как установить setLayoutParams для элементов линейного макета

Как установить setLayoutParams () для элементов LinearLayout . в MainActivity.java . Я написал следующий код для установки параметров макета и nullPointerException , показанного в строке № 50, и наконец вставляю файл Logcat.

layoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, 50);
imageview1.setLayoutParams(layoutParams); // line no 50

layoutParams = new LinearLayout.LayoutParams(LayoutParams.WRAP_CONTENT, 50);
textview1.setLayoutParams(layoutParams);

layoutParams = new LinearLayout.LayoutParams(LayoutParams.FILL_PARENT,LayoutParams.WRAP_CONTENT);
listview01.setLayoutParams(layoutParams);

Это мой XML-файл

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
android:orientation="vertical"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:stackFromBottom="true"
>
<ImageView
android:id="@+id/imageview1"
android:layout_width="wrap_content"
android:layout_height="50px"
android:background="@drawable/applicationbar"
android:layout_x="0px"
android:layout_y="0px"
>
</ImageView>
<TextView
android:id="@+id/textview1"
android:layout_width="wrap_content"
android:layout_height="50px"
android:text="TextView"
android:layout_x="0px"
android:layout_y="55px"
>
</TextView>
<ListView android:id="@+id/ListView01"
android:layout_width="fill_parent"
android:layout_height="wrap_content"
android:layout_marginLeft="10px" 
android:layout_marginRight="10px"
android:layout_marginTop="35px" 
android:layout_marginBottom="40px"
android:paddingLeft="0px"
android:paddingRight="0px" />
</LinearLayout>

Log cat:

04-19 16:47:22.221: ERROR/AndroidRuntime(3437): Uncaught handler: thread main exiting due to uncaught exception
04-19 16:47:22.221: ERROR/AndroidRuntime(3437): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.android/com.android.Listview}: java.lang.NullPointerException
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2496)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2512)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread.access$2200(ActivityThread.java:119)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1863)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.os.Handler.dispatchMessage(Handler.java:99)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.os.Looper.loop(Looper.java:123)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread.main(ActivityThread.java:4363)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at java.lang.reflect.Method.invokeNative(Native Method)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at java.lang.reflect.Method.invoke(Method.java:521)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:860)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:618)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at dalvik.system.NativeStart.main(Native Method)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437): Caused by: java.lang.NullPointerException
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at com.android.Listview.onCreate(Listview.java:40)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
04-19 16:47:22.221: ERROR/AndroidRuntime(3437):     at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2459)