Hide Notification bar

The answers above hide ActionBar too. If you intend to only hide notification bar, use this code:

getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);

be careful to put it before setContentView().

public void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    getWindow().requestFeature(Window.FEATURE_NO_TITLE);
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
}

You could use a theme in your AndroidManifest.xml:

android:theme="@android:style/Theme.NoTitleBar.Fullscreen"

or change parent of your AppTheme to @android:style/Theme.NoTitleBar.Fullscreen like this

<style name="AppTheme" parent="Theme.NoTitleBar.Fullscreen">
</style>

then apply this theme on activities which you want Fullscreen like

android:theme="@style/AppTheme"

or use the following code snippet:

public class FullScreen
    extends android.app.Activity
{
    @Override
    public void onCreate(android.os.Bundle savedInstanceState)
    {
        super.onCreate(savedInstanceState);

        requestWindowFeature(Window.FEATURE_NO_TITLE);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                                WindowManager.LayoutParams.FLAG_FULLSCREEN);

        setContentView(R.layout.main);
    }
}