A good answer might be:

A type cast is necessary to tell the compiler that the variable tax in this case contains a Book object:

public static void main ( String[] args ) { Book book ; Taxable tax = new Book ( "Emma", 24.95, "Austin" ); book = (Book)tax; book.display(); System.out.println( "Tax on item 1 "+ book.calculateTax() ); }

There are features of interfaces that the example did not show: A class can implement several interfaces:

class SomeClass extends SomeParent implements InterfaceA, InterfaceB, InterfaceC { }

Now SomeClass must implement all the methods listed in all the interfaces.

QUESTION 18:

Could the interfaces contain several definitions of the same constant?