Android再按一次后退键退出应用程序

private static Boolean isExit = false; 
    private static Boolean hasTask = false; 
    Timer tExit = new Timer(); 
    TimerTask task = new TimerTask() { 
          
        @Override 
        public void run() { 
            isExit = false; 
            hasTask = true; 
        } 
    }; 
      
    @Override 
    public boolean onKeyDown(int keyCode, KeyEvent event) { 
        System.out.println("TabHost_Index.java onKeyDown"); 
        if (keyCode == KeyEvent.KEYCODE_BACK) { 
            if(isExit == false ) { 
                isExit = true; 
                Toast.makeText(this, "再按一次后退键退出应用程序", Toast.LENGTH_SHORT).show(); 
                if(!hasTask) { 
                    tExit.schedule(task, 2000); 
                } 
            } else { 
                finish(); 
                System.exit(0); 
            } 
        } 
        return false; 
    }