Immersive fullscreen on Android 11

For compatibility, use WindowCompat and WindowInsetsControllerCompat. You'll need to upgrade your gradle dependency for androidx.core to at least 1.6.0-alpha03 so that there will be support for setSystemBarsBehavior on SDK < 30. mainContainer is the top-level ConstraintLayout in my activity.

private fun hideSystemUI() {
    WindowCompat.setDecorFitsSystemWindows(window, false)
    WindowInsetsControllerCompat(window, mainContainer).let { controller ->
        controller.hide(WindowInsetsCompat.Type.systemBars())
        controller.systemBarsBehavior = WindowInsetsControllerCompat.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE
    }
}

private fun showSystemUI() {
    WindowCompat.setDecorFitsSystemWindows(window, true)
    WindowInsetsControllerCompat(window, mainContainer).show(WindowInsetsCompat.Type.systemBars())
}

You can find out more information about WindowInsets by watching this YouTube video

EDIT:

I didn't consider display cutouts or in-display cameras in this answer previously. In the app theme style, i added the following to display my content above the cutout (to the top of the screen when in portrait mode):

<item name="android:windowLayoutInDisplayCutoutMode">shortEdges</item>

You can read more at this link: Display Cutout

This works for me, mixed two answers:

First create method :

private void setFullScreen(){
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
        getWindow().setDecorFitsSystemWindows(false);
        WindowInsetsController controller = getWindow().getInsetsController();
        if(controller != null) {
            controller.hide(WindowInsets.Type.statusBars() | WindowInsets.Type.navigationBars());
            controller.setSystemBarsBehavior(WindowInsetsController.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE);
        }
    }
    else {
        //noinspection deprecation
        getWindow().getDecorView().setSystemUiVisibility(
                View.SYSTEM_UI_FLAG_FULLSCREEN
                        | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                        | View.SYSTEM_UI_FLAG_IMMERSIVE
                        | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                        | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                        | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION);
    }
}

then on create just call it after setContentView like this :

  @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main_menu);
        setFullScreen();
    }

and it works like a charm :D

For the deprecation, you can try the below code. Worked for me.

  if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
        final WindowInsetsController controller = getWindow().getInsetsController();

        if (controller != null)
            controller.hide(WindowInsets.Type.statusBars());
    }
    else {
        //noinspection deprecation
                getWindow().getDecorView().setSystemUiVisibility(
            View.SYSTEM_UI_FLAG_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                    | View.SYSTEM_UI_FLAG_IMMERSIVE
                    | View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                    | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                    | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION);
    }

Here's the answer in Kotlin, in case if you don't want to increase the size of your app by using the androidx library.

private fun setFullscreen() {
    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.P) {
        window.attributes.layoutInDisplayCutoutMode =
            WindowManager.LayoutParams.LAYOUT_IN_DISPLAY_CUTOUT_MODE_SHORT_EDGES
    }

    if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.R) {
        window.setDecorFitsSystemWindows(false)
        window.insetsController?.apply {
            hide(WindowInsets.Type.statusBars() or WindowInsets.Type.navigationBars())
            systemBarsBehavior = WindowInsetsController.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE
        }
    } else if (Build.VERSION.SDK_INT >= Build.VERSION_CODES.KITKAT) {
        @Suppress("DEPRECATION")
        window.decorView.systemUiVisibility = (View.SYSTEM_UI_FLAG_FULLSCREEN
                or View.SYSTEM_UI_FLAG_HIDE_NAVIGATION
                or View.SYSTEM_UI_FLAG_IMMERSIVE
                or View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                or View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                or View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION)
    }
}

Call the function your activity's onCreate method.

ORIGINAL ANSWER:

As documentation suggests, you should use WindowInsetsController.

getWindow().setDecorFitsSystemWindows(false);
WindowInsetsController controller = getWindow().getInsetsController();
if (controller != null) {
    controller.hide(WindowInsets.Type.statusBars() | WindowInsets.Type.navigationBars());
    controller.setSystemBarsBehavior(WindowInsetsController.BEHAVIOR_SHOW_TRANSIENT_BARS_BY_SWIPE);
}