Answer:

  1. The programmer can write a no-argument constructor for a class.
  2. If the programmer does not write any constructors for a class, then a no-argument constructor (called the default constructor) is automatically supplied.
  3. If the programmer writes even one constructor for a class then the default constructor is not automatically supplied.

Instantiating Movie

In the example program, the class definition for Video includes a constructor, so the default constructor was not automatically supplied. So the constructor proposed for Movie causes a syntax error. Let us not use it.

Here is an example program that makes use of the two classes:

public class VideoStore
{
  public static void main ( String args[] )
  {
    Video item1 = new Video("Microcosmos", 90 );
    Movie item2 = new Movie("Jaws", 120, "Spielberg", "PG" );
    item1.show();
    item2.show();
  }
}

(You can run this program by using a text editor to copy and paste the class definitions from the previous pages into a source file.) The program prints:

Microcosmos, 90 min. available:true
Jaws, 120 min. available:true

The statement item2.show() calls the show() method of item2. This method was inherited without change from the class Video. This is what it looks like:

public void show()
{
  System.out.println( title + ", " + length + " min. available:" + avail );
}

It does not mention the new variables that have been added to objects of type Movie, so nothing new is printed out.

QUESTION 14:

Why not change show() in Video to include the line

System.out.println( "dir: " + director + rating );