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Android Studio - Chat not displaying

Main_activity.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="16dp"
android:paddingLeft="16dp"
android:paddingRight="16dp"
android:paddingTop="16dp"
android:id="#+id/activity_main"
tools:context="com.example.syafiq.mychatapp.MainActivity">
<android.support.design.widget.FloatingActionButton
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:clickable="true"
android:src="#drawable/ic_send"
android:id="#+id/fab"
android:tint="#android:color/white"
android:layout_alignParentBottom="true"
android:layout_alignParentLeft="true"
app:fabSize="mini"
/>
<android.support.design.widget.TextInputLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_toLeftOf="#+id/fab"
android:layout_alignParentBottom="true"
android:layout_alignParentEnd="true"
>
<EditText
android:id="#+id/messageinput"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignBottom="#+id/fab"
android:layout_centerHorizontal="true"
android:hint="Message..." />
</android.support.design.widget.TextInputLayout>
<ListView
android:id="#+id/list_of_message"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
android:layout_above="#+id/fab"
android:dividerHeight="16dp"
android:divider="#android:color/transparent"
android:layout_marginBottom="16dp"
android:layout_width="match_parent"
android:layout_height="match_parent">
</ListView>
Maincode
private static int SIGN_IN_REQUEST_CODE =1;
//private List<ChatMessage> list = new ArrayList<ChatMessage>();
FirebaseListAdapter<ChatMessage> adapter;
RelativeLayout activity_main;
FloatingActionButton fab;
FirebaseDatabase db = FirebaseDatabase.getInstance();
DatabaseReference myRef = db.getReference("message");
#Override
public boolean onOptionsItemSelected(MenuItem item)
{
if (item.getItemId() == R.id.menu_signout)
{
AuthUI
.getInstance()
.signOut(this)
.addOnCompleteListener(
new OnCompleteListener<Void>() {
#Override
public void onComplete(#NonNull Task<Void> task)
{
Snackbar
.make(
activity_main,
"You have been signed out.",
Snackbar.LENGTH_SHORT
).show()
;
finish();
}
}
)
;
}
return true;
}
#Override
protected void onActivityResult(int requestCode, int resultCode, Intent data)
{
super.onActivityResult(requestCode, resultCode, data);
if (resultCode==SIGN_IN_REQUEST_CODE)
{
if (resultCode==RESULT_OK)
{
Snackbar
.make(
activity_main,
"Successfully signed in!",
Snackbar.LENGTH_SHORT
).show()
;
displayChatMessage();
}
else
{
Snackbar
.make(
activity_main,
"We couldn't sign you in. Please try again lter!",
Snackbar.LENGTH_SHORT
).show()
;
finish();
}
}
}
#Override
public boolean onCreateOptionsMenu(Menu menu)
{
getMenuInflater().inflate(R.menu.main_menu,menu);
return true;
}
#Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
activity_main = (RelativeLayout) findViewById(R.id.activity_main);
fab = (FloatingActionButton) findViewById(R.id.fab);
fab
.setOnClickListener(
new View.OnClickListener()
{
#Override
public void onClick(View v)
{
EditText input = (EditText) findViewById(R.id.messageinput);
FirebaseDatabase
.getInstance()
.getReference()
.push()
.setValue(
new ChatMessage(
input.getText().toString(),
FirebaseAuth.getInstance().getCurrentUser().getEmail()
)
)
;
input.setText("");
displayChatMessage();
}
}
)
;
if (FirebaseAuth.getInstance().getCurrentUser()== null)
{
startActivityForResult(
AuthUI
.getInstance()
.createSignInIntentBuilder()
.build(),
SIGN_IN_REQUEST_CODE
);
}
else
{
Snackbar
.make(
activity_main,
"Welcome " + FirebaseAuth
.getInstance()
.getCurrentUser()
.getEmail(),
Snackbar.LENGTH_SHORT
).show()
;
//displayChatMessage();
}
}
DisplaychatMessage() function to display my chat message
private void displayChatMessage() {
Query query = FirebaseDatabase.getInstance().getReference().child("Chats");
ListView listofmsgs = (ListView) findViewById(R.id.list_of_message);
FirebaseListOptions<ChatMessage> options = new FirebaseListOptions
.Builder<ChatMessage>()
.setQuery(query, ChatMessage.class)
.setLayout(R.layout.list_item)
.build()
;
//adapter.startListening();
Log.d("ErrorCheck", "1");
adapter = new FirebaseListAdapter<ChatMessage>(options) {
#Override protected void populateView(View v, ChatMessage model, int position) {
//ChatMessage cm = (ChatMessage) model;
TextView messageText, messageUser, messageTime;
messageText = (TextView) v.findViewById(R.id.messageinput);
messageUser = (TextView) v.findViewById(R.id.message_user);
messageTime = (TextView) v.findViewById(R.id.message_time);
messageText.setText(model.getMessageText().toString());
messageUser.setText(model.getMessageUser());
messageTime
.setText(
android.text.format.DateFormat.format(
"dd-mm-yyyy (HH:mm:ss)",
model.getMessageTime()
)
)
;
Log.d("ErrorCheck", "2");
}
};
listofmsgs.setAdapter(adapter);
adapter.startListening();
}
Hi guys, i did this but it doesn't seem like anything appear on my APP. But the then, when i press send, in my database, my chat appears there but again it doesn't appear on my Chat app. I did a debug log. Errorcheck 1, and 2 to see where the code ends. When i checked, looks like the debug log only display up till ErrorCheck 1 and does not display display 2. How do i solve this?

You've put ErrorCheck 2 in something called an anonymous class. That's why calling your displayChatMessage() will only log ErrorCheck 1. The code in the anonymous FirebaseListAdapter class you defined will only run when it's populateView() method is called. You don't call that method. You call adapter.startListening(); Something, somewhere, needs to call adapter.populateView() before you'll see ErrorCheck 2 logged.
You likely don't want to call it here. You can call it here just for a test but you should track down what is supposed to be calling it.
According to the docs about FirebaseListAdapter
This class is a generic way of backing an Android ListView with a Firebase location. It handles all of the child events at the given Firebase location. It marshals received data into the given class type. Extend this class and provide an implementation of populateView, which will be given an instance of your list item mLayout and an instance your class that holds your data. Simply populate the view however you like and this class will handle updating the list as the data changes.
So trying changing the data and see if that makes you log ErrorCheck 2

How to start an Activity in onClick method from ArrayAdapter with custom rows

So I wrote a custom ArrayAdapter to have a ListView populate with custom row data, and also to set the OnClickListener to each row. I'm facing two major problems here:
the specified Activity (InvoiceActivity) does not start up on click.
The reaction time on clicks when I only output the logmessage (and comment the intent and startActivity part) is really slow.
For now there are only 3 rows of dummy data in the ListView.
I would be grateful for any help here, thanks!
public class InvoiceListAdapter extends ArrayAdapter<InvoiceShort> {
public InvoiceListAdapter(Context context, ArrayList<InvoiceShort> invoices) {
super(context, R.layout.list_elem, invoices);
}
#Override
public View getView(int position, View convertView, ViewGroup parent) {
final InvoiceShort invoice = getItem(position);
if (convertView == null) {
convertView = LayoutInflater.from(getContext()).inflate(R.layout.list_elem, parent, false);
convertView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(final View parent) {
Log.d("item click", "invoice id: " + invoice.getId());
Intent intent = new Intent(getContext(), InvoiceActivity.class);
intent.putExtra("id", invoice.getId());
getContext().startActivity(intent);
}
});
}
// Lookup view for data population
TextView date = (TextView) convertView.findViewById(R.id.textDate);
TextView total = (TextView) convertView.findViewById(R.id.textTotal);
CheckBox paid = (CheckBox) convertView.findViewById(R.id.checkBoxPaid);
// populate data
DateFormat formatter = new SimpleDateFormat("yyyy-MM-dd");
date.setText(formatter.format(invoice.getDate()));
total.setText("€ " + new Long(invoice.getTotal()).toString());
paid.setChecked(invoice.isPaid());
return convertView;
}
}
EDIT: Here is the xml for one row
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="50dp"
android:layout_centerHorizontal="false"
android:orientation="horizontal">
<CheckBox
android:id="#+id/checkBoxPaid"
android:layout_width="wrap_content"
android:layout_height="match_parent"
android:checked="true"
android:elevation="0dp"
android:inputType="none" />
<EditText
android:id="#+id/textDate"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="5"
android:background="#null"
android:ems="10"
android:focusable="false"
android:inputType="none"
android:text="2017-12-30" />
<EditText
android:id="#+id/textTotal"
android:layout_width="0dp"
android:layout_height="match_parent"
android:layout_weight="2"
android:background="#null"
android:ems="10"
android:focusable="false"
android:inputType="none"
android:text="10€"
android:textAlignment="viewStart" />
</LinearLayout>

I have found the solution, problem here is you have edittext in your xml but your fetching it as Textview, you can change textview to edittext in list_elem layout your click listener will work . Otherwise you can change textview to edittext in your customAdapter and set two click listener for edittext as below to make it work
date.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
View parent = (View) v.getParent();
parent.performClick();
}
});
total.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
View parent = (View) v.getParent();
parent.performClick();
}
});

Start activity using view context
Intent intent = new Intent(convertView.getContext(), InvoiceActivity.class);
intent.putExtra("id", invoice.getId());
convertView.getContext().startActivity(intent);

Implement OnClickListener in your Activity class in which you are setting the Adapter and showing the ListView.

Try to modify your constructor like this:-
Context mContext;
public InvoiceListAdapter(Context context, ArrayList<InvoiceShort>
invoices) {
super(context, R.layout.list_elem, invoices);
mContext = context;
}
and use mContext instead of getContext();
this part:-
#Override
public void onClick(View parent) {
Log.d("item click", "invoice id: " + invoice.getId());
Intent intent = new Intent(getContext(), InvoiceActivity.class);
intent.putExtra("id", invoice.getId());
getContext().startActivity(intent);
}
try by writing the onclick listner outside the
if else
convertView.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View parent) {
Log.d("item click", "invoice id: " + invoice.getId());
Intent intent = new Intent(getContext(), InvoiceActivity.class);
intent.putExtra("id", invoice.getId());
getContext().startActivity(intent);
}
});`

So I finally found the solution, in short it is setting this listener to all the views inside one row of the ListView:
/** A definition of a listener that does nothing but to let the click be handled by the parent **/
View.OnClickListener listener = new View.OnClickListener() {
#Override
public void onClick(View v) {
((ListView) parent).performItemClick(v, position, 0);
}
};
date.setOnClickListener(listener);
total.setOnClickListener(listener);
paid.setOnClickListener(listener);
And also to implement the OnItemClickedListener for the ListView inside the MainActivity

changing the setText value in a fragment from main activity

I've been trying to change my textViews text in a fragmentActivity based on whether or not user has clicked the button in the first main activity. I have 2 xml files and 2 .java files and the code at the moment crashes when the app starts.
This is the button code
public void getNum(View view) {
buttonCheck = 1;
}
And this is the code from FragmentTab
public class FragmentTab extends Fragment {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View v = inflater.inflate(R.layout.fragment_layout, container, false);
TextView tv = (TextView) v.findViewById(R.id.text);
tv.setText("nothing has been input yet");
if (FirstActivity.buttonCheck == 1){
tv = (TextView) v.findViewById(R.id.text);
tv.setText("value");}
FirstActivity.buttonCheck = 0;
return v;
}
The first error i get from my code from logcat is: "java.lang.NullPointerException
at com.ezentertainment.dietabialkowa.FragmentTab.onCreateView(FragmentTab.java:23)" and line 23 is tv.setText("nothing has been input yet");
any help at all is greatly appreciated, I have been fighting with this issue for quite some time now..
tl;dr how to change fragment value based upon input from mainActivity?
edit: here is the fragment_layout.xml
<LinearLayout
xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:orientation="vertical"
android:background="#eaecee">
<TextView
android:id="#+id/textResult"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:text="value"
android:textAppearance="?android:attr/textAppearanceMedium" />

Your textview id is
android:id="#+id/textResult"
So, that's why you're getting a null pointer exception.
Also, it's probably a good idea to do a null check when finding a view by ID, just in case something isn't instantiated yet, so it doesn't kill your app. Something like:
TextView tv = (TextView) v.findViewById(R.id.textResult);
if (tv != null){
tv.setText("nothing has been input yet");
} else {
// log something if you want.
}

How about using setArguement:
In Activity:
Bundle bundle = new Bundle();
bundle.putString("email", email);
FragmentA fragment = new FragmentA();
fragment.setArguments(bundle);
getSupportFragmentManager().beginTransaction().add(container, fragment, tag).addToBackStack(tag).commit(); //please input container and tag yourself
In FragmentA:
String email;
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
Bundle arguments = getArguments();
if (arguments != null)
email = bundle.getString("email");
...
}

Restore views in a fragment added programmatically

I have a single activity with a navigation drawer (the basic one provided by Eclipse new app wizard). I have a FrameLayout as a container for the different fragments of the app, which are replaced when selecting an item in the navigation drawer. They are also added to the BackStack.
These fragments contain a LinearLayout, which has some EditTexts and a Button. If the button is pressed, a new LinearLayout is created and a couple TextViews are added to it with the content of the EditTexts. The user can repeat this option more than once, so I cannot tell how many LinearLayouts I'll need, therefore I need to add them programmatically.
One of these fragments xml:
<LinearLayout
android:id="#+id/pen_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="vertical" >
<LinearLayout
android:id="#+id/new_pen_layout"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_marginTop="#dimen/activity_vertical_margin"
android:background="#drawable/border"
android:orientation="vertical"
android:paddingBottom="#dimen/home_section_margin_bottom"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/home_section_margin_top" >
<EditText
android:id="#+id/new_pen_round"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:inputType="number"
android:hint="#string/new_pen_round_hint"
android:textSize="#dimen/normal_text_size" />
<LinearLayout
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:orientation="horizontal"
android:weightSum="2" >
<Button
android:id="#+id/new_pen_cancel_button"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginRight="#dimen/new_item_button_margin_right"
android:layout_weight="1"
android:background="#drawable/button_bg"
android:paddingBottom="#dimen/new_item_button_padding_bottom"
android:paddingTop="#dimen/new_item_button_padding_top"
android:text="#string/new_item_cancel_button"
android:textSize="#dimen/normal_text_size" />
<Button
android:id="#+id/new_pen_insert_button"
android:layout_width="0dp"
android:layout_height="wrap_content"
android:layout_marginLeft="#dimen/new_item_button_margin_left"
android:layout_weight="1"
android:background="#drawable/button_bg"
android:paddingBottom="#dimen/new_item_button_padding_bottom"
android:paddingTop="#dimen/new_item_button_padding_top"
android:text="#string/new_pen_insert_button"
android:textSize="#dimen/normal_text_size" />
</LinearLayout>
</LinearLayout>
</LinearLayout>
There are actually many other EditTexts but I removed them here to keep it short, the result is the same. It's java file:
public class PenaltiesFragment extends Fragment {
#Override
public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_penalties, container, false);
Button insertNewPen = (Button) view.findViewById(R.id.new_pen_insert_button);
insertNewPen.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View v) {
TextView round = (TextView) getActivity().findViewById(R.id.new_pen_round);
LinearLayout layout = (LinearLayout) getActivity().findViewById(R.id.pen_layout);
int numChilds = layout.getChildCount();
CustomPenaltyLayout penalty = new CustomPenaltyLayout(getActivity(), round.getText());
layout.addView(penalty, numChilds - 1);
}
});
return view;
}
}
I removed some useless methods, which are just the default ones. CustomPenaltyLayoutis a subclass of LinearLayout which I created, it just creates some TextViews and adds them to itself.
Everything works fine here. The user inserts data in the EditText, presses the Insert button and a new layout is created and added in the fragment.
What I want to achieve is: say that I open the navigation drawer and select another page, the fragment gets replaced and if I go back to this fragment (via navigation drawer or via Back button) I want the text, that the user added, to be still there.
I do not call PenaltiesFragment.newInstance() everytime I switch back to this fragment, I instead create the PenaltiesFragment object once and keep using that one. This is what I do:
Fragment fragment;
switch (newContent) {
// various cases
case PEN:
if(penFragment == null) // penFragment is a private field of the Main Activity
penFragment = PenaltiesFragment.newInstance();
fragment = penFragment;
break;
}
getSupportFragmentManager()
.beginTransaction()
.replace(R.id.container, fragment)
.addToBackStack("fragment back")
.commit();
I understand that onCreateView() is called again when the fragment is reloaded, right? So that is probably why a new, blank fragment is what I see. But how do I get the inserted CustomPenaltyLayout back? I cannot create it in the onCreateView() method.

I found a solution to my problem. I replaced the default FrameLayout that Android automatically created as a container for my fragments, with a ViewPager, then created a FragmentPagerAdapter like this:
public static class MyAdapter extends FragmentPagerAdapter {
public MyAdapter(FragmentManager fm) {
super(fm);
}
#Override
public Fragment getItem(int position) {
Fragment fragment;
switch (position) {
// ...other cases
case PEN:
fragment = PenaltiesFragment.newInstance();
break;
// ...other cases
}
return fragment;
}
#Override
public int getCount() {
return 6;
}
}
Then the only thing left to do to keep all the views at all times has been to add this line to my activity onCreate method.
mPager.setOffscreenPageLimit(5);
See the documentation for details on how this method works.
This way, though, I had to reimplement all the back button logic, but it's still simple, and this is how I did it: I create a java.util.Stack<Integer> object, add fragment numbers to it (except when you use the back button, see below), and override onBackPressed() to make it pop the last viewed fragment instead of using the back stack, when my history stack is not empty.
You want to avoid pushing elements on the Stack when you press the back button, otherwise you will get stuck between two fragments if you keep using the back button, instead of eventually exiting.
My code:
MyAdapter mAdapter;
ViewPager mPager;
Stack<Integer> pageHistory;
int currentPage;
boolean saveToHistory;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mAdapter = new MyAdapter(getSupportFragmentManager());
mPager = (ViewPager)findViewById(R.id.container);
mPager.setAdapter(mAdapter);
mPager.setOffscreenPageLimit(5);
pageHistory = new Stack<Integer>();
mPager.setOnPageChangeListener(new OnPageChangeListener() {
#Override
public void onPageSelected(int arg0) {
if(saveToHistory)
pageHistory.push(Integer.valueOf(currentPage));
}
#Override
public void onPageScrolled(int arg0, float arg1, int arg2) {
}
#Override
public void onPageScrollStateChanged(int arg0) {
}
});
saveToHistory = true;
}
#Override
public void onBackPressed() {
if(pageHistory.empty())
super.onBackPressed();
else {
saveToHistory = false;
mPager.setCurrentItem(pageHistory.pop().intValue());
saveToHistory = true;
}
};

Null pointer when setting text/background on android app [duplicate]

This is a canonical question for a problem frequently posted on StackOverflow.
I'm following a tutorial. I've created a new activity using a wizard. I get NullPointerException when attempting to call a method on Views obtained with findViewById() in my activity onCreate().
Activity onCreate():
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}
Layout XML (fragment_main.xml):
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:paddingBottom="#dimen/activity_vertical_margin"
android:paddingLeft="#dimen/activity_horizontal_margin"
android:paddingRight="#dimen/activity_horizontal_margin"
android:paddingTop="#dimen/activity_vertical_margin"
tools:context="packagename.MainActivity$PlaceholderFragment" >
<View
android:layout_width="100dp"
android:layout_height="100dp"
android:id="#+id/something" />
</RelativeLayout>

The tutorial is probably outdated, attempting to create an activity-based UI instead of the fragment-based UI preferred by wizard-generated code.
The view is in the fragment layout (fragment_main.xml) and not in the activity layout (activity_main.xml). onCreate() is too early in the lifecycle to find it in the activity view hierarchy, and a null is returned. Invoking a method on null causes the NPE.
The preferred solution is to move the code to the fragment onCreateView(), calling findViewById() on the inflated fragment layout rootView:
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
View something = rootView.findViewById(R.id.something); // not activity findViewById()
something.setOnClickListener(new View.OnClickListener() { ... });
return rootView;
}
As a side note, the fragment layout will eventually be a part of the activity view hierarchy and discoverable with activity findViewById() but only after the fragment transaction has been run. Pending fragment transactions get executed in super.onStart() after onCreate().

Try OnStart() method and just use
View view = getView().findViewById(R.id.something);
or Declare any View using getView().findViewById method in onStart()
Declare click listener on view by anyView.setOnClickListener(this);

Try to shift your accessing views to the onViewCreated method of fragment because sometimes when you try to access the views in onCreate method they are not rendered at the time resulting null pointer exception.
#Override
public void onViewCreated(View view, #Nullable Bundle savedInstanceState) {
super.onViewCreated(view, savedInstanceState);
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}

Agreed, this is a typical error because people often don't really understand how Fragments work when they begin working on Android development. To alleviate confusion, I created a simple example code that I originally posted on Application is stopped in android emulator , but I posted it here as well.
An example is the following:
public class ContainerActivity extends FragmentActivity implements ExampleFragment.Callback
{
#Override
public void onCreate(Bundle saveInstanceState)
{
super.onCreate(saveInstanceState);
this.setContentView(R.layout.activity_container);
if (saveInstanceState == null)
{
getSupportFragmentManager().beginTransaction()
.add(R.id.activity_container_container, new ExampleFragment())
.addToBackStack(null)
.commit();
}
getSupportFragmentManager().addOnBackStackChangedListener(new OnBackStackChangedListener()
{
public void onBackStackChanged()
{
int backCount = getSupportFragmentManager().getBackStackEntryCount();
if (backCount == 0)
{
finish();
}
}
});
}
#Override
public void exampleFragmentCallback()
{
Toast.makeText(this, "Hello!", Toast.LENGTH_LONG).show();
}
}
activity_container.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent"
android:orientation="vertical" >
<FrameLayout
android:id="#+id/activity_container_container"
android:layout_width="match_parent"
android:layout_height="match_parent" />
</RelativeLayout>
ExampleFragment:
public class ExampleFragment extends Fragment implements View.OnClickListener
{
public static interface Callback
{
void exampleFragmentCallback();
}
private Button btnOne;
private Button btnTwo;
private Button btnThree;
private Callback callback;
#Override
public void onAttach(Activity activity)
{
super.onAttach(activity);
try
{
this.callback = (Callback) activity;
}
catch (ClassCastException e)
{
Log.e(this.getClass().getSimpleName(), "Activity must implement Callback interface.", e);
throw e;
}
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
{
View rootView = inflater.inflate(R.layout.fragment_example, container, false);
btnOne = (Button) rootView.findViewById(R.id.example_button_one);
btnTwo = (Button) rootView.findViewById(R.id.example_button_two);
btnThree = (Button) rootView.findViewById(R.id.example_button_three);
btnOne.setOnClickListener(this);
btnTwo.setOnClickListener(this);
btnThree.setOnClickListener(this);
return rootView;
}
#Override
public void onClick(View v)
{
if (btnOne == v)
{
Toast.makeText(getActivity(), "One.", Toast.LENGTH_LONG).show();
}
else if (btnTwo == v)
{
Toast.makeText(getActivity(), "Two.", Toast.LENGTH_LONG).show();
}
else if (btnThree == v)
{
callback.exampleFragmentCallback();
}
}
}
fragment_example.xml:
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:layout_width="match_parent"
android:layout_height="match_parent" >
<Button
android:id="#+id/example_button_one"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_centerHorizontal="true"
android:layout_marginTop="30dp"
android:text="#string/hello"
android:layout_marginLeft="20dp"
android:layout_marginRight="20dp"/>
<Button
android:id="#+id/example_button_two"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/example_button_one"
android:layout_alignRight="#+id/example_button_one"
android:layout_below="#+id/example_button_one"
android:layout_marginTop="30dp"
android:text="#string/hello" />
<Button
android:id="#+id/example_button_three"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignLeft="#+id/example_button_two"
android:layout_alignRight="#+id/example_button_two"
android:layout_below="#+id/example_button_two"
android:layout_marginTop="30dp"
android:text="#string/hello" />
</RelativeLayout>
And that should be a valid example, it shows how you can use an Activity to display a Fragment, and handle events in that Fragment. And also how to communicate with the containing Activity.

The view "something" is in fragment and not in activity, so instead of accessing it in activity you must access it in the fragment class like
In PlaceholderFragment.class
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View root = inflater.inflate(R.layout.fragment_main, container,
false);
View something = root .findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... });
return root;
}

You are trying to access UI elements in the onCreate() but , it is too early to access them , since in fragment views can be created in onCreateView() method.
And onActivityCreated() method is reliable to handle any actions on them, since activity is fully loaded in this state.

Add the following in your activity_main.xml
<fragment
android:id="#+id/myFragment"
android:name="packagename.MainActivity$PlaceholderFragment"
android:layout_width="wrap_content"
android:layout_height="wrap_content" >
</fragment>

Since you have declared your View in the fragment_main.xml,move that piece of code where you get the NPE in the onCreateView() method of the fragment.
This should solve the issue.

in the posted code above in the question there is a problem :
you are using R.layout.activity_main in oncreate method, but the xml files name is "fragment_main.xml" , means you are trying to get the view of fragment_main.xml file which is not being shown so it gives null pointer exception. change the code like :
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.fragment_main);// your xml layout ,where the views are
View something = findViewById(R.id.something);
something.setOnClickListener(new View.OnClickListener() { ... }); // NPE HERE
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new PlaceholderFragment()).commit();
}
}

You have to remember important thing is :
NullPointerException occurs when you have declared your variable and trying to retreive its value before assigning value to it.

Use onViewCreated() Method whenever using or calling views from fragments.
override fun onViewCreated(view: View?, savedInstanceState: Bundle?) {
super.onViewCreated(view, savedInstanceState)
View v = view.findViewById(R.id.whatever)
}

I've got the same NullPointerException initializing a listener after calling findViewById() onCreate() and onCreateView() methods.
But when I've used the onActivityCreated(Bundle savedInstanceState) {...} it works. So, I could access the GroupView and set my listener.
I hope it be helpful.

Most popular library for finding views which is used by almost every developer.
ButterKnife
As I can their are enough answers explaining finding views with proper methodology. But if you are android developer and code frequently on daily basis then you can use butter-knife which saves a lot time in finding views and you don't have write code for it, With in 2-3 steps you can find views in milliseconds.
Add dependency in app level gradle:
implementation 'com.jakewharton:butterknife:8.8.1'
annotationProcessor 'com.jakewharton:butterknife-compiler:8.8.1'
Add plugin for butter knife:
File -> Settings -> plugins->
Then search for Android ButterKnife Zelezny and install plugin and restart your studio and you are done with it.
Now just go to Oncreate method of your activity and right click on your layout_name and tap on generate button and select butterknife injection option and your views references will be automatically created like mention below:
#BindView(R.id.rv_featured_artist)
ViewPager rvFeaturedArtist;
#BindView(R.id.indicator)
PageIndicator indicator;
#BindView(R.id.rv_artist)
RecyclerView rvArtist;
#BindView(R.id.nsv)
NestedScrollingView nsv;
#BindView(R.id.btn_filter)
Button btnFilter;