按两次返回键退出

@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(), "再按一次退出程序", Toast.LENGTH_SHORT).show();
				exitTime = System.currentTimeMillis();
			} else {
				finish();
				System.exit(0);
			}
			return true;
		}
		return super.onKeyDown(keyCode, event);
	}