HelloWorld.java

/**
 *
 */
package me.jackkingsman.demoapplet;

import javacard.framework.Applet;
import javacard.framework.ISO7816;
import javacard.framework.ISOException;
import javacard.framework.APDU;
import javacard.framework.Util;

/**
 * @author Robert
 *
 */
public class HelloWorld extends Applet {

	private final static byte[] hello=
	{(byte)'H', (byte)'e', (byte)'l', (byte)'l', (byte)'o', (byte)':', (byte)')'} ;

	public static void install(byte[] buffer, short offset, byte length)

	{
		// GP-compliant JavaCard applet registration
		new HelloWorld().register();
	}

	public void process(APDU apdu) {
		// Good practice: Return 9000 on SELECT
		if (selectingApplet()) {
			return;
		}

		byte[] buf = apdu.getBuffer();
		switch (buf[ISO7816.OFFSET_INS]) {
		case (byte) 0x40:
			Util.arrayCopy(hello, (short)0, buf, ISO7816.OFFSET_CDATA, (short) hello.length);
				apdu.setOutgoingAndSend(ISO7816.OFFSET_CDATA, (short) hello.length);
			break;
		default:
			// good practice: If you don't know the INStruction, say so:
			ISOException.throwIt(ISO7816.SW_INS_NOT_SUPPORTED);
		}
	}
}

MainActivity.kt

package me.jackkingsman.myapplication

import android.nfc.NfcAdapter
import android.nfc.Tag
import android.nfc.tech.IsoDep
import android.os.Bundle
import android.widget.TextView
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity(), NfcAdapter.ReaderCallback {
    private var nfcAdapter: NfcAdapter? = null

    override fun onCreate(savedInstanceState: Bundle?) {
        super.onCreate(savedInstanceState)
        setContentView(R.layout.activity_main)
        nfcAdapter = NfcAdapter.getDefaultAdapter(this);
    }

    public override fun onResume() {
        super.onResume()
        nfcAdapter?.enableReaderMode(this, this,
            NfcAdapter.FLAG_READER_NFC_A or NfcAdapter.FLAG_READER_SKIP_NDEF_CHECK,
            null)
    }

    public override fun onPause() {
        super.onPause()
        nfcAdapter?.disableReaderMode(this)
    }

    fun ByteArray.toHexString() = asUByteArray().joinToString("") { it.toString(16).padStart(2, '0') }

    override fun onTagDiscovered(tag: Tag?) {
        val textView: TextView = findViewById(R.id.theTextView) as TextView
        val tag = IsoDep.get(tag)
        tag.connect()

        val select: ByteArray = byteArrayOf(
            0x00, 0xA4.toByte(), 0x04, 0x00, 0x06, 0xA0.toByte(), 0x00, 0x42, 0x06, 0x94.toByte(), 0x21
        )

        var select_result: ByteArray = tag.transceive(select)
        runOnUiThread {textView.append("\nSelect applet got back " + select_result.toHexString())}

        val hallo_fetch: ByteArray = byteArrayOf(
            0x01, 0x40.toByte(), 0x00, 0x00, 0x00
        )

        var hallo_result: ByteArray = tag.transceive(hallo_fetch)
        runOnUiThread {textView.append("\nInteract got back " + hallo_result.size + " bytes:" + hallo_result.toHexString())}

        tag.close()
    }
}