public class Store
{

  public static void main ( String[] args )
  {
    Goods   toy ;
    Taxable tax = new Toy ( "Building Blocks", 1.49, 6 );

    toy = (Toy)tax;
    System.out.println( toy );
    System.out.println( "Tax: "+ ((Taxable)toy).calculateTax() );
  }
}

Yet More Practice

The first type cast in the answer must cast tax to a type Toy (or the parent class Goods). The second must cast toy to Taxable or to a class that implements Taxable.

When you think about such problems, remember that at compile time objects do not exist. The compiler knows what methods are available with a reference variable based only on the type of the variable.

For example, in the above, only the calculateTax() method and methods of Object are available with tax. At run time the object tax happens to point to has more methods avaialble. To say this in the code, use a type cast.

It seems obvious in this program that toy points to an object with a calculateTax() method. But with more complicated programs it is not obvious.

Here is another possible answer:

public class Store
{
  public static void main ( String[] args )
  {
    Goods   toy ;
    Taxable tax = new Toy ( "Building Blocks", 1.49, 6 );

    toy = (Goods)tax;
    System.out.println( toy );
    System.out.println( "Tax: "+ ((Toy)toy).calculateTax() );
  }
}