Develop Reference

Implement onBackPressed for 2 fragments

I've looked at multiple questions, haven't found solution yet.
I've youtube player SDK imported and a webview. One is in Live_fragment and another Article_Fragment.
This is the onBackPressed code that causes the crash:
#Override
override fun onBackPressed() {
if (webViewE.canGoBack() || youTubePlayer.isFullScreen()) {
youTubePlayer.exitFullScreen()
webViewE.goBack()
} else {
super.onBackPressed()
}
}
Now, webViewE.canGoBack and youTubePlayer.isFullScreen() alone works fine (if I only put in one of them), but if they're both together in the function, I get:
Attempt to invoke virtual method 'boolean android.webkit.WebView.canGoBack()' on a null object reference" and app crashes

This means your WebView variable is null at the time of the onBackPressed invocation. You need to make sure you properly initialise it and ensure it hasn't been nullified somehow up until possible calls to onBackPressed. If you want to axe this condition in the event that the WebView may be null, you can just add a check for null alongside your current check, i.e.:
#Override
override fun onBackPressed() {
if ((webViewE != null && webViewE.canGoBack()) || youTubePlayer.isFullScreen()) {
youTubePlayer.exitFullScreen()
webViewE.goBack()
} else {
super.onBackPressed()
}
}

Crash on getSystemService(Context.CAMERA_SERVICE)

I have a widget with a button that enables the camera service.
It works quite well but after some time (maybe with the home screen app exit) it loses the status (bein on or off).
I have a crash with a NullPointerException from, I believe, Context.CAMERA_SERVICE. It is really hard to reproduce and so far I haven't found the sequence that leads to the crash.
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Object android.content.Context.getSystemService(java.lang.String)' on a null object reference
at android.content.ContextWrapper.getSystemService(ContextWrapper.java:714)
at com.widget.tst.Widget.CallbackWidgetService.isCameraInUse(CallbackWidgetService.java:163)
at com.widget.tst.Widget.CallbackWidgetService.startCameraInUse(CallbackWidgetService.java:242)
//** button click-->
at com.widget.tst.Widget.CameraWidgetReceiver.updateWidgetButton(CameraWidgetReceiver.java:55)
at com.widget.tst.Widget.CameraWidgetReceiver.onReceive(CameraWidgetReceiver.java:32)
at android.app.ActivityThread.handleReceiver(ActivityThread.java:3632)
at android.app.ActivityThread.-wrap18(Unknown Source:0) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1975) 
at android.os.Handler.dispatchMessage(Handler.java:109) 
at android.os.Looper.loop(Looper.java:166) 
at android.app.ActivityThread.main(ActivityThread.java:7367) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:469) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:963)
How can Context.CAMERA_SERVICE be null?
private void isCameraInUse(){
if(context == null) return;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.LOLLIPOP) {
--> line 163 mCameraManager = (CameraManager)getSystemService(Context.CAMERA_SERVICE);
}
else{
Toast.makeText(context, "You need to run Android version "+
Build.VERSION_CODES.M+" or above",
Toast.LENGTH_SHORT).show();
return;
}
 

It looks like somewhere inside your current class (this) it's own context gets nulled, and that's why you get your NPE.
But since you have a 100% non-null context in a variable (if(context == null) return;), why don;t you use it instead of this's own context?:
mCameraManager = (CameraManager)context.getSystemService(Context.CAMERA_SERVICE);

unable to create a service

I have a music player application and untill now, it was working smoothly.
So, i learnt about Android Focus from android documentation and i thought i would try this on my app for learning purposes.
App: minimum api level :16; Maximum:26;
I basically implemented the AudioManager.OnAudioFocusChangeListener in my service class.
In it i included this code:
if(Build.VERSION.SDK_INT>=26){
mAudioFocusRequest = new AudioFocusRequest.Builder(AudioManager.AUDIOFOCUS_GAIN)
.setAudioAttributes(mAudioAttributes)
.setAcceptsDelayedFocusGain(true)
.setOnAudioFocusChangeListener(this)
.build();
int res = mAudioManager.requestAudioFocus(mAudioFocusRequest);
synchronized(mFocusLock) {
if (res == AudioManager.AUDIOFOCUS_REQUEST_FAILED) {
mPlaybackNowAuthorized = false;
} else if (res == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
mPlaybackNowAuthorized = true;
playSong();
} else if (res == AudioManager.AUDIOFOCUS_REQUEST_DELAYED) {
mPlaybackDelayed = true;
mPlaybackNowAuthorized = false;
}
}
} else if(Build.VERSION.SDK_INT<26){
int result = 0;
if (mAudioManager != null) {
result = mAudioManager.requestAudioFocus(this,AudioManager.STREAM_MUSIC,AudioManager.AUDIOFOCUS_GAIN);
}
if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
mPlaybackNowAuthorized = true;
playSong();
}
}
My onAudioFocusChange(int focusChange) looks something like this:
if(Build.VERSION.SDK_INT<26){
//<26 code
}else {
//26 and above api level
}
I tried implementing audiofocus in my application and this is how i did it. The issue is when i try to run my app now , it gives me this error
unable to create a service
What's the problem with it? i think i read the documentation clearly.
Am i missing out something?
Update:
Removing the below code does make it work
else if(Build.VERSION.SDK_INT<26){
int result = 0;
if (mAudioManager != null) {
result = mAudioManager.requestAudioFocus(this,AudioManager.STREAM_MUSIC,AudioManager.AUDIOFOCUS_GAIN);
}
if (result == AudioManager.AUDIOFOCUS_REQUEST_GRANTED) {
mPlaybackNowAuthorized = true;
playSong();
}
Stacktrace:
12-18 16:27:47.740 30183-30183/com.example.tilak.imusicplay E/AndroidRuntime:
12-18 16:27:47.740 30183-30183/com.example.tilak.imusicplay E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.tilak.imusicplay, PID: 30183
java.lang.RuntimeException: Unable to create service com.example.tilak.imusicplay.MusicService: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference
at android.app.ActivityThread.handleCreateService(ActivityThread.java:2920)
at android.app.ActivityThread.access$2000(ActivityThread.java:153)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1458)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:5529)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629)
Caused by: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object reference
at com.example.tilak.imusicplay.MusicService.playSong(MusicService.java:239)
at com.example.tilak.imusicplay.MusicService.initMusicPlayer(MusicService.java:115)
at com.example.tilak.imusicplay.MusicService.onCreate(MusicService.java:66)
at android.app.ActivityThread.handleCreateService(ActivityThread.java:2910)
at android.app.ActivityThread.access$2000(ActivityThread.java:153) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1458) 
at android.os.Handler.dispatchMessage(Handler.java:102) 
at android.os.Looper.loop(Looper.java:154) 
at android.app.ActivityThread.main(ActivityThread.java:5529) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:739) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:629) 
12-18 16:27:47.742 30183-30183/com.example.tilak.imusicplay E/MQSEventManagerDelegate: failed to get MQSService.
Whole Class Code

look your Error Msg!this is a NullPointerException. List is a null object! check you code
java.lang.RuntimeException: Unable to create service com.example.tilak.imusicplay.MusicService: java.lang.NullPointerException: Attempt to invoke interface method 'java.lang.Object java.util.List.get(int)' on a null object

The problem is that your application does not initialise songs. (Sure, there is a setSongs method, but you don't call it!)
Solution: add code to initialize songs in the appropriate place. That will fix this error ...
I strongly advise you to read the Question and Answers for:
What is a NullPointerException, and how do I fix it?

Error while trying to get data from a webservice with retrofit

I have an app that sometimes get crushed, it has never happened to me, some users have reported it... this is the stack trace that google sends me:
java.lang.NullPointerException:
at com.mal.saul.preciosbitcoinmexico.Fragment.FragmentPrincipal.
realizarCambioPrecio(FragmentPrincipal.java:181)
at com.mal.saul.preciosbitcoinmexico.Fragment.FragmentPrincipal.
access$200(FragmentPrincipal.java:42)
at com.mal.saul.preciosbitcoinmexico.Fragment.FragmentPrincipal$2.
onResponse(FragmentPrincipal.java:160)
at retrofit2.ExecutorCallAdapterFactory$ExecutorCallbackCall$1$1.
run(ExecutorCallAdapterFactory.java:68)
at android.os.Handler.handleCallback(Handler.java:815)
at android.os.Handler.dispatchMessage(Handler.java:104)
at android.os.Looper.loop(Looper.java:194)
at android.app.ActivityThread.main(ActivityThread.java:5637)
at java.lang.reflect.Method.invoke(Native Method:0)
at java.lang.reflect.Method.invoke(Method.java:372)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.
run(ZygoteInit.java:959)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:754)
i think my error is on the onResponse Method, here it is:
#Override
public void onResponse(Call<BtcValuesResponse> call,
Response<BtcValuesResponse> response) {
BtcValuesResponse btcValuesResponse =
response.body();
if(btcValuesResponse != null){
btcValues =
btcValuesResponse.getBtcValues();
showBtcValues();
}
else {
Toast.makeText(getActivity(), "Bitso No Está Disponible Por
el Momento", Toast.LENGTH_SHORT).show();
}
}
any idea?

You are requesting data by Retrofit asynchronously, after request is completed, you show data by calling showBtcValues().
In a case that your request has not completed yet but users change to other screen. So, when it finish, it still calls showBtcValues, then you will get crash because there is no UI component there.
Please try that to recreate the crash. I think it is the issue here.
If it is, the solution is that you should check the state of Fragment (or Activity) where you call request.

As stated before maybe your there is no UI component there
or
You are just assuming the the response Response<BtcValuesResponse> response is valid, but you should check if the API response is not rate limited, or due connectivity errors, the response was an invalid nonce or something else. You need to verify response status before unwrapping it.
if(response.isSuccessful()){
// Get the body, where the BTCValueResponse is
BtcValuesResponse responseBody = response.body();
btcValuesResponse.getBtcValues();
showBtcValues();
}else{
// Check the error stream
System.out.println(Utils.inputStreamToString(wrappedServiceResponse.errorBody().byteStream()));
}

Android mediaplayer using uri causes app crash

I have been working to resolve this problem for the last three days and cannot get it working. These are the steps of my app:
Open file browser to select and audio file
Audio file is selected
Selected audio file is added to the database
from a list of items the added file is selected
a play button is pressed to play the sound file
at this point the sound file will play, however if I close the app then open it again and attempt to play the same sound file the app crashes with the following error:
FATAL EXCEPTION: main
Process: com.arcitech.developer.ultimatesoundboard, PID: 22967
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.media.MediaPlayer.start()' on a null object reference
at com.arcitech.developer.ultimatesoundboard.activities.ItemDetailFragment$4.onClick(ItemDetailFragment.java:153)
at android.view.View.performClick(View.java:5156)
at android.view.View$PerformClick.run(View.java:20755)
at android.os.Handler.handleCallback(Handler.java:739)
at android.os.Handler.dispatchMessage(Handler.java:95)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:5835)
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:1399)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1194)
the filepath(uri) I am saving to the database is:
Content://com.android.providers.media.documents/document/audio%3A15790
and the code I am using is located in the onCreateView of a fragment:
MediaPlayer m;
playSound.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
if (m != null) {
m.stop();
m.release();
m = null;
}
m = MediaPlayer.create(getActivity(), Uri.parse(mItem.filePath));
m.start();
m.setOnCompletionListener(new MediaPlayer.OnCompletionListener() {
#Override
public void onCompletion(MediaPlayer m) {
countdownTimerHandler.removeCallbacks(countdownTimerRunnable);
}
});
}
});

According to the doc for MediaPlayer.create(), it can return null if for some reason MediaPlayer cannot be created. You should be checking the return value for null every time unless you are very confident that it will not return null (but even then, do it for safety).
You may also want to check the log at the time create() is called to see if Android is leaving any clues as to why creation failed.