If the implementation, CODE(!), is of course a formalized description of methods of operation, the declaring code (or API) is providing a means of access to it. Something that takes quite a bit of thought and creativity to do well.
Google didn't just copy the "heading" java.lang.Math.max() they also copied the contract that calling this method should always return the bigger of the two arguments. Ergo, the "content" under the heading is the same, or rather the any differance between the content of Oracle's book and Google's book is irrelevant
As any programmer knows, you code against interfaces not implementations. And if you don't realize that declaring those interfaces take both effort and creativity then you probably aren't a very good programmer.
Irrelevant, the contract is not part of the trial
If any language has a function max that takes two integer values and doesn't return bigger, that language isn't worth bothering.
And as any (competent) programmer knows, the "interface" in "code against interfaces not implementations" is merely a homonym of the "interface" in "application programming interface".