Question

I am implementing fragments for the first time so please help me.

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    <fragment android:name="com.example.news.ArticleListFragment"
            android:id="@+id/list"
            android:layout_weight="1"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
    <fragment android:name="com.example.news.ArticleReaderFragment"
            android:id="@+id/viewer"
            android:layout_weight="2"
            android:layout_width="0dp"
            android:layout_height="match_parent" />
</LinearLayout>

I want that fragment having the id as ‘list’ should remain constant but the the fragment having id ‘viewer’ should be able to call different classes.

(Note that the classes extend Activity.)

My question is simple: I have four classes(Extending ACTIVITY). I want to divide the screen into two parts. The left Side remains constant which contains the listview. On list view’s click I want to open my Class(Extending ACTIVITY), but only in the right portion(remaining screen).

Answer 1

According to the android documentation of fragments:
A Fragment represents a behavior or a portion of user interface in an Activity. You can combine multiple fragments in a single activity to build a multi-pane UI and reuse a fragment in multiple activities. You can think of a fragment as a modular section of an activity, which has its own lifecycle, receives its own input events, and which you can add or remove while the activity is running (sort of like a “sub activity” that you can reuse in different activities).

http://developer.android.com/guide/components/fragments.html

what i understood from your question is that you want to the content of your fragement viewer at run-time. a possible solution which i can suggest for this is:
Instead of your four classes extending Activity, extend Fragment, each having its own layout. Modify the main layout file to look something like this:

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="horizontal"
android:layout_width="match_parent"
android:layout_height="match_parent">
<fragment android:name="com.example.news.ArticleListFragment"
        android:id="@+id/list"
        android:layout_weight="1"
        android:layout_width="0dp"
        android:layout_height="match_parent" />
<FrameLayout xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/viewer"
        android:layout_weight="2"
        android:layout_width="0dp"
        android:layout_height="match_parent" >
</FrameLayout>

</LinearLayout> 

the FrameLayout will basically act as a container for your fragments, which you can dynamically load at run-time(by clicking the ListView). This tutorial will help you out with it:

http://developer.android.com/training/basics/fragments/fragment-ui.html

Hope my answer helps you in some way.