android:layout_height 50% of the screen size

Question

I just implemented a ListView inside a LinearLayout, but I need to define the height of the LinearLayout (it has to be 50% of the screen height).

<LinearLayout
    android:id="@+id/widget34"
    android:layout_width="300px"
    android:layout_height="235px"
    android:orientation="vertical"
    android:layout_below="@+id/tv_scanning_for"
    android:layout_centerHorizontal="true">

    <ListView
        android:id="@+id/lv_events"
        android:textSize="18sp"         
        android:cacheColorHint="#00000000"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tv_scanning_for"
        android:layout_centerHorizontal="true">
    </ListView>

</LinearLayout>

Is that possible?

I did something similar for a button and an EditText, but doesn't seem to work on Layouts.

This is my Code:

    //capture the size of the devices screen
    Display display = getWindowManager().getDefaultDisplay();
    double width = display.getWidth();

    //my EditText will be smaller than full screen (80%)        
    double doubleSize = (width/5)*4;
    int editTextSize = (int) doubleSize;

    //define the EditText 
    userName = (EditText) this.findViewById(R.id.userName);
    password = (EditText) this.findViewById(R.id.password);

    //set the size
    userName.setWidth(editTextSize);
    password.setWidth(editTextSize);

Answer 1

Set its layout_height="0dp"*, add a blank View beneath it (or blank ImageView or just a FrameLayout) with a layout_height also equal to 0dp, and set both Views to have a layout_weight="1"

This will stretch each View equally as it fills the screen. Since both have the same weight, each will take 50% of the screen.

*See adamp's comment for why that works and other really helpful tidbits.