Understunding this code for having two instanciation of applets

Question

this an exemple i would like to understund how this code work and how it make difference between register() and register(buffer, (short)(offset + 1), (byte)buffer[offset]);
protected MyApplet1(byte[] buffer, short offset, byte length) {
    // data offset is used for application specific parameter.
    // initialization with default offset (AID offset).
    short dataOffset = offset;
    boolean isOP2 = false;
    if (length >= 9) {
      // Install parameter detail. Compliant with OP 2.0.1.
      // | size | content
      // |------|---------------------------
      // |  1   | [AID_Length]
      // | 5-16 | [AID_Bytes]
      // |  1   | [Privilege_Length]
      // | 1-n  | [Privilege_Bytes] (normally 1Byte)
      // |  1   | [Application_Proprietary_Length]
      // | 0-m  | [Application_Proprietary_Bytes]
      // shift to privilege offset
      dataOffset += (short)( 1 + buffer[offset]);
      // finally shift to Application specific offset
      dataOffset += (short)( 1 + buffer[dataOffset]);
      /**@todo: IF NECESSARY, USE COMMENTS TO CHECK LENGTH*/
      // // checks wrong data length
      // if(buffer[dataOffset] !=  <PUT YOUR PARAMETERS LENGTH> )
      //     // return received proprietary data length in the reason
      //     ISOException.throwIt((short)(ISO7816.SW_WRONG_LENGTH + offset + length - dataOffset));
      // go to proprietary data
      dataOffset++;
      // update flag
      isOP2 = true;
    else {
      // Install parameter compliant with OP 2.0.
      /**@todo: IF NECESSARY, USE COMMENTS TO CHECK LENGTH*/
      // if(length != <PUT YOUR PARAMETERS LENGTH> )
      //     ISOException.throwIt((short)(ISO7816.SW_WRONG_LENGTH + length));
    /**@todo: PUT YOUR CREATION ACTION HERE*/
    // register this instance
    if (isOP2) {
      register(buffer, (short)(offset + 1), (byte)buffer[offset]);
    else {
      register();
  }

Answer 1

On a GlobalPlatform compliant card, the constructor of an applet is called during the last step of the applet's installation process.
The "buffer" parameter given as argument to the constructor usually contains: the instance AID, the applet's privileges and applet's specific bytes.
When installing an applet on a card you can specify 3 different kind of AIDs:
- the package AID
- the applet AID
- the applet instance AID
register(buffer, (short)(offset + 1), (byte)buffer[offset]); will register the applet with the JCRE using the applet instance AID specified in the buffer parameter
register(); will register the applet with the JCRE using the applet AID