is it possible to hide the system bar

You can’t hide it but you can disable it, except home. For that you can give your application as home category and let the user choose.

<category android:name="android.intent.category.HOME" />

Rest all can be disable.

add this in manifest.

<uses-permission android:name="android.permission.EXPAND_STATUS_BAR"/>

inside onCreate()

this.requestWindowFeature(Window.FEATURE_NO_TITLE); 
    getWindow().setFlags(WindowManager.LayoutParams.FLAG_FULLSCREEN, WindowManager.LayoutParams.FLAG_FULLSCREEN);
    setContentView(R.layout.activity_home);
    View v = findViewById(R.id.home_view);
    v.setSystemUiVisibility(View.SYSTEM_UI_FLAG_LOW_PROFILE);

where home_view is the parent view of xml file.

 @Override
     public boolean onKeyDown(int keyCode, KeyEvent event) {
            return false;
        }

 public void onWindowFocusChanged(boolean hasFocus)
     {
             try
             {
                if(!hasFocus)
                {
                     Object service  = getSystemService("statusbar");
                     Class<?> statusbarManager = Class.forName("android.app.StatusBarManager");
                     Method collapse = statusbarManager.getMethod("collapse");
                     collapse .setAccessible(true);
                     collapse .invoke(service);
                }
             }
             catch(Exception ex)
             {
             }
     }