Writing drop-in replacements may very well fall under fair use and be allowed, but this is not what happened in this particular case the court ruled on:
> It was not ... intended to permit third party interoperability, since Google had made no substantial efforts to use them for the purpose of third party interoperability. (In fact it found that Google had tried to prevent interoperability with other Java and had previously been refused a license by Sun for that reason.) It was not transformative in the sense of a new platform either, since other Java smartphones predated Android.
> It was not ... intended to permit third party interoperability, since Google had made no substantial efforts to use them for the purpose of third party interoperability. (In fact it found that Google had tried to prevent interoperability with other Java and had previously been refused a license by Sun for that reason.) It was not transformative in the sense of a new platform either, since other Java smartphones predated Android.
https://en.wikipedia.org/wiki/Oracle_America,_Inc._v._Google...