Many feared the legal issues surrounding APIs would come up again, and they were right. Oracle is appealing last year’s ruling that its APIs weren’t copyrightable, and the appeal is being heard by the U.S. Court of Appeals for the Federal Circuit in Washington D.C.

To refresh your memory, Oracle attempted to sue Google in 2010 for its use of 37 Java APIs in its Android mobile operating system, which Oracle argued constituted patent and copyright infringement. Google fought back, claiming that it had every right to use the patents in question because the Java programming language is free to use and APIs are necessary to use the language. In 2012, U.S. District Judge William Alsup ruled in favor of Google, stating Oracle couldn’t assert copyright over application programming interfaces.

Now, Oracle is appealing and arguing that Google took the most popular and important parts of the APIs to create Android.

(The legal soup this fight is serving to developers)

“Google took the code for its own uses, and it did it to leverage Oracle’s fan base,” Oracle lawyer Josh Rosenkranz said, according to Bloomberg. “Google was very careful to only use what was structural. No one was able to use the Java language as a smartphone platform.”

During a hearing held Wednesday, Federal Circuit Judge Kathleen O’Malley questioned if Alsup’s ruling meant Google would have been able to use APIs developed from other companies such as Apple or Microsoft.

Google responded yes, but only the command structure, and that it would have had to rewrite millions of lines of code if it did use Apple or Microsoft’s interfaces. Google also stated that when it used Java’s APIs, it did in fact rewrite 15 million lines of Android code.

Bloomberg reported Microsoft, NetApp and EMC have filed arguments supporting Oracle, while Rackspace and the Application Developers Alliance have sided with Google.

If the previous ruling is overturned, Google could owe Oracle more than US$1 billion in damages, but the appeals court is not expected to make its decision for another several months.