launch sms application with an intent

To start launch the sms activity all you need is this:

Intent sendIntent = new Intent(Intent.ACTION_VIEW);         
sendIntent.setData(Uri.parse("sms:"));

You can add extras to populate your own message and such like this

sendIntent.putExtra("sms_body", x); 

then just startActivity with the intent.

startActivity(sendIntent);

Intent smsIntent = new Intent(Intent.ACTION_VIEW);
smsIntent.setType("vnd.android-dir/mms-sms");
smsIntent.putExtra("address", "12125551212");
smsIntent.putExtra("sms_body","Body of Message");
startActivity(smsIntent);

If android version is Kitkat or above, users can change default sms application. This method will get default sms app and start default sms app.

private void sendSMS() {    
      if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) // At least KitKat
      {
         String defaultSmsPackageName = Telephony.Sms.getDefaultSmsPackage(this); // Need to change the build to API 19

         Intent sendIntent = new Intent(Intent.ACTION_SEND);
         sendIntent.setType("text/plain");
         sendIntent.putExtra(Intent.EXTRA_TEXT, "text");

         if (defaultSmsPackageName != null)// Can be null in case that there is no default, then the user would be able to choose
         // any app that support this intent.
         {
            sendIntent.setPackage(defaultSmsPackageName);
         }
         startActivity(sendIntent);

      }
      else // For early versions, do what worked for you before.
      {
         Intent smsIntent = new Intent(android.content.Intent.ACTION_VIEW);
         smsIntent.setType("vnd.android-dir/mms-sms");
         smsIntent.putExtra("address","phoneNumber");         
         smsIntent.putExtra("sms_body","message");
         startActivity(smsIntent);
      }
   }

Intent intent = new Intent(Intent.ACTION_MAIN);
intent.addCategory(Intent.CATEGORY_DEFAULT);
intent.setType("vnd.android-dir/mms-sms");
startActivity(intent);

That's all you need.