Android and arduino, send a string how it works

Question

I am using a bluetooth serial connection to send data from Arduino to Android. This is the code that I use in my arduino to send data:

char toSend = (char)Serial.read();
bluetooth.print(toSend);

and this code on my Android application to read this data:

 void inputthread() {

final Handler handler = new Handler(); 
stopWorker = false;
workerThread = new Thread(new Runnable()
{
  public void run() 
  {
     while(!Thread.currentThread().isInterrupted() && !stopWorker)
     {
          try
          { 
            int bytesAvailable = mmInputStream.available();     
            if(bytesAvailable > 0) 
            {
              byte[] packetBytes = new byte[bytesAvailable];
              mmInputStream.read(packetBytes);
              final String s = new String(packetBytes);
              final String t = new String("s");
              if (s==t)
                  {
                      handler.post(new Runnable() 
                      {

                          public void run()
                          {
                              try {
                                  closeBT();
                                  }
                                catch (IOException ex) { }
                          }
                      });
                  }

              handler.post(new Runnable() 
              {
                  public void run()
                  {
                   // txtaggiorna.setText(s);
                  }
              }); 

            }
          } catch (IOException ex) 
          {
            stopWorker = true;
          }
     }
  }
});

When I send "Ciao" from Arduino serial monitor, Android shows "C" first and then "iao", but I don't know why.

"// txtaggiorna.setText(s);" is not commented in the original — user3250541
– user3250541, Commented Mar 25, 2014 at 18:15

Chris Stratton · Accepted Answer · 2014-03-25 19:56:45Z

This is an expected result that you must allow for. Any time a stream of data is transitioned between character-by-character transports and batched or packetized transports, without the use of logic intended to preserve grouping, there is a possibility for a message to be split across multiple batches/packets.

Your project involves both character-by-character serial transports and packetized bluetooth transport, so this type of split is quite likely to happen.

A relatively poor way to reassemble messages is to assume anything which arrives with small time gaps to be a single message, and then assume a new message starts when there is a longer time gap.

A much more sound mechanism is to declare that messages will be terminated, for example with a newline character. In that case, you re-assemble everything obtained between newlines into a single message. Note that it's entirely possible for a packet / read call to give you the tail end of one message, a newline, and then the start of a new message.

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