I'm studying on lambda expressions, the book illustrates an example that uses `toString()` method of lambda expressions.
It explains meaning of the output as,
This actually does mean something. Our
test class is named BuiltIns ,
and it is in a package that we created named functionalinterface .
Then comes `$$` , which means that
the class doesn’t exist in a class
file on the file system. It exists only in memory.
I don't understand the meaning of the last sentences. Could you express it?