At the end of my testimony in the recent Oracle v Google trial in San Francisco, Judge Alsup asked me to explain what an API is. My answer aimed to simplify the answer for a general listener while remaining recognizable to most programmers. Here’s what I said.

The Java source code of OpenJDK usefully follows a layout convention. Up front of each file is a copyright and license statement. After that come a sequence of definitions of the various standard functions that complete the Java programming language. Together, a set of related definitions comprise a class library.

Each of those definitions comprise three parts. There is a function declaration, which defines the name of the function and the order and data types of the parameters used by the function. After that is a comment block with a summary of the specification for the function, tagged to allow it to be…