Android save file to internal storage

Question

Hi i have an app at start show me login button, when i click this it shows me a dialog when input username and password, then i click ok and it should save username into a file in my internal storage. But when i press ok my app crashes giving me error in logcat.

Here mainclass:

    public class MainActivity extends ActionBarActivity {

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

        public void loginMethod(View v){
            MyDialog dialog = new MyDialog();
            dialog.show(getSupportFragmentManager(),"my_dialog");
        }

        public void writeToFile(String data) {
            try {
                OutputStreamWriter outputStreamWriter = new OutputStreamWriter(openFileOutput("config.txt", Context.MODE_PRIVATE));
                outputStreamWriter.write(data);
                outputStreamWriter.close();
            }
            catch (IOException e) {
                Log.e("Exception", "File write failed: " + e.toString());
            }
        }

    }


Here DialogClass:

    public class MyDialog extends DialogFragment{

    LayoutInflater inflater;
    View v;
    EditText user, password;

    MainActivity main;

    @NonNull
    @Override
    public Dialog onCreateDialog(Bundle savedInstanceState) {

        inflater = getActivity().getLayoutInflater();
        v = inflater.inflate(R.layout.my_dialog_layout,null);

        AlertDialog.Builder builder = new AlertDialog.Builder(getActivity());
        builder.setView(v).setPositiveButton("OK", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                user = (EditText) v.findViewById(R.id.username);
                password = (EditText) v.findViewById(R.id.password);
                if (user.getText().toString().equals("hello") && password.getText().toString().equals("123")) {   //here it checks if username and password are "hello" and "123"; if true it should save into file

                    main.writeToFile(user.getText().toString());
                    Intent i = new Intent("user_activity");
                    startActivity(i);
                    Toast.makeText(getActivity(), "Welcome: " + user.getText().toString(), Toast.LENGTH_LONG).show();
                } else {
                    Toast.makeText(getActivity(), "Username invalid", Toast.LENGTH_LONG).show();
                }
            }
        }).setNegativeButton("CANCEL", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {

            }
        });

        return builder.create();
    }
}

This is the logcat:

    01-07 10:53:32.000: E/AndroidRuntime(13341): FATAL EXCEPTION: main
01-07 10:53:32.000: E/AndroidRuntime(13341): java.lang.NullPointerException
01-07 10:53:32.000: E/AndroidRuntime(13341):    at com.example.digitpassword2.MyDialog$1.onClick(MyDialog.java:44)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at com.android.internal.app.AlertController$ButtonHandler.handleMessage(AlertController.java:166)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at android.os.Handler.dispatchMessage(Handler.java:99)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at android.os.Looper.loop(Looper.java:137)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at android.app.ActivityThread.main(ActivityThread.java:5103)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at java.lang.reflect.Method.invokeNative(Native Method)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at java.lang.reflect.Method.invoke(Method.java:525)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:737)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
01-07 10:53:32.000: E/AndroidRuntime(13341):    at dalvik.system.NativeStart.main(Native Method)

Thanks

Answer 1

Here:

main.writeToFile(user.getText().toString());

NullPointerException because main instance of MainActivity is Null.

if you want to call method from Activity to Fragment then do it as:

((MainActivity)getActivity()).writeToFile(user.getText().toString());