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;  
    }