Google argues the APIs are a fundamental requirement for using the Java programming language, and that because the language is free for anyone to use without a license, the APIs also cannot require a license. Google also characterizes the APIs as just "names" and "short phrases" that programmers use to invoke other parts of the platform.

Oracle disagrees. It argues that the "structure, sequence and organization" of the APIs took years for Sun's engineers to develop and are therefore subject to copyright. Mitchell, the expert witness for Oracle, backed up its claim Monday.

"API design is a really creative process," he testified.

Mitchell said the Android APIs at issue in the case are "essentially identical" to those created by Sun. "I don't think there's any way a separate team could have come up with so many things that are identical except by copying the original APIs," Mitchell said.

On cross-examination, however, Google's lawyer got Mitchell to confirm that some of the Java APIs at issue in the case, such as java.io, are a requirement for the most basic functions in Java a program, such as networking to another computer.