Change fragments textview from activity

Question

I want to change fragments textview from activity(not fragment class).How can I do it ? I am using this code:

public void startChat() {
    FrameLayout layout = (FrameLayout)findViewById(R.id.container);
    layout.setVisibility(View.VISIBLE); 
    FragmentManager fragmentManager = getSupportFragmentManager();
    FragmentTransaction fragmentTransaction = fragmentManager.beginTransaction();
    fragmentTransaction.add(R.id.container, new ConversationFragment());
    fragmentTransaction.commit();
    viewPager.setVisibility(View.GONE);
    actionBar.setNavigationMode(ActionBar.NAVIGATION_MODE_STANDARD);
    actionBar.setDisplayHomeAsUpEnabled(true);
    TextView nameView=(TextView)findViewById(R.id.user_name);
    nameView.setText("asd");
}

This code loads conversation_fragment.xml and wants change textview,but my app is crashing.

Here conversation_fragment.xml :

<?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:orientation="vertical"
    android:background="#878787" >

        <TextView android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="dfgdfgdf"
        android:textSize="20dp"
        android:layout_centerInParent="true"
        android:id="@+id/user_name"/>

    <EditText
    android:id="@+id/message"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
   />

    <Button 
        android:text="Gönder"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:onClick="getFromUser"
        android:layout_marginTop="40dp"
        />

</RelativeLayout>

Answer 1

You can either use findFragmentById to find your fragment and call a public method that changes the text

(ConversationFragment) getFragmentManager().findFragmentById(R.id.yourid);

or keep an instance of the class when you create it before you add it to the fragment manager

conv = new ConversationFragment()
fragmentTransaction.add(R.id.container, conv);

then just call the public method using conv or whatever you call it

EDIT:

you use a bundle to send in data to the fragment

Bundle b = new Bundle()
b.putString("text",data)
conv.setArguments(b);

then in your fragment get the arguments with getArguments() and pull the data from the bundle and use it however you need