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Toolbar in Fragment crashes app on launch

This seems like a common issue with lots of accepted answers but none of them seem to be working for me.
Most answers suggest using this piece of code to add a toolbar into a fragment:
mToolbar = (Toolbar)rootView.findViewById(R.id.toolbar);
if (mToolbar != null) {
((AppCompatActivity) getActivity()).setSupportActionBar(mToolbar);
}
Alas, this still crashes my app even though I'm using AppCompatActivity. I think the error may lie in the fact that my Fragment class extends Fragment and not AppCompatActivity but I don't know enough about android as of yet to be sure of this. It crashes on the setSupportActionBar line.
My Fragment code:
package erikligai.ribbitapplication;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
/**
* Created by erik on 2017-06-07.
*/
public class MessageFragment extends Fragment {
Toolbar mToolbar;
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, #Nullable ViewGroup container, #Nullable Bundle savedInstanceState) {
// return inflater.inflate(R.layout.message_fragment_layout, container, false);
View rootView = inflater.inflate(R.layout.message_fragment_layout, container, false);
mToolbar = (Toolbar)rootView.findViewById(R.id.toolbar);
if (mToolbar != null) {
((AppCompatActivity) getActivity()).setSupportActionBar(mToolbar);
}
return rootView;
}
Would appreciate any suggestions.

You need to cast your activity from getActivity() to AppCompatActivity
((AppCompatActivity) getActivity()).getSupportActionBar().setTitle();
Toolbar getting from Activity, not from Fragment
mToolbar = (Toolbar)rootView.findViewById(R.id.toolbar);

Cannot create widget in Android Fragment

I am trying to create edittext using Java in bottom fragment class based on input that is to be passed from top fragment. But when I type
Button add_submit = new Button(this);
I get error for the this for parameter. However I can use this code in MainActivity.java.
Why is this so? What is causing the error and how to fix it?
The following are the complete code for the class
package com.test.gpacalc;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
public class AddActivityBottom extends Fragment {
#Nullable
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
View view = inflater.inflate(R.layout.activity_add_bottom,container,false);
return view;
}
public void createAddInput(String number_of_subjects){
Button add_submit = new Button(this);
}
}

Button constructor requires context as an argument. Fragment doesn't implement it, activity does. try new Button(getActivity())

Open activity on button click in NavigationDrawer

I have created project in Android Studio with Navigation Drawer Activity and I have successfully created it,On navigation drawer activity I have created 3 section, In 1st section I put Button and I want to setOnClickListener method on that button which lead to start new Activity (ex. "xyz.class") I have used code
startActivity(new Intent(this,xyz.class));
but "this" keyword is not working and gives me error.
So, I changed code like
Context c;
startActivity(new Intent(c,xyz.class));
which gives NullPointerException,
My Section1 code is
import android.app.Fragment;
import android.content.Context;
import android.content.Intent;
import android.os.Build;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v4.widget.DrawerLayout;
import android.util.Log;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.RelativeLayout;
import android.widget.Toast;
import com.example.pratik.crm50.R;
import com.gc.materialdesign.views.ButtonFloat;
public class Dashboard extends Fragment implements View.OnClickListener {
View rootView;
Context c;
private ButtonFloat float_btn;
private Button but;
private Button btn;
Context c;
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState)
View v;
if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
v = inflater.inflate(R.layout.dashboard_layout,container, false);
} else {
v = super.onCreateView(inflater,container, savedInstanceState);
}
View vv=v.findViewById(R.id.button_simple);
vv.setOnClickListener(this);
return v;
}
#Override
public void onClick(View v) {
// Toast.makeText(c,"Float Button",Toast.LENGTH_SHORT).show();
Log.e("ssas","sasafa");
//startActivity(new Intent(c,xyz.class));
}
}
and I can get successfully log Log.e("ssas","sasafa") on above code.
So how to do this?

Fragment does not extend Context while the Intent constructor expects the first parameter to be exactly that.
The Context c variable is redundant because a Fragment saves the parent activity by default.
You need to use the activity (class that extends Context) that the fragment is attached to:
startActivity(new Intent(getActivity(), xyz.class));

You get a NullPointerException, because you´re never assining a value to Context c.
So as user3249477 also mentioned use the method getActivity() to assign a value to c like c=getActivity();
Or call it directly while starting the Intent. This would look like this: startActivity(new Intent(getActivity(),xyz.class));

Constructor within activity is undefined

I have been fighting with this error for a couple of hours and I am not able to continue. I need your help here!
I am trying to replace my previous TabNavigation at ActionBar by a ViewPager in SDK21, looking at comments within StackOverflow I found this webpage, where the use of PagerTabStrip is described with an example, so I tried to implement it in my activity, however I am getting an strange error.
I tried to google the problem and all the suggestions are not really applicable to my problem (wrong parameters in the constructor are the usual errors) . I also reproduced the error with a simple Activity in order to avoid anything I have in my previous activity, but the error keeps. I attach you here the code I replicated isolated:
import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
public class TestActivity extends Activity {
CustomPagerAdapter mCustomPagerAdapter;
ViewPager mViewPager;
#Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.adding_users_to_list);
mCustomPagerAdapter = new CustomPagerAdapter(getFragmentManager(), this.getApplicationContext());
mViewPager = (ViewPager) findViewById(R.id.pager);
mViewPager.setAdapter(mCustomPagerAdapter);
}
class CustomPagerAdapter extends FragmentPagerAdapter {
Context mContext;
public CustomPagerAdapter(FragmentManager fm, Context context) {
super(fm);
mContext = context;
}
#Override
public Fragment getItem(int position) {
// Create fragment object
Fragment fragment = new DemoFragment();
// Attach some data to the fragment
// that we'll use to populate our fragment layouts
Bundle args = new Bundle();
args.putInt("page_position", position + 1);
// Set the arguments on the fragment
// that will be fetched in the
// DemoFragment#onCreateView
fragment.setArguments(args);
return fragment;
}
#Override
public int getCount() {
return 3;
}
#Override
public CharSequence getPageTitle(int position) {
return "Page " + (position + 1);
}
}
class DemoFragment extends Fragment {
#Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// Inflate the layout resource that'll be returned
View rootView = inflater.inflate(R.layout.fragment_users, container, false);
// Get the arguments that was supplied when
// the fragment was instantiated in the
// CustomPagerAdapter
Bundle args = getArguments();
((TextView) rootView.findViewById(R.id.text_option)).setText("Page " + args.getInt("page_position"));
return rootView;
}
}
}
I am getting in the call to the constructor the following error:
"The constructor TestActivity.CustomePageAdapter(FragmentManager, Context) is undefined"
In here:
mCustomPagerAdapter = new CustomPagerAdapter(getFragmentManager(), this.getApplicationContext());
I have tried to introduce the Adapter outside and inside the activity definition and is still not working. Is likely to be something simple, but... I can't see it and I need other eyes. Any idea what I am doing wrong?

change the import of the activity to:
import android.support.v4.app.FragmentActivity;
Also change constructor to:
mCustomPagerAdapter = new CustomPagerAdapter(getSupportFragmentManager(), this.getApplicationContext());

The reason is that your activity is used from the ADT:
import android.app.Activity;
and the Fragments used from the support package:
import android.support.v4.app.FragmentManager;
change the import of the activity to:
import android.support.v4.app.FragmentActivity;

Android Fragment Type Mismatch

I am new to android and am following the tutorial at Dartmouth. http://www.cs.dartmouth.edu/~campbell/cs65/lecture08/lecture08.html
I am following all the codes and at the MainActivity.java,
// create the fragments
Fragment mFindFragment = new FindFragment();
Fragment mChatFragment = new ChatFragment();
// bind the fragments to the tabs - set up tabListeners for each tab
mFindTab.setTabListener(new MyTabsListener(mFindFragment,
getApplicationContext()));
mChatTab.setTabListener(new MyTabsListener(mChatFragment,
getApplicationContext()));
I have encountered this error: Type mismatch: cannot convert from FindFragment to Fragment. So I follow the fix error suggestions and change the code to
// create the fragments
FindFragment mFindFragment = new FindFragment();
// bind the fragments to the tabs - set up tabListeners for each tab
mFindTab.setTabListener(new MyTabsListener(mFindFragment,
getApplicationContext()));
Now, there is a new error: The constructor MyTabsListener(FindFragment, Context) is undefined.
Just in case the imports are critical, here they are:
import android.app.ActionBar;
import android.app.ActionBar.Tab;
import android.app.Activity;
import android.app.Fragment;
import android.app.FragmentTransaction;
import android.content.Context;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuInflater;
import android.view.MenuItem;
import android.widget.Toast;
For myTabsListener:
class MyTabsListener implements ActionBar.TabListener {
public Fragment fragment;
public Context context;
public MyTabsListener(Fragment fragment, Context context) {
this.fragment = fragment;
this.context = context;
}
#Override
public void onTabReselected(Tab tab, FragmentTransaction ft) {
Toast.makeText(context, "Reselected!", Toast.LENGTH_SHORT).show();
}
#Override
public void onTabSelected(Tab tab, FragmentTransaction ft) {
Toast.makeText(context, "Selected!", Toast.LENGTH_SHORT).show();
ft.replace(R.id.container, fragment);
}
#Override
public void onTabUnselected(Tab tab, FragmentTransaction ft) {
Toast.makeText(context, "Unselected!", Toast.LENGTH_SHORT).show();
ft.remove(fragment);
}
}
For my FindFragment class:
import android.os.Bundle;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
public class FindFragment extends Fragment {
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
// Inflate the layout for this fragment
return inflater.inflate(R.layout.findfragment, container, false);
}
}
I am pretty confused here. I am not sure if this is related to my import, lib setup or other problems. Thanks in advance!

You need to change this
import android.support.v4.app.Fragment;
// you will use this import when you want fragment from support library
// in that case you will extend FragmentActivity which is the base class for support based fragments
to
import android.app.Fragment;
in FindFragment.java.
Similarly do the same in ChatFragment.java also