /* will this center horiz? NO doesnt */
 max-width: 1000px;          /*  <IMG has style=??? */
 max-height: 600px;          /*  THIS WORKS */
}
/* Position the image container (needed to position the left and right arrows) */
.container {
 position: relative;
}
/* Hide the images by default */
.mySlides {
 display: none;
}
/* Add a pointer when hovering over the thumbnail images */
.cursor {
 cursor: pointer;
}
/* Next & previous buttons */
.prev,
.next {
 cursor: pointer;
 position: absolute;
 top: 40%;
 width: auto;
 padding: 16px;
 margin-top: -50px;
 color: white;
 font-weight: bold;
 font-size: 20px;
 border-radius: 0 3px 3px 0;
 user-select: none;
 -webkit-user-select: none;
}
/* Position the "next button" to the right */
.next {
 right: 0;
 border-radius: 3px 0 0 3px;
}
/* On hover, add a black background color with a little bit see-through */
.prev:hover,
.next:hover {
 background-color: rgba(0, 0, 0, 0.8);
}
/* Number text (1/3 etc) */
.numbertext {
 color: #f2f2f2;
 font-size: 12px;
 padding: 8px 12px;
 position: absolute;
 top: 0;
}
/* Container for image text */
.caption-container {
 text-align: center;
 background-color: #222;
 padding: 2px 16px;
 color: white;
}
</style>
<script language="JavaScript" src="LoadImageRefs.js">
</script>
<body>

<div class="container">
 
 
  <div class="mySlides" id="theImageDiv">
     <div class="numbertext">? / ?</div>
     <IMG SRC="jpg/imageFNs[0]" id="theImage" style="width:100%">
     
  </div>
   
 
 <a class="prev" onclick="plusSlides(-1)">❮</a>
 <a class="next" onclick="plusSlides(1)">❯</a>
 <div class="caption-container">
   <p id="caption"></p>
 </div>
</div>
<script>
var slideIndex = 1;
showSlides(slideIndex);
function plusSlides(n) {
 showSlides(slideIndex += n);
}
function currentSlide(n) {
 showSlides(slideIndex = n);
}
function showSlides(n) {
 var i;
 var slides = document.getElementsByClassName("mySlides");
 var captionText = document.getElementById("caption");
 //  Keep index in bounds
 if (n > imageFNs.length) {slideIndex = 1}
 if (n < 1) {slideIndex = imageFNs.length}
 for (i = 0; i < slides.length; i++) {
     slides[i].style.display = "none";
 }
 slides[0].style.display = "block";    // <<<<<<<<<<WORKS!!!
>
 var theImageRef = document.getElementById('theImage');
 theImageRef.src = imagesFolder+imageFNs[slideIndex-1];       //   Not shown???
 captionText.innerHTML = imageFNs[slideIndex-1];
}
</script>
   
</body>
</html>

Also posted here: https://www.dreamincode.net/forums/topic/416918-prevent-portrait-images-from-overflowing-screen/

4 weeks ago

HTML Pages with CSS and JavaScript

Checking Runtimes of Algorithms

Exception in thread "main" java.lang.StackOverflowError
at project1.Project1.quicksort(Project1.java:43)
at project1.Project1.quicksort(Project1.java:47)

Looks like there is a recursive call at line 47. Can you post the code that shows lines 43 and 47

4 weeks ago

Look at the code and see where those lines are printed.
Is it inside of a loop? What variables control if that loop continues looping?
What conditional test controls if the loop continues?

4 weeks ago

The problem is the Transportation Menu is repeating after I make a selection.

Can you post the program's output that shows the problem? Be sure to add some comments where the program's output goes wrong.

4 weeks ago

I'm not quite sure where the problem is

Where is there any problem?
Is that output what you want the program to do?
I asked for some comments (<<<< This is wrong because ...) be added to the output to point out to us where the problem is I don't see any comments like that so I assume the output is correct.

4 weeks ago

Stuck on Head First Java Chapter 12

it coudn't compile
... reasons causing this problem
1) my program doesnt have main method,

A class does NOT need a main method to compile. There shouldn't be any compiler errors for a class because it does not have a main method.

A class usually needs a main method to be executed with the java command. There are a few exceptions.

1 month ago

Swing / AWT / SWT

Can you copy the contents of the console window and paste it here so we can see what is happening?

Add some comments (<<<< Here it repeats) to show where the problem is.

1 month ago