This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 2 years ago.
Hi i am new to android studio and I am trying to start a new activity - however, I am having endless issues with getting Context - I have tried a few different methods posted on stack overflow but It just keeps throwing a null pointer please help. See onCreate method and exception below.
AddRoutine.class is just a blank activity
MainActivity
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
MainActivity.mContext = this.getApplicationContext();
Toolbar toolbar = findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
//Add Routine
FloatingActionButton fab = findViewById(R.id.addRoutine);
fab.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
startActivity(new Intent(MainActivity.this, AddRoutine.class));
}
});
generateRoutineListing(getAppContext());
//if returnListing.length > 0
// recyclerView.addItems
//else
// Show Jumbotron/Message board explaining that no routines have been created
}
Exception
E/AndroidRuntime: FATAL EXCEPTION: main
Process: za.co.freelanceweb.routines, PID: 14648
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:130)
at android.content.Intent.<init>(Intent.java:5780)
at za.co.freelanceweb.routines.MainActivity$1.onClick(MainActivity.java:38)
at android.view.View.performClick(View.java:6294)
at android.view.View$PerformClick.run(View.java:24774)
at android.os.Handler.handleCallback(Handler.java:790)
at android.os.Handler.dispatchMessage(Handler.java:99)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6518)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
Thanks so Much
Instead of:
MainActivity.mContext = this.getApplicationContext();
Use:
Context mcontext = MainActivity.this; //Also, set mcontext as a global variable.
Also,
Instead of:
generateRoutineListing(getAppContext());
Use:
generateRoutineListing(mcontext);
The problem is probably in this line:
MainActivity.mContext = this.getApplicationContext();
Activity extends Context so you can always use this to refer to the activity's context.
I am not sure what you are trying to do with that but you should not be using the application's context on one activity. The application context lives through out the entire lifetime of the application.
If you want want to start a new activity do this.
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Intent intent = new Intent(this, AddRoutineActivity.class);
startActivity(intent);
}
And then register AddRoutineActivity in your AndroidManifest.xml file like so:
<activity android:name=".AddRoutineActivity" />
If you are new to android you may want to check out Kotlin.
Related
I'm trying to make my app handle device rotation but it always crashes when I add the below code on the onCreate method from the mainActivity. Here is the error that I am getting. how to I fix this? :
java.lang.NullPointerException: Attempt to invoke virtual method 'void
android.widget.ArrayAdapter.clear()' on a null object reference
2019-09-25 06:58:16.743 10444-10444/com.example.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.myapplication, PID: 10444
java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.ArrayAdapter.clear()' on a null object reference
at com.example.myapplication.MainActivity$1.onChanged(MainActivity.java:48)
at com.example.myapplication.MainActivity$1.onChanged(MainActivity.java:44)
at androidx.lifecycle.LiveData.considerNotify(LiveData.java:113)
at androidx.lifecycle.LiveData.dispatchingValue(LiveData.java:131)
at androidx.lifecycle.LiveData.setValue(LiveData.java:289)
at androidx.lifecycle.LiveData$1.run(LiveData.java:91)
at android.os.Handler.handleCallback(Handler.java:883)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:214)
at android.app.ActivityThread.main(ActivityThread.java:7356)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:492)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:930)
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
contactListView = (ListView) findViewById(R.id.contactsListView);
adapter = new ArrayAdapter<Contact>(this,android.R.layout.simple_list_item_1, contacts);
contactListView.setAdapter(adapter);
contactListView.setOnItemClickListener(this);
setContentView(R.layout.activity_main);
}
You should better place your LiveData in a ViewModel and observe it in the activity, probably in the onCreate() method. Let me show you how can you properly use LiveData in a ViewModel,
YourViewModel
public class YourViewModel extends ViewModel {
// Create a LiveData with a List of Contact
private MutableLiveData<List<Contact>> contactList = new MutableLiveData<>();
// encapsulated with immutable live data
public LiveData<List<Contact>> getContactList() {
return contactList;
}
// call this method with a list of new contacts whenever you need to refresh your list
public void updateContactList(List<Contact> list) {
contactList.setValue(list);
}
// Rest of the ViewModel...
}
YourActivity
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
contactListView = (ListView) findViewById(R.id.contactsListView);
yourViewModel = ViewModelProviders.of(this).get(YourViewModel.class);
if(contacts==null) contacts = new ArrayList<>();
adapter = new ArrayAdapter<Contact>(this,android.R.layout.simple_list_item_1, contacts);
contactListView.setAdapter(adapter);
contactListView.setOnItemClickListener(this);
yourViewModel.getContactList().observe(this, contactObserver);
}
Observer<List<Contact>> contactObserver = new Observer<List<Contact>>() {
#Override
public void onChanged(#Nullable List<Contact> newContacts) {
if(adapter != null && newContacts != null) {
adapter.clear();
adapter.addAll(newContacts);
}
}
};
Above way you are ensuring the liveData is tied with the activity lifecycle and it gets a non null adapter in the onChanged() method. I hope my answer will help you solve your issue.
when the device is rotated android tears down the activity and recreates it. So the entire life cycle is called.
so save everything in onSavedInstanceState then extract data in onRetainInstanceState
for more details read here :https://developer.android.com/guide/topics/resources/runtime-changes
Also here I believe here the issue is setContentView(R.layout.activity_main) is called after contactListView = (ListView) findViewById(R.id.contactsListView)
try this
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
contactListView = (ListView) findViewById(R.id.contactsListView);
adapter = new ArrayAdapter<Contact>(this,android.R.layout.simple_list_item_1, contacts);
contactListView.setAdapter(adapter);
contactListView.setOnItemClickListener(this);
}
It should work
Handling lifecycle changes is a pain in the butt. I strongly recommend you to take a look at architecture component ViewModel. It will handle orientation changes and other lifecycle changes like a charm. Add together with DataBinding and you have a much more robust application that will handle lifecycle changes automatically for you.
This question already has answers here:
findViewByID returns null
(33 answers)
Closed 3 years ago.
This is a basic activity swapping.
The app does not crash if i declare a local button inside the configureActivitySwap() method like this:
Button voiceBtn = (findViewById(R.id.goToVoice));
But I have to declare the button in the global scope instead so I can use the button in other methods, mainly activating and deactivating the button when it should/should not be pressed.
I also noticed that if I remove the finish(); method and replace it with something else the app functions normally, but I have to have the finish(); method one way or another.
public class RecogActivity extends AppCompatActivity {
private Button voiceBtn;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
voiceBtn = findViewById(R.id.goToVoice);
setContentView(R.layout.main_layout);
// some unrelated code
configureActivitySwap();
}
public void configureActivitySwap(){
// Button voiceBtn = (findViewById(R.id.goToVoice));
voiceBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
finish();
}
});
}
}
My runtime error logs:
E/AndroidRuntime: FATAL EXCEPTION: main
Process: tk.gandriks.gaaudiotransform, PID: 23125
java.lang.RuntimeException: Unable to start activity ComponentInfo{tk.gandriks.gaaudiotransform/tk.gandriks.gaaudiotransform.RecogActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2957)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6944)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at tk.gandriks.gaaudiotransform.RecogActivity.configureActivitySwap(RecogActivity.java:140)
at tk.gandriks.gaaudiotransform.RecogActivity.onCreate(RecogActivity.java:124)
at android.app.Activity.performCreate(Activity.java:7183)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1220)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2910)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3032)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1696)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6944)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
You need to call the setContentView() before calling voiceBtn = findViewById(R.id.goToVoice); Since you don't specify the layout the findViewById method will not get the button instance
public class RecogActivity extends AppCompatActivity {
private Button voiceBtn;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
// set the layout first
setContentView(R.layout.YOUR_LAYOUT_XML_FILE_NAME)
voiceBtn = findViewById(R.id.goToVoice);
// some unrelated code
configureActivitySwap();
}
public void configureActivitySwap(){
// Button voiceBtn = (findViewById(R.id.goToVoice));
voiceBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
finish();
}
});
}
Try I guess) In your // some unrelated code is contains setContentView method?
public class RecogActivity extends AppCompatActivity {
private Button voiceBtn;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
voiceBtn = findViewById(R.id.goToVoice);
setContentView(R.layout.some_layout)
// some unrelated code
configureActivitySwap();
}
public void configureActivitySwap(){
// Button voiceBtn = (findViewById(R.id.goToVoice));
voiceBtn.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View view) {
finish();
}
});
}
}
You caught NPE because of findViewById is calling on inflated view. You are have been calling findViewById before setContentView in the first case and got the exception. And in the second case - in configureActivitySwap, that going after setContentView. Move setContentView after super.onCreate(savedInstanceState) and all will be working fine.
Are you setting layout before trying to find view with findViewById?
setContentView(R.layout.main_layout);
voiceBtn = (Button) findViewById(R.id.goToVoice);
replace the statement in your onCreate() method with the above. It should work.
and use
super.finish() instead of finish()
This question already has answers here:
What is a NullPointerException, and how do I fix it?
(12 answers)
Closed 5 years ago.
I am trying to create a user profile section in my app. In my Login Activity, I ask for the username from FirebaseAUTH.getCurrentUser().getDisplayName() and save it in a string.
In my xml file for the main activity, I put a textView with a string containing a placeholder.
I tried both in the Login and the Main Activity Java files to .setText .format etc, but the text simply doesnt change..
Any hints on how to solve this?
public class MainActivity extends AppCompatActivity
implements NavigationView.OnNavigationItemSelectedListener {
private HomeFragment hf;
private WebViewFragment wvf;
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
firebaseAuth = FirebaseAuth.getInstance();
if (firebaseAuth.getCurrentUser().getDisplayName() != null) {
String name = firebaseAuth.getCurrentUser().getDisplayName();
final TextView profile = (TextView)findViewById(R.id.profile_section);
profile.setText("Test");
}
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.vreeni.firebaseauthentiction, PID: 6060
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.vreeni.firebaseauthentiction/com.example.vreeni.firebaseauthentication.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2822)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2897)
at android.app.ActivityThread.-wrap11(Unknown Source:0)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1598)
at android.os.Handler.dispatchMessage(Handler.java:105)
at android.os.Looper.loop(Looper.java:251)
at android.app.ActivityThread.main(ActivityThread.java:6563)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$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 android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at com.example.vreeni.firebaseauthentication.MainActivity.onCreate(MainActivity.java:52)
at android.app.Activity.performCreate(Activity.java:6975)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1214)
You can find a simple example right at the top of this resource.
public class MainActivity extends Activity {
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
final TextView helloTextView = (TextView) findViewById(R.id.text_view_id);
helloTextView.setText(R.string.user_greeting);
}
}
If you still have problems, please provide your code.
I am trying to change elements such TextViews etc. that are parts of the Fragment (which is used for SlidingTabLayout). I can access TextView from the Tab1 class:
public class Tab1 extends Fragment {
public static TextView serverName;
#Override
public View onCreateView(LayoutInflater inflater, #Nullable ViewGroup container, #Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.tab_1,container,false);
serverName = (TextView) view.findViewById(R.id.serverName);
serverName.setText("This works, but I can't change text from outside the Tab1 class");
return view;
}
But when I want access the serverName TextView from anywhere I am always getting null value. Here I am trying to change the text from the activity which contains Sliding Tabs (Tab1 is a part of it):
public class Dashboard2 extends AppCompatActivity {
Toolbar toolbar;
ViewPager pager;
ViewPagerAdapter adapter;
SlidingTabLayout tabs;
CharSequence tabsTitles[] = {"Info", "Options"};
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_dashboard2);
InitializeToolbarAndTabs();
Tab1.serverName.setText("This doesn't work");
}
private void InitializeToolbarAndTabs()
{
toolbar = (Toolbar) findViewById(R.id.tool_bar);
setSupportActionBar(toolbar);
adapter = new ViewPagerAdapter(getSupportFragmentManager(), tabsTitles, tabsTitles.length);
pager = (ViewPager) findViewById(R.id.pager);
pager.setAdapter(adapter);
tabs = (SlidingTabLayout) findViewById(R.id.tabs);
tabs.setDistributeEvenly(true);
tabs.setCustomTabColorizer(new SlidingTabLayout.TabColorizer() {
#Override
public int getIndicatorColor(int position) {
return getResources().getColor(R.color.tabsScrollColor);
}
});
tabs.setViewPager(pager);
}
}
Logs from the Android Studio:
java.lang.RuntimeException: Unable to start activity ComponentInfo{ASD.Dashboard2}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3119)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218)
at android.app.ActivityThread.access$1000(ActivityThread.java:198)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6837)
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:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference
at ASD.Dashboard2.onCreate(Dashboard2.java:54)
at android.app.Activity.performCreate(Activity.java:6500)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1120)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3072)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3218)
at android.app.ActivityThread.access$1000(ActivityThread.java:198)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1676)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:145)
at android.app.ActivityThread.main(ActivityThread.java:6837)
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:1404)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1199)
Attempt to invoke virtual method 'void android.widget.TextView.setText(java.lang.CharSequence)' on a null object reference - how to solve this problem?
Not sure what you are doing but you can always find your fragment by FragmentManage.findFragmentByTag() or FragmentManager.findFragmentById().
Once found just access your field.
I don't think you have initialized the Tab1 fragment, at least I can't see it there.
Accessing fragment variables from an Activity through static declarations is a horrible idea, use voids in the fragment class.
I am sorry, the error you are getting is not related with accessing or not a static object of the Fragment. The error you are receiving is because at the moment you call Tab1.serverName.setText("This doesn't work"); your fragment still didnt inflate the view or still didn't charge the TextView (didn't arrive yet to serverName = (TextView) view.findViewById(R.id.serverName);). The fragments are charged into the view in a asynchrounous way, so even if you declare it in your layout as a tag, it may be possible that the Fragment's view is still not fully loaded.
If you absolutely want to be sure the fragment's view is fully loaded, use the protected void onResumeFragments() method:
#Override
protected void onResumeFragments() {
super.onResumeFragments();
Tab1.serverName.setText("This doesn't work");
}
Anyway, I strongly recommend you NOT to access fragment objects statically, but to use the findFragmentById() or findFragmentByTag() methods and then to access a public method inside the given fragment.
Hope it helps.
I am new to android programming and I am trying to create a simple home screen replacement/launcher using a viewpager.
When I try to run the application, it starts up and then immediately force closes. I think it is something to do with a null pointer exception but I am really new to programming and can't find the source of the error.
This is my logcat.
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.dntmdp.matthewhuckel.simplephonelauncher/com.dntmdp.matthewhuckel.simplephonelauncher.MainActivity}: java.lang.NullPointerException
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
// etc...
Caused by: java.lang.NullPointerException
at com.dntmdp.matthewhuckel.simplephonelauncher.MainActivity.onCreate(MainActivity.java:47)
at android.app.Activity.performCreate(Activity.java:5231)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
// etc...
08-13 20:10:58.585 1805-1805/com.dntmdp.matthewhuckel.simplephonelauncher I/Process﹕ Sending signal. PID: 1805 SIG: 9
This is my main activity java class.
public class MainActivity extends ActionBarActivity {
Button phone;
// more buttons
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ImageButton phone = (ImageButton) findViewById(R.id.imageButton);
phone.setOnClickListener(new View.OnClickListener() { // Exception thrown on this line
#Override
public void onClick(View view) {
Intent LaunchIntent = getPackageManager().getLaunchIntentForPackage("com.android.phone");
startActivity(LaunchIntent);
}
});
//setting more onclicklisteners in exactly the same manner
MyPagerAdapter adapter = new MyPagerAdapter();
ViewPager myPager = (ViewPager) findViewById(R.id.viewpager_layout);
myPager.setAdapter(adapter);
myPager.setCurrentItem(0);
}
// menu initialization
}
I found the answer. I was trying to find the image button id's in the activity_main xml, but they were in other activities.