NullpointerException on MifareClassic.connect()

Question

So i want to read from a Tag, but get an NullPointerException when connecting to it. I know that my Tag is not null, as my Activity starts upon detecting it and an additional Test in the Code proves it.

public void getDataFromTag()
{
    tag = intent.getParcelableExtra(NfcAdapter.EXTRA_TAG);
    text.setText(tag.toString());
    text.append("\n\n");
    // get NDEF tag details
    MifareClassic mfc = MifareClassic.get(tag);
    byte[] data;
    if (tag == null)
        text.append("\n Tag is null");
    try
    {
        mfc.connect();
        ...
    }
    catch (IOException e) 
    { 
        text.append("\n " + "Error while reading!");
    } 
    catch (NullPointerException e)
    {
        text.append("\n " + e.toString());
    }
}

So what is the problem here? Btw, I am testing my app on a Nexus 7 Tablet.

On popular demand... here's a logCat

04-13 15:56:47.101: D/libEGL(2046): loaded /system/lib/egl/libEGL_tegra.so
04-13 15:56:47.121: D/libEGL(2046): loaded /system/lib/egl/libGLESv1_CM_tegra.so
04-13 15:56:47.131: D/libEGL(2046): loaded /system/lib/egl/libGLESv2_tegra.so
04-13 15:56:47.151: D/OpenGLRenderer(2046): Enabling debug mode 0
04-13 15:56:52.161: D/AndroidRuntime(2046): Shutting down VM
04-13 15:56:52.161: W/dalvikvm(2046): threadid=1: thread exiting with uncaught exception (group=0x41037930)
04-13 15:56:52.171: E/AndroidRuntime(2046): FATAL EXCEPTION: main
04-13 15:56:52.171: E/AndroidRuntime(2046): java.lang.RuntimeException: Unable to resume activity {bsp.app/bsp.app.Main_Menue}: java.lang.NullPointerException
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2742)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.handleResumeActivity(ActivityThread.java:2771)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2235)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.access$600(ActivityThread.java:141)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1234)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.os.Looper.loop(Looper.java:137)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.main(ActivityThread.java:5041)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at java.lang.reflect.Method.invokeNative(Native Method)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at java.lang.reflect.Method.invoke(Method.java:511)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at dalvik.system.NativeStart.main(Native Method)
04-13 15:56:52.171: E/AndroidRuntime(2046): Caused by: java.lang.NullPointerException
04-13 15:56:52.171: E/AndroidRuntime(2046):     at bsp.app.Main_Menue.getDataFromTag(Main_Menue.java:81)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at bsp.app.Main_Menue.onResume(Main_Menue.java:65)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.Instrumentation.callActivityOnResume(Instrumentation.java:1185)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.Activity.performResume(Activity.java:5182)
04-13 15:56:52.171: E/AndroidRuntime(2046):     at android.app.ActivityThread.performResumeActivity(ActivityThread.java:2732)
04-13 15:56:52.171: E/AndroidRuntime(2046):     ... 12 more

Answer 1

You do not need to get the Tag object to get to the NDEF content, Android reads NDEf content per default and you can get it using

Parcelable[] messages = intent.getParcelableArrayExtra(NfcAdapter.EXTRA_NDEF_MESSAGES);
if (messages != null) {
    NdefMessage[] ndefMessages = new NdefMessage[messages.length];
    for (int i = 0; i < messages.length; i++) {
        ndefMessages[i] = (NdefMessage) messages[i];
    }
    // process messages, usually only a single is present
}

Note that getting any particular tag technology class (MifareClassic) does not work if the tag is in fact not of that technology - that is what you are seeing. So you should first check which tag type you have.

Answer 2

You should check whether mfc is not null after the line MifareClassic mfc = MifareClassic.get(tag);. If it is null, the tag is perhaps not MIFARE Classic (perhaps you should try MifareUltralight instead).