next up previous
Next: About this document

Adaptive Programming with Traversals and Visitors

by Doug Orleans
for Dr. Karl J. Lieberherr

Adaptive programming using the Demeter Method is a programming methodology (with tool support) that lets you work less, but achieve more. By specifying as little as possible about your program, you can make it

The Demeter Method has three steps, performed iteratively:

Using the Demeter system, you specify structure and behavior succinctly and separately. The path directives only mention the classes at the endpoints of the traversal, and the visitor methods are only specified on the classes where the behavior needs to be performed; in this way, the class graph, traversal methods, and visitor methods are only loosely coupled, allowing the program to automatically adapt to evolutionary changes in the structure or the behavior.

In the current version of the tool, Demeter/Java, behavior is encapsulated in methods on visitor classes, allowing object-oriented reuse through inheritance and polymorphism. Visitor classes are defined separately from traversal methods, so they can be reused in different combinations. (The idea of visitor classes comes from the Visitor pattern in the widely-used book Design Patterns, by Gamma, Helm, Johnson & Vlissides.)

A class graph is specified with a class dictionary file, with the filename extension .cd. You can define three kinds of classes in a class dictionary:

A class dictionary may also be annotated with syntax strings, which are used to construct a parser that can read a text stream representing data and build corresponding object graphs. Object graphs can also be printed as text using the same syntax.

Class definitions may refer to classes (for example, as a part class) that are not defined in the class dictionary; the class must be defined in another source file or library package. This is useful for built-in Java classes such as Integer or String.

Program behavior is specified in a behavior file, with the filename extension .beh. You can define three kinds of behavior methods:

Visitor classes are defined in the class dictionary, just as any other class. There's nothing special about a visitor class except that it has before and after methods.

A visitor class may have data parts that can be referred to in the visitor methods; these are useful for transporting data from one part of the traversal subgraph to another.

A visitor class may itself define a traversal method, which could be called with other visitor objects; traversing a visitor may often be necessary if the visitor class has composite data members.

Here's an example class dictionary file:

// A construction class.
Computer = <parts> EquipmentList.

// A repetition class.
EquipmentList ~ { Equipment }.

// An alternation class with common parts,
// and some empty construction classes.
Equipment : Card | Drive | FloppyDisk | CompositeEquipment
    *common* <netPrice> Currency "discount" <discountPrice> Currency.
Card = "card".   Drive = "drive".   FloppyDisk = "floppy".

CompositeEquipment : Cabinet | Chassis | Bus
    *common* "(" <parts> EquipmentList ")".
Cabinet = "cabinet".   Chassis = "chassis".   Bus = "bus".

// Some visitor classes.
PricingVisitor = <total> Currency.
InventoryVisitor = <inventory> Inventory.

Here's an example behavior file to go with it:

Computer {
  traversal allEquip(PricingVisitor price, InventoryVisitor inv) {
    bypassing -> CompositeEquipment,parts,* to Equipment;

PricingVisitor {
  before Equipment (@ total = total.add(host.get_netPrice()); @)
  // Chassis are on sale...
  before Chassis (@ total = total.add(host.get_discountPrice()); @)

InventoryVisitor {
  before Equipment (@ inventory.accumulate(host); @)

(example behavior file continued)

Main {
  static public void main(String args[]) throws Exception {
    Computer comp = Computer.parse(;

    PricingVisitor price = PricingVisitor.parse("$0");
    InventoryVisitor inv = InventoryVisitor.parse("");

    comp.allEquip(price, inv);

    System.out.println("Price = $" + price.get_total());
    System.out.println("Inventory: " + inv.get_inventory());

Notice a few things about this example program:

Directions for future development:

next up previous
Next: About this document

Karl Lieberherr
Wed Jan 15 09:27:34 EST 1997