About Question enthuware.ocpjp.v7.2.1304 :

[hamada.yamasaki]

This will not help because it does not solve the basic problem of bad hashCode() method that exists with Book class. Moreover, if TreeMap is used, the second map.add() call will throw a ClassCastException because Book does not implement Comparable interface.

Even the first call to method map.put() will throw a ClassCastException. And more over ther is no method map.add() , i think you mean map.put() ?

[admin]

You are right. Even the first call throws ClassCastException now. With previous versions of Java, the first call did not require any comparison and so there was no need to cast it to Comparable.

[renzo2939]

why

Book b = new Book("A111"); // in BookStore constructor

Book b = new Book("A111"); // in public static void main

has different hashCode?

[admin]

That's because the Book class doesn't implement the hashCode method. Therefore, the inherited version from the Object class is used. Object's hashCode returns a different value for a different object.