Would the following work?

item1.display() ;

A good answer might be:

No---all the compiler has been told is that item1 implements the methods in the interface Taxable. The display() method is not in the interface.

When you use a variable of type Taxable you are asking to use the "taxable" aspect of the object. Many different kinds of objects might be referred to by the variable. (In a larger program there may be Taxable classes that are not Goods.) The compiler can only use the methods it knows that the object must have---those in the interface

However, you can use a type cast to tell the compiler that in a particular statement in the program the variable will refer to an object of a specific class:

public static void main ( String[] args ) { Taxable item1 = new Book ( "Emma", 24.95, "Austin" ); System.out.println( "Tax on item 1 "+ item1.calculateTax() ); ((Book)item1).display(); }

This program is not very sensibly written, since if the variable item1 were of type Book everything would work without the need for a type cast. But in programs with more complicated logic such casts are sometimes needed.

QUESTION 16:

Why are the red parentheses needed in:

    ((Book)item1).display();