实现“两次按返回键退出程序”的效果

private static Boolean isExit = false;  
    private static Boolean hasTask = false;  
    Timer tExit = new Timer();  
    TimerTask task = new TimerTask() {  
        @Override  
        public void run() {  

            isExit = true;  

            hasTask = true;  
        }  
    };  

    public  boolean onKeyDown(int keyCode, KeyEvent event) {  

                // TODO Auto-generated method stub  

                if(keyCode == KeyEvent.KEYCODE_BACK){  

                        System.out.println(isExit);  

                        if(isExit == false ) {  

                                isExit = true;  
                                System.out.println("isexit1 "+isExit);  

                                Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();  
                                System.out.println("isexit2 "+isExit);  

                                if(!hasTask) {