Using Memory Diagrams When Teaching a Java-Based CS1

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Using Memory Diagrams When Teaching a Java-Based
CS1

• Mark A. Holliday
• David R. Luginbuhl
• Dept of Mathematics and Computer Science
• Western Carolina University

ACM-SE Conference March 7, 2003 Savannah, GA
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Overview

• Motivation and Introduction
• Diagramming as a teaching tool
• Objects, variables, and references
• Primitive types, fields, visibility, and
  parameter passing
• Static fields and methods
• Arrays
• Diagramming as an assessment tool
• Conclusion

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Motivation

• Problem
• How to help CS1 students visualize state of
  computer as Java program executes
• How to introduce and reinforce object-oriented
  concepts in an introductory CS course
• Solution
• Diagrams
• Abstraction visualization
• UML-like
• UML not primarily focused on state of memory
• Diagrams used in many textbooks, but without much
  emphasis

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Criteria

• Take into account features of language (for us,
  Java)
• Visualize state of computer
• Distinguish between
• Objects classes reference variables
• Reference types primitive types
• Private public
• Consistency for similar concepts
• Method invocation and object construction
• Labels, as appropriate
• Arrays vs. other objects

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Memory Diagrams

• Similar to other diagrams
• See, particularly, WU2001
• But with extensions and particular emphases
• Important aspects
• Shape matters
• Importance of reference and invocation
• Indication of values with variables
• Visibility rules

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Objects, Variables, and References

• String acolorblue
• String favColoraColor
• String otherColor

• Features
• Labels on objs and vars
• Different shapes
• Reference lines begin inside variable

String blue
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Method Invocation

• aColor aColor.concat(otherColor)

• Features
• Method indicated by line on object
• Indicates each object has method
• Method is public
• Squiggly line indicates invocation
• New object created
• New value for aColor old value removed

X
String red
String blue
concat
String bluered
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Primitive Types, Fields, Multiple Instances

• Primitive type variables have numeric value
• As opposed to ref types
• Private fields represented inside object
• Using rectangles indicates fields are just
  variables associated with objects
• Different Student objects have same fields with
  (possibly) different values

Student
score
name
87
String Sue
Student
score
name
85
String Bob
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Static fields and methods

• Static fields and methods are displayed in a
  diamond instead of a circle
• Punch clock provides a nice example for static
  fields and methods
• See paper for more examples, including passing
  parameters and diagramming arrays

Employee
(8)
advance
clock
8 16
Employee
start
end
8
16
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Student Exposure to Diagrams

• We introduce these diagrams to students on the
  first day of CS1 class
• We ask students to produce diagrams of their own
• In groupwork
• In directed lab work
• On quizzes and tests
• Bottom line reinforcement of concepts in a
  number of contexts
• But not just for learning

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Student Assessment

• By having students use diagrams themselves, we
  have them demonstrate their comprehension of
  object-oriented concepts
• We believe these diagrams have potential for
  measuring programming comprehension
• Anecdotal evidence from several semesters of
  application

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Example Test Problem From Last Fall

• Diagram the program fragment below
• Dog spot // fig a)
• spot new Dog("spot")
• // right hand side is fig b)
• // left hand side and equal sign is fig c)
• System.out.println(spot.toString()) // fig d)
• Note last line particularly
• Static public variable (System.out)
• Method composition
• PrintStream object
• Dog object
• Creation of a String object

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Results(13 students)
Dog spot // fig a) spot new Dog("spot")
// right hand side is fig b) // left hand
side and equal sign is fig c) System.out.println(s
pot.toString()) // fig d)

• Figure a
• 11 correct
• 2 labeled rectanglewith name of class
• Figure b
• 4 correct
• 2 put spot inside var rectangle
• 4 didnt show name field
• 2 had more serious errors
• Figure c all but 1 student correct

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Results(13 students)
Dog spot // fig a) spot new Dog("spot")
// right hand side is fig b) // left hand
side and equal sign is fig c) System.out.println(s
pot.toString()) // fig d)

• Figure d
• All but 3 students showed call to toString()
  correctly
• Of those 10
• 2 students failed only to show System.out
  correctly
• 1 showed nothing else correctly
• 5 missed the creation of a String object from
  toString()
• They got the println() call right
• 2 showed the String object
• But they missed the println() call
• Summary for this exam
• Decent correlation between overall grades and
  grades on diagramming problem

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Summary for Assessment

• We have more work to do
• We are considering a more rigorous examination of
  diagramming as a student assessment tool
• Not just to measure progress
• We hope we can pinpoint problems and provide
  proper reinforcement of specific concepts

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Conclusion Memory Diagrams

• A relatively low-tech approach for teaching OO
  concepts
• Well-suited for classroom, labs, exams
• Importance of shape and placement for reinforcing
  concepts
• Having students make their own diagrams adds to
  this reinforcement
• Promise of diagrams for measuring comprehension
• If students can diagram what is happening in
  memory, they are probably understanding the
  deeper meaning of the program
• More work to do