Try to invoke virtual method on a null object reference

Question

I am trying to make a barcode scanner app, and I want to add scan results to a certain list. In the MainActivity, I have a certain button that should send me to MyList activity, but then the app crashes and I don't know how to solve it.

So here is my code:

public class MainActivity extends AbsRuntimePermission implements ZXingScannerView.ResultHandler{

private ZXingScannerView zXingScannerView;
private static final int REQUEST_PERMISSION = 10;
boolean ok = false, chk = false;
private String scanResult;

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

    chk = getIntent().getBooleanExtra("check", chk);

    if(chk) {
        chk = false;

        ok = true;
        zXingScannerView = new ZXingScannerView(getApplicationContext());
        setContentView(zXingScannerView);
        zXingScannerView.setResultHandler(this);
        zXingScannerView.startCamera();
    }
}

public void scan(View view){
    requestAppPermissions(new String[]{Manifest.permission.CAMERA}, R.string.msg, REQUEST_PERMISSION);
    ok = true;
    zXingScannerView = new ZXingScannerView(getApplicationContext());
    setContentView(zXingScannerView);
    zXingScannerView.setResultHandler(this);
    zXingScannerView.startCamera();
}

@Override
public void onPermissionGranted(int requestCode) {
    if(ok)
        Toast.makeText(getApplicationContext(), "Permission Granted", Toast.LENGTH_LONG).show();
}

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

    if(ok)
    {
        if(zXingScannerView == null)
        {
            zXingScannerView = new ZXingScannerView(this);
            setContentView(zXingScannerView);
        }
        zXingScannerView.setResultHandler(this);
        zXingScannerView.startCamera();
    }
}

@Override
protected void onPause() {
    super.onPause();
    zXingScannerView.stopCamera();
}

@Override
public void handleResult(final Result result) {
    scanResult = result.getText();
    AlertDialog.Builder builder = new AlertDialog.Builder(this);
    builder.setTitle("Result");
    builder.setPositiveButton("OK", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            zXingScannerView.resumeCameraPreview(MainActivity.this);
        }
    });
    builder.setNeutralButton("GO", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            try
            {
                Intent intent = new Intent(Intent.ACTION_VIEW, Uri.parse(scanResult));
                startActivity(intent);
            }catch (Exception ex)
            {
                Intent intent = new Intent(Intent.ACTION_WEB_SEARCH);
                intent.putExtra(SearchManager.QUERY, scanResult);
                startActivity(intent);
            }
        }
    });

    builder.setNegativeButton("Add to list", new DialogInterface.OnClickListener() {
        @Override
        public void onClick(DialogInterface dialog, int which) {
            Intent intent = new Intent(MainActivity.this, MyList.class);
            intent.putExtra("CODE", scanResult);
            startActivity(intent);
        }
    });

    builder.setMessage(scanResult);
    AlertDialog alert = builder.create();
    alert.show();
}

public void Lista(View view){
    Intent iNtent = new Intent(MainActivity.this, MyList.class);
    startActivity(iNtent);
}

}`

And this is my error report:

01-12 09:42:26.013 2670-2670/com.example.tchibo.justqr E/AndroidRuntime:        FATAL EXCEPTION: main

Process: com.example.tchibo.justqr, PID: 2670

java.lang.RuntimeException: Unable to pause activity {com.example.tchibo.justqr/com.example.tchibo.justqr.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera()' on a null object reference                                                                          at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:3976)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3942)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3916)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3890)
at android.app.ActivityThread.-wrap15(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1605)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6541)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:240)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:767)

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera()' on a null object reference

Actually, it says that is trying to invoke virtual method void me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera() on a null object reference.

(sorry for the post style but i'm not familiar with it)

this is part of the basics. if you don't know how to fix that, don't try to start with applications yet. — Stultuske, Jan 12 '18 at 07:55

score 1 · Accepted Answer · answered Jan 12 '18 at 07:59

Well you create method scan(View view) in this method you initialize your camera(ZXingScannerView ) object but din't call anywhere. have look for solution

in onCreate():

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

    chk = getIntent().getBooleanExtra("check", chk);
    zXingScannerView = new ZXingScannerView(getApplicationContext());
    if(chk) {
        chk = false;
        ok = true;
        zXingScannerView = new ZXingScannerView(getApplicationContext());
        setContentView(zXingScannerView);
        zXingScannerView.setResultHandler(this);
        zXingScannerView.startCamera();
    }
}

in onPause()

@Override
protected void onPause() {
    super.onPause();
    if(zXingScannerView!=null)
    zXingScannerView.stopCamera();
}

score 0 · Answer 2 · answered Jan 12 '18 at 07:58

0

You can check zXingScannerView before using it.

if (zXingScannerView != null){
    zXingScannerView.stopCamera();
}

answered Jan 12 '18 at 07:58

Wu Yuan Chun

1,116
11
11

score 0 · Answer 3 · answered Jan 12 '18 at 08:02

java.lang.RuntimeException: Unable to pause activity 
{com.example.tchibo.justqr/com.example.tchibo.justqr.MainActivity}: 
java.lang.NullPointerException: Attempt to invoke virtual method 'void 
me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera()' on a null 
object reference

the logcat is telling you the problem, which is this line :

zXingScannerView.stopCamera();

your app is trying to stop something that does not exist !!

trivial solution :

if(zXingScannerView!=null){
   zXingScannerView.stopCamera();
}

Try to invoke virtual method on a null object reference

3 Answers3