android 双击返回退出

/**
 * 当按下Back键，会执行onBackPressed里的代码
 */
private long firstTime = 0;

@Override
public void onBackPressed()
{
    long secondTime = System.currentTimeMillis();
    if (secondTime - firstTime >= 2000)
    {
        Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
        firstTime = secondTime;
    }
    else
    {
        finish();
    }
}