class DVD

{ String title; // name of the item

int length; // number of minutes

boolean avail; // is the DVD in the store?

// constructor

public DVD( String ttl )

{title = ttl; length = 90; avail = true;

}

// constructor

public DVD( String ttl, int lngth )

{title = ttl; length = lngth; avail = true;

}

public void show()

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

}

}

The DVD class has basic information in it, and would be OK for documentaries

and instructional tapes. But movies need more information. Let us make a

class that is similar to DVD, but has the name of the director and a rating.

class Movie extends DVD

{

String director; // name of the director

String rating; // G, PG, R, or X

// constructor

public Movie( String ttl, int lngth, String dir, String rtng )

{super( ttl, lngth ); // use the super class's constructor

director = dir; rating = rtng; // initialize what's new to Movie

}

}

The class Movie is a subclass of DVD. An object of type Movie has the following

members in it:

Both classes are defined: the DVD class can be used to construct objects of

that type, and now the Movie class can be used to construct objects of the

Movie type. The method shown in the superclass DVD is inherited in the

subclass Movie.

The class definition for DVD has a constructor that initializes the member

data of DVD objects. The class Movie has a constructor that initializes the

data of Movie objects. The constructor for class Movie looks like this:

// constructor
public Movie( String ttl, int lngth, String dir, String rtng )
{
  super(ttl, lngth);               // use the super class's constuctor
  director = dir;  rating = rtng;  // initialize the members new to Movie
}

The statement super(ttl, lngth) invokes a constructor of the parent to

initialize some of the data. There are two constructors in the parent.

The one that is invoked is the one that matches the argument list in

super(ttl, lngth). Then the next two statements (on one line) initialize

the members that only Movie has.

Note: When it is used, super() must be the first statement in the

subclass's constructor.

It is called super() because the parent of a class is sometimes called

its superclass.

The Constructor Invoked by super()

A constructor for a child class always starts with an invocation of one

of the constuctors in the parent class. If the parent class has several

constructors then the one which is invoked is determined by matching

argument lists.

For example, we could define a second constructor for Movie that does

not include an argument for length. It starts out by invoking the parent

constructor that does not have an argument for length:

// alternate constructor
public Movie( String ttl, String dir, String rtng )
{
  super( ttl );    // invoke the matching parent class constructor  
  director = dir;  rating = rtng;     // initialize members unique to Movie
}

Examine the following proposed constructor for Movie:

// proposed constructor
public Movie( String ttl, int lngth, String dir, String rtng )
{
  title = ttl;       // do what the parent's constuctor does.
  length = lngth; 
  avail = true;
 
  director = dir; rating = rtng;
}

It looks like there is no need to invoke the parent class's constructor

since all variables are initialized in this one. However a constructor

from the parent class is always invoked even if you don't explicity

ask for it. The Java compiler regards the above code as "shorthand"

for this:

// proposed constructor
public Movie( String ttl, int lngth, String dir, String rtng )
{
  super();           // invoke the parent's no-argument constructor
  title = ttl;       // do what the parent's constuctor does.
  length = lngth;  
  avail = true;
  
  director = dir; rating = rtng;
}

As always, super() comes first, even if you don't write it in. If the

parent does not have a no-argument constructor, then using this

"shorthand" causes a syntax error.

In our program the class definition for DVD (the superclass) lacks a

no-argument constructor. The proposed constructor (above) calls for

such a constructor so it would cause a syntax error.

The programmer can write a no-argument constructor for a class. If

the programmer does not write any constructors for a class, then

a no-argument constructor (called the default constructor) is

automatically supplied. If the programmer writes even one constructor

for a class then the default constructor is not supplied.

In the example program, the class definition for DVD 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:

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

When you run it, you will get the following output:

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 DVD. 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.

Because the class DVD does not have the variables director nor rating, so

its show() method can't use them.

We need a new show() method in the class Movie:

// added to class Movie
public void show()
{
  System.out.println( title + ", " + length + " min. available:" + avail );
  System.out.println( "dir: " + director + "  " + rating );  
}

Now, even though the parent has a show() method the new definition of show()

in the child class will override the parent's version.

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 will include the method given in the parent's

definition. An object of the child type will include the method given in the

child's definition.

With the change in the class Movie the following program will print out the full

information for both items.

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

The line item1.show() calls the show() method defined in DVD, and the line

item2.show() calls the show() method defined in Movie.

Microcosmos, 90 min. available:true
Jaws, 120 min. available:true
dir: Spielberg PG

Yes, it looks like we wrote the same code twice.

Using super in a Child's Method

Sometimes (as in the example) you want a child class to have its own

method, but that method includes everything the parent's method does.

You can use the super reference in this situation. For example, here is

DVD's method:

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

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

Movie's method would better be written using super:

public void show()
{
  super.show();
  System.out.println( "dir: " + director + "  " + rating );  
}

class DVD
{  String  title;    // name of the item
  int     length;   // number of minutes
  boolean avail;    // is the tape in the store?

  // constructor
  public DVD( String ttl, int lngth )
  {    title = ttl; length = lngth; avail = true; 
  }

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

class DVD
{  String  title;    // name of the item
  int     length;   // number of minutes
  boolean avail;    // is the tape in the store?

  // constructor
  public DVD( String ttl, int lngth )
  {    title = ttl; length = lngth; avail = true; 
  }

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

class MusicDVD extends DVD
{  String artist;
  String category;

  // The constructor will go here

  // The show() method will go here
}

class DVD
{  String  title;    // name of the item
  int     length;   // number of minutes
  boolean avail;    // is the tape in the store?

  // constructor
  public DVD( String ttl, int lngth )
  {    title = ttl; length = lngth; avail = true; 
  }

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

class MusicDVD extends DVD
{  String artist;
  String category;

  // The constructor will go here

  // The show() method will go here

}

class DVD
{  String  title;    // name of the item
  int     length;   // number of minutes
  boolean avail;    // is the tape in the store?

  // constructor
  public DVD( String ttl, int lngth )
  {    title = ttl; length = lngth; avail = true; 
  }

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

class MusicDVD extends DVD
{  String artist;
  String category;
  
  // constructor
  public MusicDVD ( String ttl, int len, String art, String cat )
  {    super( ttl, len );
    artist   = art;
    category = cat;
  }

  // The show() method will go here

}

class DVD
{
  // stuff ommitted here
  public void show()
  {    System.out.println( title + ", " + length + " min. available:" + avail );
  }
  
}

class MusicDVD extends DVD
{  String artist;
  String category;
 
  // constructor
  public MusicDVD ( String ttl, int len, String art, String cat )
  {    super( ttl, len );
    artist   = art;
    category = cat;
  }
  
  public void show()
  {    super.show();
    System.out.println( "artist:" + artist + " style: " + category );
  }
}

class DVD
{  String  title;    // name of the item
  int     length;   // number of minutes
  boolean avail;    // is the tape in the store?

  // constructor
  public DVD( String ttl, int lngth )
  {    title = ttl; length = lngth; avail = true; 
  }

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

  // constructor
  public DVD( String ttl, int lngth )
  {    super();     // use the super class's constuctor
    title = ttl; length = lngth; avail = true; 
  }

class DVD extends Object
{
. . .
}