android - NullPointerException:尝试在空对象引用上调用虚方法 enableForegroundDispatch(...)

Question

我正在开发一个包含两个 Activity 的示例 Android 应用

1. 登录页面
2. 将 NDEF 消息写入 NFC 标签的 Activity 。

点击登录按钮后，我得到了 NullPointerException。这是整个日志:

12-08 11:57:32.510: E/AndroidRuntime(1162): FATAL EXCEPTION: main
12-08 11:57:32.510: E/AndroidRuntime(1162): Process: com.example.garageconnect, PID: 1162
12-08 11:57:32.510: E/AndroidRuntime(1162): java.lang.RuntimeException: Unable to resume activity {com.example.garageconnect/com.example.garageconnect.WriteActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.nfc.NfcAdapter.enableForegroundDispatch(android.app.Activity, android.app.PendingIntent, android.content.IntentFilter[], java.lang.String[][])' on a null object reference
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3103)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:3134)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2481)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.-wrap11(ActivityThread.java)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.os.Handler.dispatchMessage(Handler.java:102)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.os.Looper.loop(Looper.java:148)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.main(ActivityThread.java:5417)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at java.lang.reflect.Method.invoke(Native Method)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
12-08 11:57:32.510: E/AndroidRuntime(1162): Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.nfc.NfcAdapter.enableForegroundDispatch(android.app.Activity, android.app.PendingIntent, android.content.IntentFilter[], java.lang.String[][])' on a null object reference
12-08 11:57:32.510: E/AndroidRuntime(1162):     at com.example.garageconnect.WriteActivity.WriteModeOn(WriteActivity.java:134)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at com.example.garageconnect.WriteActivity.onResume(WriteActivity.java:129)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1258)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.Activity.performResume(Activity.java:6312)
12-08 11:57:32.510: E/AndroidRuntime(1162):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:3092)
12-08 11:57:32.510: E/AndroidRuntime(1162):     ... 10 more

MainActivity.java

public class MainActivity extends Activity {

    EditText t1,t2;
    Button b1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        t1=(EditText)findViewById(R.id.editText1);
        t2=(EditText)findViewById(R.id.editText2);
        b1=(Button)findViewById(R.id.button1);
        b1.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                String username=t1.getText().toString(),pwd=t2.getText().toString();
                if( username.equals("Siva") && pwd.equals("pwd123")){
                    Intent i = new Intent(getBaseContext(), WriteActivity.class);
                    startActivity(i);   
               }
            }
        });
    }
}

WriteActivity.java

public class WriteActivity extends Activity {

    boolean writeMode;
    NfcAdapter adapter;
    PendingIntent pendingIntent;
    IntentFilter writeTagFilters[];
    EditText t1,t2;
    Button b1;
    String nfcmsg;
    Tag tag;
    Context ctx;
    String str;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.write_activity);
        t1=(EditText)findViewById(R.id.editText1);
        t2=(EditText)findViewById(R.id.editText2);
        b1=(Button)findViewById(R.id.button1);
        ctx=this;

        b1.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                // TODO Auto-generated method stub
                try {
                    if(tag==null){
                        Toast.makeText(ctx, "No tag found", Toast.LENGTH_LONG ).show();
                    }
                    else{
                        write(t1.getText().toString(),t2.getText().toString(),tag);
                        Toast.makeText(ctx, "Done Successfully", Toast.LENGTH_LONG ).show();
                    }
                } 
                catch (IOException e) {
                    Toast.makeText(ctx, "Error in Writing", Toast.LENGTH_LONG ).show();
                    e.printStackTrace();
                } 
                catch (FormatException e) {
                    Toast.makeText(ctx, "Error in Writing FF" , 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 locid,String secid, Tag tag) throws IOException, FormatException {

        //Code that calls createRecord method to create NDEFRecord and write NDEF message to NFC tag
    }



    private NdefRecord createRecord(String text) throws UnsupportedEncodingException {
        //Code to create NDEF record and returns the record to write method
    }


    @Override
    protected void onNewIntent(Intent intent){
        if(NfcAdapter.ACTION_TAG_DISCOVERED.equals(intent.getAction())){
            tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);    
            Toast.makeText(this, "Tag Detected. Now Write!", 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);
    }
}

Answer 1

在

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

检查id适配器是否初始化，

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

对 WriteModeOff 做同样的事情。