NullPointerException异常：尝试在空对象引用

Question

我正在开发一个样本Android应用程序有两个活动

1. 一个登录页面
2. 的活动写入调用虚拟方法enableForegroundDispatch（...） NDEF消息到NFC标签。

点击登录按钮后，出现NullPointerException。这里是整个日志：

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); 
    } 

查身份证适配器初始化与否，

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

为WriteModeOff做的一样。