Enthuware

What is going on here?
The list will not be printed, only the object references. Or should we assume there is a toString()?

There is no need to assume. All objects have a toString method already. It may not print the value of the fields of the object, but it does print a unique string for that object (No, it doesn't print object reference. It prints the hashcode).

Further, what the code actually prints is not really relevant here because all it says is that the list is printed, which it does. It doesn't claim anything about the actual format or content of what is printed.

The book class is not implementing Comparable interface then how the compareTo function is working on line //1

Book doesn't implement Comparable. But the code passes a Comparator instance that is able to compare two Book objects.
This Comparator uses the compareTo method of String class.
Observe the lambda used to create the Comparator. It calls b1.getGenre().compareTo and not b1.compareTo.

Thanks for the explaination. Got it.