Because the instance variables director, and rating
are members only of Movie objects, so code defined in Video cannot use them.
We need a new toString() method in the class Movie:
// added to class Movie
public String toString()
{
return getTitle() + ", " + getLength() + " min. available:" + getAvailable() +
" dir: " + director + ", rating: " + rating ;
}
Now, Movie has its own toString() method that uses variables
it inherits and variables it defines.
Even though the parent has a toString() method,
the new definition of toString() in the child class will override
the parent's version.
It is used when a String based on the child object is needed.
A child's method overrides a parent's method when it has the same signature as a parent method. Now the parent has its method, and the child has its own method with the same signature. (Remember that the signature of a method is the name of the method and its parameter list.)
An object of the parent type
includes the method given in
the parent's definition.
An object of the child type
includes both the method given in
the parent's definition
and the method given in
the child's definition.
The parent's method can be invoked using super.
(See below.)
With the change in the class Movie the
following program will print out the full information for both items.
class VideoStore
{
public static void main ( String args[] )
{
Video item1 = new Video("Microcosmos", 90 );
Movie item2 = new Movie("Jaws", 120, "Spielberg", "PG" );
System.out.println( item1.toString() );
System.out.println( item2.toString() );
}
}
The line item1.toString() calls the toString() method
defined in Video, and
the line item2.toString() calls the toString() method
defined in Movie.
Microcosmos, 90 min. available:true Jaws, 120 min. available:true dir: Spielberg PG, rating: PG
Would the following main() work? (toString() has been removed.)
class VideoStore
{
public static void main ( String args[] )
{
Video item1 = new Video("Microcosmos", 90 );
Movie item2 = new Movie("Jaws", 120, "Spielberg", "PG" );
System.out.println( item1 );
System.out.println( item2 );
}
}