getSupportFragmentManager().findFragmentById gives me a nullPointer

Question

I have app with viewpager.I need to call other fragment methon when i click button on first fragment.

this is my mail Activity

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package src.com.programmingmobile.pageviewer;  

package src.com.programmingmobile.pageviewer;  

import java.util.ArrayList;  
import java.util.List;  

import android.os.Bundle;  
import android.support.v4.app.Fragment;  
import android.support.v4.app.FragmentActivity;  
import android.support.v4.app.FragmentManager;  
import android.support.v4.app.FragmentPagerAdapter;  
import android.support.v4.view.ViewPager;  
import android.widget.Toast;  

public class PageViewActivity extends FragmentActivity implements Communicator{  
    MyPageAdapter pageAdapter;  
    @Override  
    public void onCreate(Bundle savedInstanceState) {  
        try{  
        super.onCreate(savedInstanceState);  

        setContentView(R.layout.activity_page_view);          
        List<Fragment> fragments = getFragments();          
        pageAdapter = new MyPageAdapter(getSupportFragmentManager(),  fragments);          
        ViewPager pager = (ViewPager)findViewById(R.id.viewpager);  
        pager.setAdapter(pageAdapter);  

        }catch(Exception e){  
            e.printStackTrace();  
        }  

    }  

    private List<Fragment> getFragments(){  
        List<Fragment> fList = new ArrayList<Fragment>();  

        fList.add(MyFragment.newInstance("Fragment 1"));  
        fList.add(MyFragment2.newInstance("Fragment 2"));  
        //fList.add(MyFragment.newInstance("Fragment 3"));  

        return fList;  
    }  

    private class MyPageAdapter extends FragmentPagerAdapter {  

        private List<Fragment> fragments;  

        public MyPageAdapter(FragmentManager fm, List<Fragment> fragments) {  
            super(fm);  
            this.fragments = fragments;  
        }  
        @Override  
        public Fragment getItem(int position) {  
            return this.fragments.get(position);  
        }  

        @Override  
        public int getCount() {  
            return this.fragments.size();  
        }  
    }  

    @Override  
    public void respond(String data) {  
        // TODO Auto-generated method stub  
        MyFragment2 fragment = (MyFragment2) getSupportFragmentManager().findFragmentById(R.id.Fragment2);   
        fragment.print();  

    }  
}  

this is my fragment1

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package src.com.programmingmobile.pageviewer;  

import java.io.InputStream;  

import javax.net.ssl.ManagerFactoryParameters;  

import com.epapyrus.plugpdf.SimpleDocumentReader;  
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;  
import com.epapyrus.plugpdf.SimpleReaderFactory;  
import com.epapyrus.plugpdf.core.viewer.DocumentState;  

import android.app.Activity;  
import android.content.res.AssetManager;  
import android.os.Bundle;  
import android.support.v4.app.Fragment;  
import android.support.v4.app.FragmentManager;  
import android.util.Log;  
import android.view.LayoutInflater;  
import android.view.View;  
import android.view.View.OnClickListener;  
import android.view.ViewGroup;  
import android.widget.Button;  
import android.widget.LinearLayout;  
import android.widget.TextView;  

public class MyFragment extends Fragment{  
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";  

    Communicator com;  

    public void onAttach(Activity a) {  
        super.onAttach(a);  
        com = (Communicator) a;  
    }  

    public static final MyFragment newInstance(String message) {  
        MyFragment f = new MyFragment();  
        Bundle bdl = new Bundle(1);  
        bdl.putString(EXTRA_MESSAGE, message);  
        f.setArguments(bdl);  
        return f;  
    }  

    @Override  
    public View onCreateView(LayoutInflater inflater, ViewGroup container,  
            Bundle savedInstanceState) {  
        String message = getArguments().getString(EXTRA_MESSAGE);  
        View v = inflater.inflate(R.layout.myfragment_layout, container, false);  
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);  
        messageTextView.setText(message);  
        // OpenPdf();  

        LinearLayout mLinearLayout = (LinearLayout) inflater.inflate(  
                R.layout.myfragment_layout, container, false);  

        // note that we're looking for a button with id="@+id/myButton" in your  
        // inflated layout  
        // Naturally, this can be any View; it doesn't have to be a button  
        Button mButton = (Button) mLinearLayout.findViewById(R.id.button1);  
        mButton.setOnClickListener(new OnClickListener() {  
            @Override  
            public void onClick(View v) {  
                com.respond("Fragment ");  
            }  
        });  

        return mLinearLayout;  
    }  



} 

this is fragment2

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package src.com.programmingmobile.pageviewer;  

import java.io.InputStream;  

import com.epapyrus.plugpdf.SimpleDocumentReader;  
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;  
import com.epapyrus.plugpdf.SimpleReaderFactory;  
import com.epapyrus.plugpdf.core.viewer.DocumentState;  

import android.content.res.AssetManager;  
import android.os.Bundle;  
import android.support.v4.app.Fragment;  
import android.util.Log;  
import android.view.LayoutInflater;  
import android.view.View;  
import android.view.ViewGroup;  
import android.widget.TextView;  
import android.widget.Toast;  

public class MyFragment2 extends Fragment {  
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";  

    // create a listener for receiving provide pdf loading results  
    SimpleDocumentReaderListener m_listener = new SimpleDocumentReaderListener() {  

        @Override  
        public void onLoadFinish(DocumentState.OPEN state) {  
        }  
    };  

    public static final MyFragment2 newInstance(String message) {  
        MyFragment2 f = new MyFragment2();  
        Bundle bdl = new Bundle(1);  
        bdl.putString(EXTRA_MESSAGE, message);  
        f.setArguments(bdl);  
        return f;  
    }  

    @Override  
    public View onCreateView(LayoutInflater inflater, ViewGroup container,  
            Bundle savedInstanceState) {  
        String message = getArguments().getString(EXTRA_MESSAGE);  
        View v = inflater.inflate(R.layout.myfragment_layout2, container, false);  
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);  
        messageTextView.setText(message);  
        //OpenPdf();  
        return v;  
    }  

    public void print(){  
        Toast.makeText(getActivity(), " inside Respond ", 100);  
    }  

}  

this is myfragment_layout xml file

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Note: Text content in the code blocks is automatically word-wrapped
package src.com.programmingmobile.pageviewer;  

import java.io.InputStream;  

import javax.net.ssl.ManagerFactoryParameters;  

import com.epapyrus.plugpdf.SimpleDocumentReader;  
import com.epapyrus.plugpdf.SimpleDocumentReaderListener;  
import com.epapyrus.plugpdf.SimpleReaderFactory;  
import com.epapyrus.plugpdf.core.viewer.DocumentState;  

import android.app.Activity;  
import android.content.res.AssetManager;  
import android.os.Bundle;  
import android.support.v4.app.Fragment;  
import android.support.v4.app.FragmentManager;  
import android.util.Log;  
import android.view.LayoutInflater;  
import android.view.View;  
import android.view.ViewGroup;  
import android.widget.TextView;  

public class MyFragment extends Fragment {  
    public static final String EXTRA_MESSAGE = "EXTRA_MESSAGE";  

    Communicator com;  

    public void onAttach(Activity a) {  
        super.onAttach(a);  
        com = (Communicator) com;  
    }  

    public static final MyFragment newInstance(String message) {  
        MyFragment f = new MyFragment();  
        Bundle bdl = new Bundle(1);  
        bdl.putString(EXTRA_MESSAGE, message);  
        f.setArguments(bdl);  
        return f;  
    }  

    @Override  
    public View onCreateView(LayoutInflater inflater, ViewGroup container,  
            Bundle savedInstanceState) {  
        String message = getArguments().getString(EXTRA_MESSAGE);  
        View v = inflater.inflate(R.layout.myfragment_layout, container, false);  
        TextView messageTextView = (TextView) v.findViewById(R.id.textView);  
        messageTextView.setText(message);  
        // OpenPdf();  
        return v;  
    }  

    public void onclick(View v) {  

        com.respond("Fragment ");  
    }  

}  

this is my_fragment2 xml filee

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Note: Text content in the code blocks is automatically word-wrapped
<?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:id="@+id/Fragment2"  
    android:orientation="vertical" >  

    <TextView  
        android:id="@+id/textView"  
        android:layout_width="wrap_content"  
        android:layout_height="wrap_content"  
        android:layout_centerHorizontal="true"  
        android:layout_centerVertical="true"  
        android:textAppearance="?android:attr/textAppearanceLarge"/>  

</RelativeLayout> 

when i run the app it gives me the first fragment with a button.when i clcick button it gives me below error

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10-07 09:37:52.610: E/AndroidRuntime(10907): FATAL EXCEPTION: main  
10-07 09:37:52.610: E/AndroidRuntime(10907): java.lang.NullPointerException  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at src.com.programmingmobile.pageviewer.PageViewActivity.respond(PageViewActivity.java:66)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at src.com.programmingmobile.pageviewer.MyFragment$1.onClick(MyFragment.java:63)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.view.View.performClick(View.java:3620)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.view.View$PerformClick.run(View.java:14322)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Handler.handleCallback(Handler.java:605)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Handler.dispatchMessage(Handler.java:92)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.os.Looper.loop(Looper.java:137)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at android.app.ActivityThread.main(ActivityThread.java:4507)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at java.lang.reflect.Method.invokeNative(Native Method)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at java.lang.reflect.Method.invoke(Method.java:511)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:978)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:745)  
10-07 09:37:52.610: E/AndroidRuntime(10907):    at dalvik.system.NativeStart.main(Native Method)  

The line which gives null pointer is below

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Note: Text content in the code blocks is automatically word-wrapped
MyFragment2 fragment = (MyFragment2) getSupportFragmentManager().findFragmentById(R.id.Fragment2); 

Answer 1

Looking at the source for FragmentPagerAdapter, they use FragmentManager.findFragmentByTag(name) where name is in the form of

"android:switcher:" + viewId + ":" + id;

where viewId is the android:id of the ViewPager and id is the result from getItemId(position) (which in your case is probably just the position).

NOTE: this is not safe and could change from version to version of the support library. It has remained constant throughout all previous releases, but you should consider instead setting up an event callback interface between your two components (technically, between your first fragment, the activity, and then the second fragment).