android按两次退出

	/**
	 * 连续按两次返回键就退出
	 */
	private long firstTime;

	@Override
	public void onBackPressed() {
		if (System.currentTimeMillis() - firstTime < 3000) {
			finish();
		} else {
			firstTime = System.currentTimeMillis();
			Toast.makeText(this, R.string.press_again_exit, Toast.LENGTH_SHORT)
					.show();
		}
	}