There is a case between Google and Oracle over the use of Java and it's standard library on the Google Android operating system. From [this article]:
To recap: Oracle accused Google of copying some of its Java computer code when it wrote Android. Android itself wasn't the issue. Android is different than Java. But Google wanted developers who work with Java, a popular language for web apps, to jump to Android. So it incorporated Java's application programming interfaces (APIs) into Android. This allowed them to quickly convert their apps to Android and it meant that the millions of programmers trained on Java would be familiar with Android, too.
Oracle sued claiming that the APIs were copyrighted. But the judge ruled that APIs are not subject to copyright laws
On Friday, an appellate court just overturned that loss, and said APIs are subject to copyright.
My question is, Java is Open Source, and was at the time Google forked ( used loosely to mean copied some pieces of code), AFAIK. I'm not interested in whether APIs are copyright-able, let's assume they are (since that's where the case left off). How is what Google did not permissible under the Open Source license that it's distributed under (AFAIK GPLv2 (or 3?) with Classpath exception)?