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Using UML, Patterns, and Java
Object-Oriented Software Engineering
Chapter 8, Design Patterns
Visitor
A Pattern Taxonomy
Pattern
Structural
Pattern
Behavioral
Pattern
Creational
Pattern
Composite
Decorator
Adapter
Bridge
Façade
Proxy
Iterator
Visitor
Command
Observer
Template
Strategy
Singleton
Abstract Factory
Builder
Factory
Prototype
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
2
the basics
Intent: Represent an operation to be performed on the
elements of an object structure.
 Visitor lets you define a new operation without
changing the classes of the elements on which it
operates
 aka: none!

These slides were borrowed from
lecture notes on Design Patterns by
Brian Malloy at Clemson University
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
3
Motivation
Consider a compiler that represents programs as ASTs
 need to perform operations on ASTs

 type checking
 code generation
 printing

One option: place all of these operations in the nodes
of the AST
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
4
Node class hierarchy
Node
typeCheck()
generateCode();
prettyPrint();
VariableRefNode
AssignmentNode
typeCheck()
generateCode();
prettyPrint();
typeCheck()
generateCode();
prettyPrint();
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
5
Problem with Node class hierarchy
Distributing operations across Node classes leads to a
system that’s hard to understand, maintain and change.
 it’s confusing to have type-checking code in the same
class with code to perform pretty printing.
 also, adding a new operation, e.g. data flow analysis,
requires recompiling all of the classes!

Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
6
Visitor
allows us to separate Node classes from operations
“on” Node classes
 allows each new operation to be added separately,
without changing Node class!

Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
7
Nodes “accept” visitors
Node
program
accept(NodeVisitor)
v.visitAssignment(this)
VariableRefNode
AssignmentNode
accept(NodeVisitor)
accept(NodeVisitor v)
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
8
Visitor for Nodes
NodeVisitor
visitAssignment(node);
visitVariableRef(node);
VariableRefNode
AssignmentNode
visitAssignment(node);
visitVariableRef(node);
visitAssignment(node)
visitVariableRef(node)
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
9
Visitor Pattern

Define two class hierarchies
 one for elements being operated on (nodes)
 one for visitors that define operations on the elements

create new operations by adding a new subclass to
Visitor class hierarchy
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
10
Structure of Visitor Pattern
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
11
Collaborations
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
12
Circularity
inherent in Visitor pattern: Visitor needs elements
& elements need Visitor
 emanates from double dispatch
 In Java, it’s much easier, don’t have to worry about
forward references, or quick compiles
 C++ strives for efficiency, at a cost in programmer
expertise

Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
13
Single Dispatching

Two criteria determine the operation:
 name of the request
 type of the receiver
 Example: calling node->generateCode() on an
assignment Node will dynamically bind to the
generateCode() function in AssignmentNode
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
14
Double Dispatch

operation that gets executed depends on
 kind of request
 types of two receivers
 example: accept() is double dispatched operation. Its
meaning depends on two types: Visitor’s and Element’s
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
15
More implementation issues
A visitor must visit each element of the structure
 Who is responsible for traversing the structure?

 object structure -- most often
 an iterator
 the visitor -- problem: you will likely dupe traversal code
in each concrete visitor
Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
16
Accumulating State
Visitors can accumulate state as they traverse
 without a visitor, this state information might have to
be passed during traversal

Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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Consequences
Easy to add new operations
 Visitor gathers related operations and separates
unrelated operations
 Adding new ConcreteElement classes takes extra
work -- need new abstract operation on Visitor

Bernd Bruegge & Allen H. Dutoit
Object-Oriented Software Engineering: Using UML, Patterns, and Java
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