Question

我是新来的。我为我的英语和我的职位感到抱歉。我有下一个问题“无法恢复活动nullpointerexception”我尝试在NFC标签上写这是我的代码。

package com.example.chips;

import java.io.IOException;
import java.io.UnsupportedEncodingException;

import android.annotation.SuppressLint;
import android.app.Activity;
import android.app.PendingIntent;
import android.content.Context;
import android.content.Intent;
import android.content.IntentFilter;
import android.nfc.FormatException;
import android.nfc.NdefMessage;
import android.nfc.NdefRecord;
import android.nfc.NfcAdapter;
import android.nfc.Tag;
import android.nfc.tech.Ndef;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.View;
import android.view.Window;
import android.widget.Button;
import android.widget.Toast;

@SuppressLint({ "ParserError", "ParserError" })
public class MyDialog extends Activity {

    Button writeBtn;
    NfcAdapter adapter;
    PendingIntent pendingIntent;
    IntentFilter writeTagFilters[];
    boolean writeMode;
    Tag mytag;
    Context ctx;

    @Override
    protected void onCreate(Bundle savedInstanceState) {

        super.onCreate(savedInstanceState);
        requestWindowFeature(Window.FEATURE_NO_TITLE);
        setContentView(R.layout.writing_dialog);
        writeBtn = (Button) findViewById(R.id.writeBtn);
        ctx = this;

        writeBtn.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                try {
                    String message = "hola mundo";
                    if (mytag == null) {

                        Toast.makeText(ctx, "error mytag null", Toast.LENGTH_LONG).show();
                    } else {
                        write(message, mytag);
                        Toast.makeText(ctx, "writing", Toast.LENGTH_LONG).show();
                    }
                } catch (IOException e) {
                    Toast.makeText(ctx, "IO error", Toast.LENGTH_LONG).show();
                    e.printStackTrace();
                } catch (FormatException e) {
                    Toast.makeText(ctx, "error format", Toast.LENGTH_LONG).show();
                    e.printStackTrace();
                }

            }
        });

        adapter = NfcAdapter.getDefaultAdapter(this);
        pendingIntent = PendingIntent.getActivity(this, 0,
                new Intent(this, getClass()).addFlags(Intent.FLAG_ACTIVITY_SINGLE_TOP), 0);
        IntentFilter tagDetected = new IntentFilter(NfcAdapter.ACTION_TAG_DISCOVERED);
        tagDetected.addCategory(Intent.CATEGORY_DEFAULT);
        writeTagFilters = new IntentFilter[] { tagDetected };

    }

    private void write(String text, Tag tag) throws IOException, FormatException {

        NdefRecord[] records = { createRecord(text) };
        NdefMessage message = new NdefMessage(records);
        // Get an instance of Ndef for the tag.
        Ndef ndef = Ndef.get(tag);
        // Enable I/O
        ndef.connect();
        // Write the message
        ndef.writeNdefMessage(message);
        // Close the connection
        ndef.close();
    }

    private NdefRecord createRecord(String text) throws UnsupportedEncodingException {
        String lang = "en";
        byte[] textBytes = text.getBytes();
        byte[] langBytes = lang.getBytes("US-ASCII");
        int langLength = langBytes.length;
        int textLength = textBytes.length;
        byte[] payload = new byte[1 + langLength + textLength];

        // set status byte (see NDEF spec for actual bits)
        payload[0] = (byte) langLength;

        // copy langbytes and textbytes into payload
        System.arraycopy(langBytes, 0, payload, 1, langLength);
        System.arraycopy(textBytes, 0, payload, 1 + langLength, textLength);

        NdefRecord recordNFC = new NdefRecord(NdefRecord.TNF_WELL_KNOWN, NdefRecord.RTD_TEXT,
                new byte[0], payload);

        return recordNFC;
    }

    @Override
    protected void onNewIntent(Intent intent) {
        if (NfcAdapter.ACTION_TAG_DISCOVERED.equals(intent.getAction())) {
            mytag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
            Toast.makeText(this, "error intent" + mytag.toString(), Toast.LENGTH_LONG).show();
        }
    }

    @Override
    public void onPause() {
        super.onPause();
        WriteModeOff();
    }

    @Override
    public void onResume() {
        super.onResume();
        WriteModeOn();
    }

    private void WriteModeOn() {
        writeMode = true;
        adapter.enableForegroundDispatch(this, pendingIntent, writeTagFilters, null);
    }

    private void WriteModeOff() {
        writeMode = false;
        adapter.disableForegroundDispatch(this);
    }

}

这里是我的androidmanifest

<activity
        android:name="com.example.chips.MainActivity"
        android:label="@string/app_name" >
        <intent-filter>
            <action android:name="android.intent.action.MAIN" />
            <category android:name="android.intent.category.LAUNCHER" />
        </intent-filter>
    </activity>

    <activity
        android:name="MyDialog"
        android:theme="@android:style/Theme.Dialog" >
        <intent-filter>
            <action android:name="com.example.chips"/>
            <category android:name="android.intent.category.DEFAULT"/>
        </intent-filter>
    </activity>

Logcat：

02-27 05:07:02.386: V/NFC(3687): this device does not have NFC support 
02-27 05:07:02.386: D/AndroidRuntime(3687): Shutting down VM 
02-27 05:07:02.386: W/dalvikvm(3687): threadid=1: thread exiting with uncaught exception (group=0xb3a4dba8) 
02-27 05:07:02.406: E/AndroidRuntime(3687): FATAL EXCEPTION: main 
02-27 05:07:02.406: E/AndroidRuntime(3687): Process: com.example.chips, PID: 3687 
02-27 05:07:02.406: E/AndroidRuntime(3687): java.lang.RuntimeException: Unable to resume activity {...}: java.lang.NullPointerException

Answer 1

private void WriteModeOn() {
        writeMode = true;
if(adapter!=null){
        adapter.enableForegroundDispatch(this, pendingIntent, writeTagFilters, null);
}
    }

    private void WriteModeOff() {
        writeMode = false;
if(adapter!=null){
        adapter.disableForegroundDispatch(this);
}
    }

无法恢复活动nullpointerexception

1 个答案: