Android手机上的返回键退出软件

private boolean isFirst = true;
@Override
public void onBackPressed() {
if (isFirst) {
isFirst = false;
firstTime = System.currentTimeMillis();
Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
} else {
long secondTime = System.currentTimeMillis();
if (secondTime - firstTime <= 1888) {
finish();
}else{
Toast.makeText(this, "再按一次退出程序", Toast.LENGTH_SHORT).show();
firstTime = secondTime;
}
}
}