原因：java.lang.NullPointerException：尝试调用虚拟方法'void android.nfc.NfcAdapter.enableForegroundDispatch

Question

（问题） 原因：java.lang.NullPointerException：尝试调用虚拟方法'void android.nfc.NfcAdapter.enableForegroundDispatch（android.app.Activity，android.app.PendingIntent，android.content.IntentFilter []，java.lang .String [] []）'引用空对象

嗨，eveyone，我是android studio的初学者，有人可以帮助我解决这个问题，并解释发生了什么吗？

import androidx.appcompat.app.AppCompatActivity;

import android.app.PendingIntent;
import android.content.Intent;
import android.content.IntentFilter;
import android.nfc.NdefMessage;
import android.nfc.NdefRecord;
import android.nfc.NfcAdapter;
import android.nfc.Tag;
import android.nfc.tech.Ndef;
import android.nfc.tech.NdefFormatable;
import android.os.Bundle;
import android.util.Log;
import android.widget.Toast;

import java.io.ByteArrayOutputStream;
import java.io.UnsupportedEncodingException;
import java.util.Locale;

public class MainActivity extends AppCompatActivity {

    NfcAdapter nfcAdapter;

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

        NfcAdapter nfcAdapter = NfcAdapter.getDefaultAdapter(this);

    }


    @Override
    protected void onResume() {

        // Intent intent = new Intent(this, MainActivity.class);
        // intent.addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING);

        // PendingIntent pendingIntent = PendingIntent.getActivity(this,0,intent,0);
        // IntentFilter[] intentFilters = new IntentFilter[]{};

        // nfcAdapter.enableForegroundDispatch(this,pendingIntent,intentFilters,null);
        super.onResume();
        enableForegroundDispatchSystem();
    }

    @Override
    protected void onPause() {
        //nfcAdapter.disableForegroundDispatch(this);

        super.onPause();
        disableForegroundDispatchSystem();
    }

    @Override
    protected void onNewIntent(Intent intent) {

        super.onNewIntent(intent);
        if (intent.hasExtra(NfcAdapter.EXTRA_TAG)) {
            Toast.makeText(this, "NFCintent", Toast.LENGTH_LONG).show();

            Tag tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
            NdefMessage ndefMessage = createNdefMessage("My string content");

            writeNdefMessage(tag, ndefMessage);
        }
    }

    private void enableForegroundDispatchSystem() {
        Intent intent = new Intent(this, MainActivity.class).addFlags(Intent.FLAG_RECEIVER_REPLACE_PENDING);

        PendingIntent pendingIntent = PendingIntent.getActivity(this, 0, intent, 0);
        IntentFilter[] intentFilter = new IntentFilter[]{};

        nfcAdapter.enableForegroundDispatch(this, pendingIntent, intentFilter, null);
    }

    private void disableForegroundDispatchSystem() {
        nfcAdapter.disableForegroundDispatch(this);
    }

    private void formatTag(Tag tag, NdefMessage ndefMessage) {
        try {

            NdefFormatable ndefFormatable = NdefFormatable.get(tag);
            if (ndefFormatable == null) {
                Toast.makeText(this, "tag is not ndef formatable", Toast.LENGTH_LONG).show();
                return;
            }

            ndefFormatable.connect();
            ndefFormatable.format(ndefMessage);
            ndefFormatable.close();


        } catch (Exception e) {
            Log.e("formatTag", e.getMessage());
        }
    }

    private void writeNdefMessage(Tag tag, NdefMessage ndefMessage) {
        try {
            if (tag == null) {
                Toast.makeText(this, "Tag object cannot be null", Toast.LENGTH_LONG).show();
                return;
            }

            Ndef ndef = Ndef.get(tag);

            if (ndef == null) {
                //format tag with the ndef format and writes the message
                formatTag(tag, ndefMessage);
            } else {
                ndef.connect();

                if (!ndef.isWritable()) {
                    Toast.makeText(this, "Tag is not writeable", Toast.LENGTH_LONG).show();
                    ndef.close();
                    return;
                }

                ndef.writeNdefMessage(ndefMessage);
                ndef.close();
                Toast.makeText(this, "Tag writen ", Toast.LENGTH_LONG).show();
            }
        } catch (Exception e) {
            Log.e("writeNdefMessage", e.getMessage());
        }
    }

    private NdefRecord createTextRecord(String content) {
        try {
            byte[] language;
            language = Locale.getDefault().getLanguage().getBytes("UTF-8");

            final byte[] text = content.getBytes("UTF-8");
            final int languageSize = language.length;
            final int textLength = text.length;
            final ByteArrayOutputStream payload = new ByteArrayOutputStream(1 + languageSize + textLength);

            payload.write((byte) (languageSize & 0x1F));
            payload.write(language, 0, languageSize);
            payload.write(text, 0, textLength);

            return new NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT, new byte[0], payload.toByteArray());
        } catch (UnsupportedEncodingException e) {
            Log.e("createTextRecord", e.getMessage());
        }
        return null;
    }

    private NdefMessage createNdefMessage(String content) {

        NdefRecord ndefRecord;
        ndefRecord = createTextRecord(content);

        NdefMessage ndefMessage = new NdefMessage(new NdefRecord[] { ndefRecord });

        return ndefMessage;
    }


}```