This document discusses the visitor pattern. It begins with an example scenario of using the visitor pattern to find Lady Gaga fans among students, professors, and staff. It then discusses naive approaches that have problems and are not object-oriented. The document explains the elements of the visitor pattern solution, including how it separates the algorithm from the class structure. Finally, it provides an example of using the visitor pattern in compiler design with the Eclipse JDT and how the AspectJ compiler reuses elements from JDT. The key benefits are allowing new operations to be added without changing existing classes and making compilers extensible.

1. Tutorial 4
Visitor Pattern
Wei Wang
Nov. 25th, 2011

2. Goals
• Scenario of using Visitor Pattern
• Why naï approaches are bad?
ve
• Elements of Visitor Pattern
• Real world application
2

3. Scenario
• Sarah wants to find out who does NOT like
Lady Gaga at UWaterloo
3

4. • For male (female) students: borrowed >1
(>3)book about Lady Gaga from school library
• For profs: purchased >2 Lady Gaga CDs
• For staff: …
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5. Essence of this scenario
• Iterate a variety of elements under one
hierarchy
– Student/prof/staff
• Customization of the iterating algorithm
– Gender/library history/purchasing history
• Data aggregation
– Total number of student fans? What about CS only?
• New algorithm may arise from future demands
– Looking for fans of Justin Bieber?
• “open for extension, but closed for modification”
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6. Naï solution 1
ve
isLadyGagaFans(IPerson){
If(Iperson instanceof
Student){
} Instanceof and type cast!
Else if (Iperson instanceof
Professor){
}
}
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7. Naï Solution 2
ve
class Student{
isLadyGagaFans(){ Algorithm defined in the student class
checklibraryRecords();
checkGender();
}
}
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8. Problems of Naï solutions
ve
• “instanceof” •“pseudo OO” solution
solution:
•Touch original code
– Instanceof or
type casting is
•Similar solution is
error prone
scattered in many
– hard coding! place!
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9. Visitor Pattern Solution
Class Student implement IPerson{
void accept(Visitor visitor){
visitor.visit(this);
}
}
Class LadyGagaFansChecker(){
visit(Student student){
student.checkLibraryHistory();
student.checkGender();
print;
}
}
…… studentA.accept(new LadyGagaFansChecker());
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10. Class Diagram of Visitor Pattern
CheckJustinBieberFa
n
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11. Real world application
• Processing syntactical elements in compiler
design
– Eclipse JDT
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12. Visitor Pattern in JDT
compiler ASTVisitor
DeclarationVisitor
MethodVisitor
If the syntax of Java changes
(such as generics and enhanced
loop), we just need add new
visitors and invoke them later
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13. What if we cannot change JDT?
• The Compiler of AspectJ reuses most of JDT
elements (because we still need to compile
the JAVA part of the code)
AspectJ
compiler
JointPointVisitor
JointPoint
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14. 14

15. Take-away of this tutorial
• Separate the algorithm with from an object
structure on which it operates
• Easy to add new operations on existing
objects
– “open for extension, but closed for modification”
• Almost all compilers implementation use
visitor pattern to iterate syntactical elements
– JDT & AspectJ compiler
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