The relevant paragraphs are as follows:
Lambda expressions, or lambdas, allow passing around blocks of code. The full syntax look like this:
The parameter types can be omitted. When only one parameter is specified without a type, the parentheses can also be omitted. The braces and return statement can be omitted for a single statement making the short form as follows:
While I understand this shows the most abbreviated form, it begs the question of where b comes from. And using the same body as the full syntax may lead to one believing the latter is intended to be the short form of the former. It may be more straightforward to use a different body (e.g. a -> a.toUpperCase())