Would the following work?
item1.display() ;
Answer:
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.
Would the following work?
item1.display() ;
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.
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() );
((Book)item1).display();
}
}
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 associated by the type of the reference variable.
In the program, only the calculateTax() method is associated with the reference variable.
However, a type cast tells the compiler that
a particular variable refers to an object of a specific class:
((Book)item1).display();
Now the display() method of the object pointed to by item1 is invoked 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.
Why are the red parentheses needed in:
((Book)item1).display();