Type Casts

A new method is added to the Book class:

// Other classes
 . . . .
 
// Book
class Book  extends Goods implements Taxable
{
  protected String author;

  public Book( String des, double pr, String auth)
  {
    super( des, pr );
    author  = auth ;
  }


  public String toString()
  {
    return super.toString() + "author: " + author ;
  }

  public double calculateTax()
  {
    return price * taxRate ;
  }
  
  // Newly added method 
  public String getAuthor()
  {
    return author;
  }
}

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

    System.out.println( "Author: " + item1.getAuthor() );
  }
}

When a variable is of type Taxable all the compiler knows is the "taxable" aspect of the object. The following statement from the above program fails to compile because Taxable objects do not necessarily have the requested method.

System.out.println( "Author: " + item1.getAuthor() );

In a larger program there may be Taxable objects that do not have getAuthor(). The compiler can only use the methods associated by the type of the reference variable. In the above program, only the calculateTax() method and methods in Object are associated with the reference variable.

However, a type cast tells the compiler that a particular variable refers to an object of a specific class. To fix the problem, replace the defective statement with the following.

System.out.println( "Author: " + 
  ((Book)item1).getAuthor() );

Now the getAuthor() method of the object pointed to by item1 can be found because the compiler has been told that item1 points to a Book.

This program is not very sensibly written. If the variable item1 always points to a Book it would be more sensible to make it of type Book. But programs with complicated logic sometimes need such casts.

Answer:

Type Casts

QUESTION 21: