MainActivity.java

import android.content.Intent;
import android.content.SharedPreferences;
import android.nfc.NfcAdapter;
import android.nfc.Tag;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import java.util.HashSet;
import java.util.Set;

public class MainActivity extends AppCompatActivity {
    private NfcAdapter nfcAdapter;
    private SharedPreferences sharedPreferences;
    private ArrayAdapter<String> adapter;
    private Set<String> nfcIdSet;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        ListView listView = findViewById(R.id.nfcListView);
        Button useNfcButton = findViewById(R.id.useNfcButton);
        Button clearButton = findViewById(R.id.clearButton);
        sharedPreferences = getSharedPreferences("NFC_PREF", MODE_PRIVATE);

        // Initialize NFC
        nfcAdapter = NfcAdapter.getDefaultAdapter(this);
        if (nfcAdapter == null) {
            Toast.makeText(this, "NFC not supported", Toast.LENGTH_SHORT).show();
            finish();
            return;
        }

        // Load existing IDs
        nfcIdSet = new HashSet<>(sharedPreferences.getStringSet("nfc_ids", new HashSet<>()));
        adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, android.R.id.text1, new ArrayList<>(nfcIdSet));
        listView.setAdapter(adapter);

        // Setup list view long-click to delete
        listView.setOnItemLongClickListener((parent, view, position, id) -> {
            String item = adapter.getItem(position);
            removeNfcId(item);
            return true;
        });

        useNfcButton.setOnClickListener(v -> useStoredNfcIds());
        clearButton.setOnClickListener(v -> clearAllIds());
    }

    private void handleNfcIntent(Intent intent) {
        Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
        if (tag != null) {
            String nfcId = bytesToHex(tag.getId());
            addNfcId(nfcId);
        }
    }

    private void addNfcId(String newId) {
        // Create mutable copy
        Set<String> newSet = new HashSet<>(nfcIdSet);
        if (newSet.add(newId)) {
            nfcIdSet = newSet;
            sharedPreferences.edit().putStringSet("nfc_ids", nfcIdSet).apply();
            adapter.clear();
            adapter.addAll(nfcIdSet);
            adapter.notifyDataSetChanged();
            Toast.makeText(this, "NFC ID added: " + newId, Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(this, "NFC ID already exists", Toast.LENGTH_SHORT).show();
        }
    }

    private void removeNfcId(String id) {
        Set<String> newSet = new HashSet<>(nfcIdSet);
        if (newSet.remove(id)) {
            nfcIdSet = newSet;
            sharedPreferences.edit().putStringSet("nfc_ids", nfcIdSet).apply();
            adapter.remove(id);
            Toast.makeText(this, "ID removed: " + id, Toast.LENGTH_SHORT).show();
        }
    }

    private void clearAllIds() {
        nfcIdSet.clear();
        sharedPreferences.edit().putStringSet("nfc_ids", nfcIdSet).apply();
        adapter.clear();
        Toast.makeText(this, "All IDs cleared", Toast.LENGTH_SHORT).show();
    }

    private void useStoredNfcIds() {
        if (nfcIdSet.isEmpty()) {
            Toast.makeText(this, "No NFC IDs stored!", Toast.LENGTH_SHORT).show();
        } else {
            String ids = String.join("\n", nfcIdSet);
            Toast.makeText(this, "Using NFC IDs:\n" + ids, Toast.LENGTH_LONG).show();
        }
    }

    @Override
    protected void onResume() {
        super.onResume();
        if (nfcAdapter != null) {
            nfcAdapter.enableForegroundDispatch(this,
                    PendingIntent.getActivity(this, 0, new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0),
                    null, null);
        }
    }

    @Override
    protected void onPause() {
        super.onPause();
        if (nfcAdapter != null) {
            nfcAdapter.disableForegroundDispatch(this);
        }
    }

    @Override
    protected void onNewIntent(Intent intent) {
        super.onNewIntent(intent);
        handleNfcIntent(intent);
    }

    private void handleNfcIntent(Intent intent) {
        Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
        if (tag != null) {
            byte[] idBytes = tag.getId();
            String nfcId = bytesToHex(idBytes);
            
            sharedPreferences.edit().putString("nfc_id", nfcId).apply();
            updateNfcIdDisplay();
            Toast.makeText(this, "NFC ID stored!", Toast.LENGTH_SHORT).show();
        }
    }

    private void updateNfcIdDisplay() {
        String storedId = sharedPreferences.getString("nfc_id", null);
        nfcIdTextView.setText(storedId != null ? 
                "Stored NFC ID: " + storedId : 
                "No NFC ID stored");
    }

    private void useStoredNfcId() {
        String storedId = sharedPreferences.getString("nfc_id", null);
        if (storedId != null) {
            // Replace this with your actual usage logic
            Toast.makeText(this, "Using NFC ID: " + storedId, Toast.LENGTH_SHORT).show();
        } else {
            Toast.makeText(this, "No NFC ID stored!", Toast.LENGTH_SHORT).show();
        }
    }

    // Helper method to convert byte array to hex string
    private static String bytesToHex(byte[] bytes) {
        StringBuilder sb = new StringBuilder();
        for (byte b : bytes) {
            sb.append(String.format("%02X", b));
        }
        return sb.toString();
    }
}