In the below example I have created a commuication between an activity and fragments classes. In the below example user enters name in SendFragment part and click submit button then in second fragment on displayFragment show user name input result.

Here I have created first two new display.xml and send.xml layout.In send.xml I have added EditText and Button. And in display.xml I have added a TextView. After then in third Step, in activity_main.xml layout I have added two fragemnts.

Now See coding area I have created SendFragment and DisplayFragment classes. In SendFragment class I have created StartCommuncation interface and used OnClickListener method. In DisplayFragment class i have used setText method. Now atLast In MainActivity part I have extend Activity and implements SendFragment class and StartCommunication interface after that I have used setComm method and toast function. You can see below program it will clearly describe you How to communication between an Activity and Fragments in android.

Step(1)Created a new send.xml layout -

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:ems="10"
        android:maxLength="15"
        android:id="@+id/edit"
        android:hint="Send Text"/>
    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Message"
        android:id="@+id/button1"/>
</LinearLayout>

Step(2)Created a new display.xml layout -
 

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="vertical" android:layout_width="match_parent"
    android:layout_height="match_parent">
    <TextView
        android:layout_width="250dp"
        android:layout_height="wrap_content"
        android:id="@+id/text"
        android:text="Type here"/>

</LinearLayout>

Step(3)activity_main.xml layout-
 

<LinearLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="horizontal"
    android:baselineAligned="false">
    <fragment
        android:layout_width="0dp"
        android:layout_height="fill_parent"
        android:layout_weight="1"
        android:id="@+id/fragment1"
        android:name="com.tiwari.rajshekhar.actfrag.DisplayFragment"/>
    <fragment
        android:layout_width="0dp"
        android:layout_height="fill_parent"
        android:layout_weight="1"
        android:id="@+id/fragment2"
        android:name="com.tiwari.rajshekhar.actfrag.SendFragment"/>

</LinearLayout>

Step(4)-Created a New SendFragment class-
 

public class SendFragment extends Fragment {
    StartCommunication mStartCommunicationListner;
    String msg="Welcome";
    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstanceState){
        View mView =(View)inflater.inflate(R.layout.send,container);
        final EditText mEditText =(EditText)mView.findViewById(R.id.edit);
        Button mButton =(Button)mView.findViewById(R.id.button1);
        mButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View arg0) {
                msg=mEditText.getText().toString();
                sendMessage();

            }
        });
        return mView;
    }
    interface StartCommunication{
            public void setComm(String msg);

    }
    @Override
    public void onAttach(Activity activity){
        super.onAttach(activity);
        if (activity instanceof StartCommunication)
        {
            mStartCommunicationListner =(StartCommunication)activity;

        }
        else
            throw new ClassCastException();
    }

    private void sendMessage() {

        mStartCommunicationListner.setComm(msg);
    }


}

Step(5)-Created a New DisplayFragment class-

public class DisplayFragment extends Fragment {
    View mView;
    @Override
    public View onCreateView(LayoutInflater inflater,ViewGroup container,Bundle savedInstaneceState){
        mView=(View)inflater.inflate(R.layout.display,container);
        return mView;
    }
    void setText(String msg){
        TextView mTextView =(TextView)mView.findViewById(R.id.text);
        mTextView.setText(msg);
    }
}

Step(6)MainActivity-
 

public class MainActivity extends AppCompatActivity implements SendFragment.StartCommunication
{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }

    @Override
    public void setComm(String msg) {


        DisplayFragment mDisplayFragment = (DisplayFragment)getSupportFragmentManager().findFragmentById(R.id.fragment1);
        if(mDisplayFragment != null && mDisplayFragment.isInLayout())
        {
            mDisplayFragment.setText(msg);
        }
        else
        {
            Toast.makeText(this, "Error Sending Message", Toast.LENGTH_SHORT).show();
        }
    }
}