Set child layout width programmatically In ANDROID?

In my Code I have a XML file having multiple child layouts. Aand I just want to set one of these child layout's width programmatically according to device's screen width. till now I had tried Layout params, setparam, and other methods but it shows null pointer exception on getting layout id.

my parent layout is a Relative Layout, and 1st Child Layout is a Linear Layout("for this layout I need to change width")

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:id="@+id/parentlayout"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent">

    <LinearLayout
        android:id="@+id/childlayout"
        android:layout_width="250dip"
        android:layout_height="fill_parent"
        android:orientation="vertical" 
        android:background="#153746">
        <LinearLayout
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:orientation="vertical"
            android:background="@drawable/sub_title"
            android:gravity="center_vertical" >
            <TextView
                android:id="@+id/menu_header_label"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textStyle="bold"
                android:text="asd"
                android:textColor="#ffffff"
                android:layout_marginLeft="10dip" />

            <ImageView
                android:id="@+id/menu_header"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:layout_gravity="center_horizontal" />
        </LinearLayout>
        <ListView
            android:id="@+id/menu_listview"
            android:layout_width="fill_parent"
            android:layout_height="wrap_content"
            android:divider="@android:color/darker_gray"
            android:divid