NetRexx overview - Extending classes

NetRexx Overview, version 2.02 Copyright (c) IBM Corporation, 2001. All rights reserved. © | 22 May 2001
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## Extending classes

It's common, when dealing with objects, to take an existing class and extend it. One way to do this is to modify the source code of the original class – but this isn't always available, and with many different people modifying a class, classes could rapidly get over-complicated.

Languages that deal with objects, like NetRexx, therefore allow new classes of objects to be set up which are derived from existing classes. For example, if you wanted a different kind of Oblong in which the Oblong had a new property that would be used when printing the Oblong as a rectangle, you might define it thus:

      /* charOblong.nrx -- an oblong class with character */
      class charOblong extends Oblong
        printchar       -- the character for display

        /* Constructor to make a new oblong with character */
        method charOblong(newwidth, newheight, newprintchar)
          super(newwidth, newheight) -- make an oblong
          printchar=newprintchar     -- and set the character

        /* 'Print' the oblong */
        method print
          loop for super.height
            say printchar.copies(super.width)
            end

There are several things worth noting about this example:

1. The ‘`extends Oblong`’ on the class instruction means that this class is an extension of the Oblong class. The properties and methods of the Oblong class are inherited by this class (that is, appear as though they were part of this class).
   Another common way of saying this is that ‘`charOblong`’ is a subclass of ‘`Oblong`’ (and ‘`Oblong`’ is the superclass of ‘`charOblong`’).
2. This class adds the `printchar` property to the properties already defined for Oblong.
3. The constructor for this class takes a width and height (just like Oblong) and adds a third argument to specify a print character. It first invokes the constructor of its superclass (Oblong) to build an Oblong, and finally sets the printchar for the new object.
4. The new charOblong object also prints differently, as a rectangle of characters, according to its dimension. The `print` method (as it has the same name and arguments – none – as that of the superclass) replaces (overrides) the `print` method of Oblong.
5. The other methods of Oblong are not overridden, and therefore can be used on charOblong objects.

The `charOblong.nrx` file is compiled just like `Oblong.nrx` was, and should create a file called `charOblong.class`.

Here's a program to try it out:

      /* trycharOblong.nrx -- try the charOblong class */

      first=charOblong(5,3,'#')  -- make an oblong
      first.print                -- show it
      first.relsize(1,1).print   -- enlarge and print again

      second=charOblong(1,2,'*') -- make another oblong
      second.print               -- and print it

This should create the two charOblong objects, and print them out in a simple ‘character graphics’ form. Note the use of the method `relsize` from Oblong to resize the charOblong object.

#### Optional arguments

All methods in NetRexx may have optional arguments (omitted from the right) if desired. For an argument to be optional, you must supply a default value. For example, if the charOblong constructor was to have a default value for `printchar`, its method instruction could have been written:

      method charOblong(newwidth, newheight, newprintchar='X')

which indicates that if no third argument is supplied then `'X'` should be used. A program creating a charOblong could then simply write:

      first=charOblong(5,3)       -- make an oblong

which would have exactly the same effect as if `'X'` were specified as the third argument.

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From The NetRexx Language by Mike Cowlishaw, mfc@uk.ibm.com (ISBN 0-13-806332-X, 197pp, Prentice-Hall, 1997).