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I have used ViewPager in MainActivity, which extends AppCompatActivity. When I remove setOnClickListener it doesn't crash but when I set click listener it crashes with given log.

Here is the log but still I don't understand why its happening

 --------- beginning of crash
2018-11-15 20:26:51.814 1654-1654/com.example.suraj.myapplication E/AndroidRuntime: FATAL EXCEPTION: main
 Process: com.example.suraj.myapplication, PID: 1654
 java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
 at android.widget.Toast.<init>(Toast.java:104)
 at android.widget.Toast.makeText(Toast.java:262)
 at com.example.suraj.myapplication.ThirdFragment.onClick(ThirdFragment.java:56)
 at android.view.View.performClick(View.java:5637)
 at android.view.View$PerformClick.run(View.java:22429)
 at android.os.Handler.handleCallback(Handler.java:751)
 at android.os.Handler.dispatchMessage(Handler.java:95)
 at android.os.Looper.loop(Looper.java:154)
 at android.app.ActivityThread.main(ActivityThread.java:6119)
 at java.lang.reflect.Method.invoke(Native Method)
 at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
 at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)

I tried many times but still It's not working. My fragment code is as

package com.example.sdgh.myapplication;

import android.content.Context;
import android.os.Bundle;
import android.support.annotation.NonNull;
import android.support.annotation.Nullable;
import android.support.v4.app.Fragment;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageView;
import android.widget.Toast;

import com.squareup.picasso.Picasso;

public class ThirdFragment extends Fragment implements View.OnClickListener
 ImageView ig3;
 View view;
 Context mContext;
 Button b;

 @Nullable
 @Override
 public View onCreateView(@NonNull LayoutInflater inflater, @Nullable ViewGroup container, @Nullable Bundle savedInstanceState) 
 /* ViewGroup rootview3 = (ViewGroup)inflater.inflate(R.layout.third_frag,container,false);
 ImageView ig3 = (ImageView)rootview3.findViewById(R.id.thirdimage);
 // Picasso.get().load("http://i.imgur.com/DvpvklR.png").into(ig3);
 container.addView(rootview3);

 return rootview3;*/

 view = inflater.inflate(R.layout.third_frag,container,false);
 ig3 = (ImageView)view.findViewById(R.id.thirdimage);
 b = (Button)view.findViewById(R.id.thirdbutton);
 Picasso.get().load("https://cloudinary.com/images//da2fd17643796658bb733d1c848454590aa303cd/developeour-list.png").into(ig3);
 b.setOnClickListener(this);
 container.addView(view);

 return view;
 

 @Override
 public void onClick(View v) 
 switch (v.getId())
 case R.id.thirdbutton:
 Toast.makeText(mContext,"Third button clicked",Toast.LENGTH_SHORT).show();
 break;
 
 //Toast.makeText(mContext,"Onclicked",Toast.LENGTH_SHORT).show();
 

 public static ThirdFragment newInstance(String text) 
 ThirdFragment f = new ThirdFragment();
 Bundle b = new Bundle();
 b.putString("msg", text);
 f.setArguments(b);
 return f;
 

You forgot init varible mContext.

Toast.makeText(mContext,"Third button clicked",Toast.LENGTH_SHORT).show();

You can use getContext() or getActivity() like that:

Toast.makeText(getContext(),"Third button clicked",Toast.LENGTH_SHORT).show();

or

Toast.makeText(getActivity(),"Third button clicked",Toast.LENGTH_SHORT).show();