How to hide the navigation bar?

Question

I used this code below to hide the Android navigation bar. The code worked, but when a new Activity is opened by an app button the bar appears and comes out fast, this is not very good. So I’d like to know how to improve or change this code so that the bar only appears when called.

// Atributo

    protected View decorView;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
        setContentView(R.layout.mundo1_fase1_0);

        decorView = getWindow().getDecorView();
        decorView.setOnSystemUiVisibilityChangeListener(new View.OnSystemUiVisibilityChangeListener() {
            @Override
            public void onSystemUiVisibilityChange(int visibility) {
                if (visibility == 0) {
                    decorView.setSystemUiVisibility(hideSystemBars());
                }
            }
        });
    }

    // Método onWindowFocusChanged

    @Override
    public void onWindowFocusChanged(boolean hasFocus) {
        super.onWindowFocusChanged(hasFocus);
        if (hasFocus) {
            decorView.setSystemUiVisibility(hideSystemBars());
        }
    }

    // Método hideSystemBars

    protected int hideSystemBars() {
        return View.SYSTEM_UI_FLAG_LAYOUT_STABLE
                | View.SYSTEM_UI_FLAG_IMMERSIVE_STICKY
                | View.SYSTEM_UI_FLAG_LAYOUT_FULLSCREEN
                | View.SYSTEM_UI_FLAG_LAYOUT_HIDE_NAVIGATION
                | View.SYSTEM_UI_FLAG_FULLSCREEN
                | View.SYSTEM_UI_FLAG_HIDE_NAVIGATION;
    }