private long exitTime = 0;  
    @Override  
    public boolean onKeyDown(int keyCode, KeyEvent event) {  
        if (keyCode == KeyEvent.KEYCODE_BACK  
                && event.getAction() == KeyEvent.ACTION_DOWN) {  
            if ((System.currentTimeMillis() - exitTime) > 2000) {  
                Toast.makeText(getApplicationContext(), "再按一下退出应用",  
                        Toast.LENGTH_SHORT).show();  
                exitTime = System.currentTimeMillis();  
            } else {  
                finish();  
                System.exit(0);  
            }  
            return true;  
        }  
        return super.onKeyDown(keyCode, event);  
    }