Using Memory Diagrams When Teaching a Java-Based CS1

1. 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

2. 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 ACM-SE

3. 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 ACM-SE

4. 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 ACM-SE

5. 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 ACM-SE

6. aColor otherColor favColor Objects, Variables, and References String acolor=“blue”; String favColor=aColor; String otherColor; Features: • Labels on objs and vars • Different shapes • Reference lines begin inside variable String “blue” ACM-SE

7. aColor otherColor ( ) 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” ACM-SE

8. first second 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” ACM-SE

9. john 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 ACM-SE

10. 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… ACM-SE

11. 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 ACM-SE

12. 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 ACM-SE

13. 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) Results(13 students) • Figure a • 11 correct • 2 labeled rectanglewith name of class • Figure b • 4 correct • 2 put “spot” inside var rectangle • 4 didn’t show name field • 2 had more serious errors • Figure c – all but 1 student correct ACM-SE

14. 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) Results(13 students) • 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 ACM-SE

15. 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 ACM-SE

16. 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 ACM-SE