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// Hide Navigation Bar (Button1)

if (Build.VERSION.SDK_INT < 16) {
            getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN,
                    WindowManager.LayoutParams.FLAG_FULLSCREEN);
}
else {
   View decorView = getWindow().getDecorView();

// Hide Navigation Bar

   int uiOptions = View.SYSTEM_UI_FLAG_HIDE_NAVIGATION;
   decorView.setSystemUiVisibility(uiOptions);
}

// Show Navigation Bar (Button2)

   if (Build.VERSION.SDK_INT < 16) {
              getWindow().clearFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN);
    }
    else {
       View decorView = getWindow().getDecorView();

// Show Navigation Bar

       int uiOptions = View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION;
       decorView.setSystemUiVisibility(uiOptions);
    }