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App crashes when I call a constructor in an Activity

I am new to android studio and I am trying to build a Notepad app.
button.setOnClickListener(new View.OnClickListener()
{
#Override
public void onClick(View v) {
String title = findViewById(R.id.textView3).toString();
String note_content = findViewById(R.id.textView).toString();
FileOutputStream outputStream;
try
{
outputStream = openFileOutput(title, Context.MODE_PRIVATE);
outputStream.write(note_content.getBytes());
}
catch (FileNotFoundException e)
{
e.printStackTrace();
}
catch (IOException e)
{
e.printStackTrace();
}
MainActivity mainActivity = new MainActivity(title);
}
});
This is the button a user clicks to save the note. Once the code saves the note, it should send the Title to MainActivity so that it can be sent to Recycleview Adapter - this will display it in viewholder as a text.
Presently, when I run the code, it crashes - however, when I remove the constructor, the app works fine.
( MainActivity mainActivity = new MainActivity(title);)
Error:
10-02 02:39:13.822 27279-27279/? D/AndroidRuntime: Shutting down VM
10-02 02:39:13.824 27279-27279/? E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.quicknote, PID: 27279
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.quicknote/com.example.quicknote.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.quicknote.MainActivity> has no zero argument constructor
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2337)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5443)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
Caused by: java.lang.InstantiationException: java.lang.Class<com.example.quicknote.MainActivity> has no zero argument constructor
at java.lang.Class.newInstance(Native Method)
at android.app.Instrumentation.newActivity(Instrumentation.java:1090)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2327)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2490) 
at android.app.ActivityThread.-wrap11(ActivityThread.java) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1354) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:148) 
at android.app.ActivityThread.main(ActivityThread.java:5443) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:728) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618) 
10-02 02:39:20.081 27279-27279/com.example.quicknote I/Process: Sending signal. PID: 27279 SIG: 9

As per the error message:
java.lang.InstantiationException: java.lang.Class has no zero argument constructor
A zero argument constructor is required for the Android system to instantiate an Activity. You should never be manually calling an Activity constructor yourself since only the system can properly create an Activity.
The Parcelables and Bundles documentation details the correct way of sending information to an Activity using the extras Bundle.

Unable to instantiate activity. Because you can't start an activity like this. You have to use intent to start activity. To sent "title" use intent extra.
Intent intent=new Intent(CurrentActivity.this, NewActivty.this);
intent.putExtra("title", title);
startActivty(intent);

Is your Activity in your AndroidManifest.xml?
If it is, you have to retrieve the title from your extras from onCreate() of MainActivity instead from the constructor.
Something like that:
//that code instead of your MainActivity mainActivity = new MainActivity(title); line
Intent intent = new Intent(YourActualActivity.this, MainActivity.class);
intent.putExtra("title", title);
startActivity(intent);
//That code in the onCreate method of your MainActivity
Bundle extras = getIntent().getExtras();
if (extras != null) {
String title = extras.getString("title");
}

app crash with android.content.Context.getPackageName()' on a null object reference

i need help.
I'm in the finish phase of development of my android application and now after a few month of develop the app magically crash with this error :
java.lang.String android.content.Context.getPackageName()' on a null object reference
I don't know what is the problem but i will post the code that cause the crash :
public static void start(Context context) {
context.startActivity(new Intent(context, ConversationsActivity.class));
}
called with this piece of code (inside a fragment):
ConversationsActivity.start(getActivity());
The last change that i've make was to add Fabric.io ( in particular Branch ) and from the history in the last commit on git i don't show nothing that can produce this error
EDIT :
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ComponentName.<init>(ComponentName.java:128)
at android.content.Intent.<init>(Intent.java:4900)
at com.exampleapp.views.messages.ConversationsActivity.start(ConversationsActivity.java:31)
at com.exampleapp.views.menu.MenuFragment.onMenuMessagesClicked(MenuFragment.java:116)
at com.exampleapp.views.menu.MenuFragment_ViewBinding$5.doClick(MenuFragment_ViewBinding.java:82)
at butterknife.internal.DebouncingOnClickListener.onClick(DebouncingOnClickListener.java:22)
at android.view.View.performClick(View.java:5637)
at android.view.View$PerformClick.run(View.java:22429)
at android.os.Handler.handleCallback(Handler.java:751)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

Or you could do this:
public static void start(Activity activity) {
activity.startActivity(new Intent(activity, ConversationsActivity.class));
}

You should use getActivity() to launch an Activity from Fragment.
From a Fragment: Context is parent activity (getActivity()).
Intent intent = new Intent(getActivity(), ConversationsActivity.class);
startActivity(intent);
From an Activity: Context is current activity (this).
Intent intent = new Intent(this, ConversationsActivity.class);
startActivity(intent);

Your problem is that you are using the intent wrong. Replace your start() code with this:
public static void start() {
Intent i = new Intent(getApplicationContext(), ConversationsActivity.class);
startActivity(i);
}
Then instead of using ConversationsActivity.start(getActivity()); to call it, just use start(); when you want to call the method.
Hope this helps!

Start another activity in another class

I have a three class: MainActivity, BackgroundActivity and roomActivity.
BackgroundActivity will receipt the php server feedback.
If it is not login fail, it will call RoomActivity class.
I want to know that why is not functioning?
BackgroundActivity:
protected void onPostExecute(String result) {
if (result.equals("<meta charset=\"utf-8\">login fail")) {
alertDialog.setMessage("Please check your login email");
alertDialog.show();
} else {
**Intent myIntent = new Intent(MainActivity.class, RoomActivity.class);
startActivity(myIntent);**
}
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.a20_1discussboard, PID: 2262
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String android.content.Context.getPackageName()' on a null object reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:132)
at android.content.ComponentName.<init>(ComponentName.java:77)
at android.content.Intent.<init>(Intent.java:4160)
at com.example.a20_1discussboard.MainActivity.check(MainActivity.java:38)
at com.example.a20_1discussboard.BackgroundWorker.onPostExecute(BackgroundWorker.java:151)
at com.example.a20_1discussboard.BackgroundWorker.onPostExecute(BackgroundWorker.java:24)
at android.os.AsyncTask.finish(AsyncTask.java:636)
at android.os.AsyncTask.access$500(AsyncTask.java:177)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:653)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5254)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

change it to
Intent myIntent = new Intent(MainActivity.this, RoomActivity.class);

Calling startActivity() from outside of an Activity may produce AndroidRuntimeException, Use Context like:
Intent myIntent = new Intent(MainActivity.class, RoomActivity.class);
getApplicationContext().startActivity(myIntent);

Intent constructor is wrong.
it should be like :
Intent myIntent = new Intent(getApplicationContext(), RoomActivity.class);
startActivity(myIntent);
could you post more info about the issue ?

android.app.ActivityThread.getApplicationThread()' on a null object reference

The microhonePopUp method will work in MainActivity, but I'd like for it to work from another class (MediaButtonIntentReceiver). The problem is with startActivityForResult(intent, REQUEST_CODE); - but I don't know how to resolve it.
in the MainActivity class
public void microphonePopUp(){
Intent intent = new Intent(RecognizerIntent.ACTION_RECOGNIZE_SPEECH);
intent.putExtra(RecognizerIntent.EXTRA_LANGUAGE_MODEL,
RecognizerIntent.LANGUAGE_MODEL_FREE_FORM);
intent.putExtra(RecognizerIntent.EXTRA_PROMPT, "Voice your answer");
startActivityForResult(intent, REQUEST_CODE);
}
in the MediaButtonIntentReceiver class
public class MediaButtonIntentReceiver extends BroadcastReceiver {
#Override
public void onReceive(Context context, Intent intent) {
if (Intent.ACTION_MEDIA_BUTTON.equals(intent.getAction())) {
KeyEvent event = (KeyEvent)intent.getParcelableExtra(Intent.EXTRA_KEY_EVENT);
if (KeyEvent.KEYCODE_HEADSETHOOK == event.getKeyCode()) {
MainActivity test = new MainActivity();
test.microphonePopUp();
}
}
}
}
12-09 11:20:14.803 19556-19556/com.timtennyson.priceaddition
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.timtennyson.priceaddition, PID: 19556
java.lang.RuntimeException: Unable to start receiver
com.timtennyson.priceaddition.MediaButtonIntentReceiver:
java.lang.NullPointerException: Attempt to invoke virtual method
'android.app.ActivityThread$ApplicationThread
android.app.ActivityThread.getApplicationThread()' on a null object
reference
at android.app.ActivityThread.handleReceiver(ActivityThread.java:3641)
at android.app.ActivityThread.access$2000(ActivityThread.java:221)
at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1876)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:158)
at android.app.ActivityThread.main(ActivityThread.java:7224)
at java.lang.reflect.Method.invoke(Native Method)
at
com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1230)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1120)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual
method 'android.app.ActivityThread$ApplicationThread
android.app.ActivityThread.getApplicationThread()' on a null object
reference
at android.app.Activity.startActivityForResult(Activity.java:4283)
at android.app.Activity.startActivityForResult(Activity.java:4230)
at
android.support.v4.app.FragmentActivity.startActivityForResult(FragmentActivity.java:842)
at
com.timtennyson.priceaddition.MainActivity.microphonePopUp(MainActivity.java:103)
at
com.timtennyson.priceaddition.MediaButtonIntentReceiver.onReceive(MediaButtonIntentReceiver.java:27)
at android.app.ActivityThread.handleReceiver(ActivityThread.java:3634)
at android.app.ActivityThread.access$2000(ActivityThread.java:221) 
at
android.app.ActivityThread$H.handleMessage(ActivityThread.java:1876) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:158) 
at android.app.ActivityThread.main(ActivityThread.java:7224) 
at java.lang.reflect.Method.invoke(Native Method)

NEVER create an instance of an activity, service, or provider yourself.
If your objective is to listen for ACTION_MEDIA_BUTTON broadcasts while MainActivity is visible:
Move MediaButtonIntentReceiver to be a nested class inside of MainActivity
Get rid of test from onReceive(), and just call microphonePopUp() (which I assume is a method on MainActivity)
Register your MediaButtonIntentReceiver using registerReceiver() in onStart() of MainActivity, and use unregisterReceiver() in onStop()
If your objective is to listen ACTION_MEDIA_BUTTON broadcasts at other points in time — by registering your receiver in the manifest — while you can do that, you cannot use microphonePopUp() from that receiver.

Android "Taking Photos Simply" tutorial does not work for me [duplicate]

This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 7 years ago.
So, I am trying to implement taking of photos in my app.
I am following the official "Taking Photos Simply" tutorial, but it just gives me errors.
The only thing I am doing differently, it that I am using a popup dialog fragment.
Below is the error I get:
java.lang.RuntimeException: Failure delivering result ResultInfo{who=null, request=131073, result=-1, data=null} to activity {com.myapp.demo/com.myapp.demo.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Bundle android.content.Intent.getExtras()' on a null object reference
at android.app.ActivityThread.deliverResults(ActivityThread.java:3605)
at android.app.ActivityThread.handleSendResult(ActivityThread.java:3648)
at android.app.ActivityThread.access$1400(ActivityThread.java:154)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1370)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:135)
at android.app.ActivityThread.main(ActivityThread.java:5289)
at java.lang.reflect.Method.invoke(Native Method)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.os.Bundle android.content.Intent.getExtras()' on a null object reference
at com.paulcanning.mote.NoteDialogFragment.onActivityResult(NoteDialogFragment.java:179)
at android.support.v4.app.FragmentActivity.onActivityResult(FragmentActivity.java:153)
at android.app.Activity.dispatchActivityResult(Activity.java:6192)
at android.app.ActivityThread.deliverResults(ActivityThread.java:3601)
            at android.app.ActivityThread.handleSendResult(ActivityThread.java:3648)
            at android.app.ActivityThread.access$1400(ActivityThread.java:154)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1370)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5289)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:904)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:699)
Button click listener
takePhoto.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Intent takePictureIntent = new Intent(MediaStore.ACTION_IMAGE_CAPTURE);
if (takePictureIntent.resolveActivity(getContext().getPackageManager()) != null) {
//startActivityForResult(takePictureIntent, 1);
File photoFile = null;
try {
photoFile = createImageFile();
} catch (IOException ex) {
// Error occurred while creating the File
}
// Continue only if the File was successfully created
if (photoFile != null) {
takePictureIntent.putExtra(MediaStore.EXTRA_OUTPUT,
Uri.fromFile(photoFile));
startActivityForResult(takePictureIntent, 1);
}
}
}
});
onActivityResult (note, it wouldn't let me use protected)
#Override
public void onActivityResult(int requestCode, int resultCode, Intent data) {
if (requestCode == 1 && resultCode == getActivity().RESULT_OK) {
Bundle extras = data.getExtras();
Bitmap imageBitmap = (Bitmap) extras.get("data");
imageView.setImageBitmap(imageBitmap);
}
}

You are supplying EXTRA_OUTPUT. Hence, camera apps do not need to supply getExtra("data"), so it will generally be null.
Quoting the documentation for ACTION_IMAGE_CAPTURE:
The caller may pass an extra EXTRA_OUTPUT to control where this image will be written. If the EXTRA_OUTPUT is not present, then a small sized image is returned as a Bitmap object in the extra field. This is useful for applications that only need a small image. If the EXTRA_OUTPUT is present, then the full-sized image will be written to the Uri value of EXTRA_OUTPUT.
Your image (hopefully) is in the location that you supplied in EXTRA_OUTPUT.