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Android Fragment Back press

I have a couple of fragments. But all the fragments lead to PeriodFragment when they press a button. My question is how do I implement so that when the user presses the BACK button on his mobile phone, the PeriodFragment will switch back to the fragment it was entered from.
This is the java code from PeriodFragment.java:
public class PeriodFragment extends Fragment {
Button btnPretrazi;
public PeriodFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
// Inflate the layout for this fragment
View view = inflater.inflate(R.layout.fragment_period, container, false);
//buttonPretraziPeriod
btnPretrazi = (Button) view.findViewById(R.id.buttonPretraziPeriod);
btnPretrazi.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
Toast.makeText(getContext(),"test", Toast.LENGTH_SHORT).show();
}
});
return view;
}
}
This is my TvrdjavaFragment.java (one of the fragments that have the button to switch to PeriodFragment.java) :
package com.example.ivanp.msnis;
public class TvrdjavaFragment extends Fragment {
Button btnIdinaperiod;
public TvrdjavaFragment() {
// Required empty public constructor
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_tvrdjava, container, false);
// Inflate the layout for this fragment
//show date
TextView datumprikaz = (TextView) view.findViewById(R.id.datumprikaz);
Date danas = new Date();
SimpleDateFormat sdf = new SimpleDateFormat("dd.MM.yyyy");
String novDatum = sdf.format(danas);
datumprikaz.setText(novDatum);
//End of show date
btnIdinaperiod = (Button) view.findViewById(R.id.buttonIdinaperiod);
btnIdinaperiod.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
PeriodFragment periodFragment = new PeriodFragment();
FragmentTransaction periodFragmentTransaction = getActivity().getSupportFragmentManager().beginTransaction();
periodFragmentTransaction.replace(R.id.frame, periodFragment);
periodFragmentTransaction.commit();
}
});
return view;
}
}
I'm new to android studio, so please tell the details.

According to android docs When using fragments in your app, individual FragmentTransaction objects may represent context changes that should be added to the back stack. For example, if you are implementing a master/detail flow on a handset by swapping out fragments, you should ensure that pressing the Back button on a detail screen returns the user to the master screen. To do so, call addToBackStack() before you commit the transaction:
// Works with either the framework FragmentManager or the
// support package FragmentManager (getSupportFragmentManager).
getSupportFragmentManager().beginTransaction()
.add(detailFragment, "detail")
// Add this transaction to the back stack
.addToBackStack()
.commit();

Adding to backstack will solve the problem:
periodFragmentTransaction.replace(R.id.frame, periodFragment);
periodFragmentTransaction.addToBackStack(null);
periodFragmentTransaction.commit();

Adding a view to a fragment from another fragment

I'm trying to add a view in fragment B from fragment A, they both belong to the same activity but they are never running simultaneously
My activity:
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
fm = getSupportFragmentManager();
Fragment_Main main = new Fragment_Main();
fm.beginTransaction()
.replace(R.id.mainContainer, main)
.commit();
/* Fragment_Main is a fragment with two buttons
* to either call fragment A or fragment B and
* mainContainer is a FrameLayout occupying the whole screen
*/
}
Fragment A:
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, #Nullable ViewGroup container, #Nullable Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.fragment_subjects, container, false);
ButterKnife.bind(this, view);
AddButton.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
FragmentB.addMyView();
}
});
return view;
}
Fragment B:
public static void addMyView(){
final RelativeLayout.LayoutParams params = new RelativeLayout.LayoutParams(ViewGroup.LayoutParams.WRAP_CONTENT, ViewGroup.LayoutParams.WRAP_CONTENT);
params.setMargins(0, 200, 0, 0);
CustomView mView = new CustomView(mContext, "english", "a1", "10:12", "14:32");
mLayout.addView(mView, params);
// CustomView is the custom view I want to add
}
Question:
I know that a fragment can't communicate with another fragment directly, how do I tell the activity to add a view permanently to FragmentB when the method addMyView() is called?

Persisting for one activity lifecycle
FragmentA should set a flag (e.g. a boolean attribute) on the Activity instance.
When FragmentB is instantiated, the Activity should pass this flag to the created instance (e.g. via Bundle). FragmentB should then decide whether or not to add the View.
Persisting permanently
FragmentA should set a SharedPreferences flag.
FragmentB should check this flag in onCreateView and potentially add the View depending on the value.
General advice:
Solving communication between instances with static values/methods is almost always bad practice.

Android Studio - Fragment button action

I tried to make an application that shows a fragment at start then you can change that fragment to an other one with a button in the first fragment. But when I put the button action in the fragments java, it won't start and I get the nullpointerexception error. Could you tell me why?
public class Fragment_main extends Fragment {
FragmentManager fm = getFragmentManager();
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
Button button_inditas = (Button) getView().findViewById(R.id.button_inditas);
button_inditas.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
fm.beginTransaction().replace(R.id.content_frame, new Fragment_1()).commit();
}
});
return inflater.inflate(R.layout.fragment_main,container,false);
}
}

But when I put the button action in the fragments java, it won't start and I get the nullpointerexception error. Could you tell me why?
Well I see some errors on your code...
First of all you can't call getView() if you haven't inflated one, it means that you MUST inflate a view as i'll put on my answer and then you can avoid the getView() and use that view itself.
And instead of returning the inflater you have to return your View
This is how your Fragment should look like:
public class Fragment_main extends Fragment {
public Fragment_main() {
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main,container,false);
Button button_inditas = (Button)rootView.findViewById(R.id.button_inditas);
button_inditas.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View v) {
FragmentManager fragmentManager = getFragmentManager ();
FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction ();
fragmentTransaction.add (R.id.content_frame, new Fragment_1());
fragmentTransaction.commit ();
});
return rootView;
}
}

You have to inflate the view first and use the view returned by that instead of getView ().
View view = inflater.inflate (...);
and then
... = (Button) view.findView(...);
This happens because the view that is returned by getView () hasn't been created yet, so getView() returns null.

Nullpointerexception thrown when trying to findViewById

I have the following Activity:
public class MainActivity extends ActionBarActivity {
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
if (savedInstanceState == null) {
getSupportFragmentManager().beginTransaction()
.add(R.id.container, new StartFragment())
.commit();
}
Button login = (Button) findViewById(R.id.loginButton);
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View arg0) {
Intent intent = new Intent(MainActivity.this,LoginActivity.class);
startActivity(intent);
}
});
}
I get a NPE when I try to invoke findViewByID for R.id.loginButton, and I'm guessing this is because loginButton is within a separate Fragment, which I have as:
public static class StartFragment extends Fragment {
public StartFragment() {
}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
return inflater.inflate(R.layout.fragment_main, container, false);
}
}
However, I am unsure of how to fix this so that I can find the loginButton ID. I haven't worked with fragments before, so I realize I may be using them/implementing them incorrectly. fragment_main contains a few buttons in a LinearLayout, and activity_main has nothing but a single FrameLayout.

Try to implement your onCreateView(...) in Fragment like
#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);
something.setOnClickListener(new View.OnClickListener() { ... });
return rootView;
}
The Button 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.

Write code to initialize button from fragment becuase your button is into fragment layout not into activity's layout.
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_main, container,
false);
Button login = (Button) rootView.findViewById(R.id.loginButton);
login.setOnClickListener(new View.OnClickListener() {
#Override
public void onClick(View arg0) {
Intent intent = new Intent(MainActivity.this,
LoginActivity.class);
startActivity(intent);
}
});
return rootView;
}
And remove the login button related code from onCreate of Activity.

findViewById() works with reference to a root view.
Without having a view in the first place will throw a null pointer exception
In any activity you set a view by calling setContentView(someView);.
Thus when you call findViewById() , its with reference to the someView.
Also findViewById() finds the id only if its in that someView. So in you case null pointer exception
For fragments, adapters, activity, .... any view's findViewById() will only find if the id exixts in the view
Alternately if you are inflating a view, then you can also use inflatedView.findViewById() to get a view from that inflatedView
In short make sure you have the id in your layout you are referring to or make findViewById() call in appropriate place(Ex. adapters getView(), activity's onCreate() or onResume() or onPause() , fragments onCreateView(), ....)
Also have an idea about UI & background thread's as you cannot efficiently update UI in bg-threads

Android how to save all views in Fragment when user click on next and prev button

i use fragment for my application,they are : Fragment A, Fragment B and Fragment C.
to move from one fragment to another fragment, I use two buttons, the first button is used to move to the next fragment, and the second button is used to return to the previous fragment.
the problems when I move from one fragment to another fragment (such as from A to B and back to A and to B again and to C). all entries in the EditText that is in a fragment is lost and look back empty,it when I go back to the previous fragment, or when I want to go back to the next fragment. I've been using code addToBackStack () when switching to another fragment, indeed EditText that I made not lost when I press the back button on my phone, but it does not affect the buttons (Next And Prev) that I've created. this is Fragment A:
public class AFragment extends Fragment {
EditText text,text2;
public AFragment(){}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View rootView = inflater.inflate(R.layout.fragment_a, container, false);
ImageView next=(ImageView)rootView.findViewById(R.id.next);
text=(EditText)rootView.findViewById(R.id.text);
text2=(EditText)rootView.findViewById(R.id.text1);
next.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View arg0) {
getFragmentManager().beginTransaction().replace(R.id.frame_container, new BFragment()).addToBackStack(null).commit();
}
});
return rootView;
}
this is Fragment B:
public class BFragment extends Fragment {
EditText cut,cut2;
public BFragment(){}
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container,
Bundle savedInstanceState) {
View Fragment B = inflater.inflate(R.layout.fragment_c, container, false);
ImageView next=(ImageView)rootView.findViewById(R.id.next);
ImageView prev=(ImageView)rootView.findViewById(R.id.prev);
cut=(EditText)rootView.findViewById(R.id.cut);
cut2=(EditText)rootView.findViewById(R.id.cut2);
next.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View arg0) {
getFragmentManager().beginTransaction().replace(R.id.frame_container, new CFragment()).addToBackStack(null).commit();
}
});
prev.setOnClickListener(new OnClickListener() {
#Override
public void onClick(View arg0) {
getFragmentManager().beginTransaction().replace(R.id.frame_container, new AFragment()).addToBackStack(null).commit();
}
});
return Fragment B;
}
my question is how can i save view in Fragment when user click on next and prev button in my application? is there something missing in my code? i hope someone can help me to solve my problem. thank you very much.

Each fragments have various life cycle methods, you can use onDestroy method and save the required edit text view value calling Activity. For this expose some methods in the calling activity which can be access via fragment. Also else create a singleton class or use Shared Prefences.
I hope this helps.

try to change:
getFragmentManager().beginTransaction().replace(R.id.frame_container, new BFragment()).addToBackStack(null).commit();
to:
getFragmentManager().beginTransaction().add(R.id.frame_container, new BFragment()).addToBackStack(null).commit();