1. MULTIVIE W Slide 1 (of 21) Removing Duplication from java.io: a Case Study using Traits Emerson R. Murphy-Hill, Philip J. Quitslund, Andrew P. Black Maseeh College of Engineering and Computer Science Portland State University

2. MULTIVIE W Slide 2 (of 21) Introduction Duplication: widespread and problematic Here we demonstrate it Look through some well-weathered code Try to factor out encountered duplication with traits

3. MULTIVIE W Slide 3 (of 21) Traits A trait is a collection of pure methods Classes “use” traits Traits can be used to supply the same methods to multiple classes in the inheritance hierarchy

4. MULTIVIE W Slide 4 (of 21) Duplication: Problem and Solution

5. MULTIVIE W Slide 5 (of 21) Some Trait Implementations Original implementation in Squeak Smalltalk Also in Perl, Scala, and C# Our implementation in Java, on top of Eclipse –Traits represented as abstract classes –No compiler modification

6. MULTIVIE W Slide 6 (of 21) java.io

7. MULTIVIE W Slide 7 (of 21) The Plan 9 AM Find duplication 9:15 AM Remove duplication using Traits 11:15 AM Pat selves on back; go home early

8. MULTIVIE W Slide 8 (of 21) Finding Duplication We identified some duplication detection tools: –Duplication Management Framework –SimScan –PMD’s Copy-Paste Detector (CPD) Some obvious duplication found, but that was just the tip of the iceberg…

9. MULTIVIE W Slide 9 (of 21) Interesting Detected Duplication Other throwers of IndexOutOfBoundsException? } else if (b == null) { throw new NullPointerException(); } else if ((off b.length) || (len b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return 0; }

10. MULTIVIE W Slide 10 (of 21) Detection Effectiveness Differences in: Semantics Structure Efficiency Miscellaneous

11. MULTIVIE W Slide 11 (of 21) Semantic differences (StringReader>>read(...)) if ((off cbuf.length) || (len < 0) || ((off + len) > cbuf.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return; } (StringWriter>>write(...)) if ((off cbuf.length) || (len < 0) || ((off + len) > cbuf.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return 0; }

12. MULTIVIE W Slide 12 (of 21) Structural Differences (ByteArrayOutputStream>>write(...)) if ((off b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return; } (ByteArrayInputStream>>read(...)) if (b == null) { throw new NullPointerException(); } else if ((off b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } if (pos >= count) { return -1; } if (pos + len > count) { len = count - pos; } if (len <= 0) { return 0; }

13. MULTIVIE W Slide 13 (of 21) Exception Handling Differences (ByteArrayOutputStream>>write(...)) if ((off b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return; } (ByteArrayInputStream>>read(...)) if (b == null) { throw new NullPointerException(); } else if ((off b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } if (pos >= count) { return -1; } if (pos + len > count) { len = count - pos; } if (len <= 0) { return 0; } Suppose b == null AND off == -1 ?

14. MULTIVIE W Slide 14 (of 21) Efficiency Differences (ByteArrayOutputStream>>write(...)) if ((off b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException(); } else if (len == 0) { return; } (PipedWriter>>write(...)) if (sink == null) { throw new IOException("Pipe not connected"); } else if ((off | len | (off + len) | (cbuf.length - (off + len))) < 0) throw new IndexOutOfBoundsException(); Bounds check changed in 1.3, but in exactly 3 of 26 cases!

15. MULTIVIE W Slide 15 (of 21) Miscellaneous Differences (PushbackReader>>read(...)) try { if (len <= 0) { if (len < 0) { throw new IndexOutOfBoundsException(); } else if ((off cbuf.length)) { throw new IndexOutOfBoundsException(); } return 0; } //...actual work... } catch (ArrayIndexOutOfBoundsException e) { throw new IndexOutOfBoundsException(); }

16. MULTIVIE W Slide 16 (of 21) What did we Learn? Duplication occurs non-uniformly –So we can’t detect it easily using tools –And we can’t easily remove it We can’t tell the difference between intentional and accidental differences

17. MULTIVIE W Slide 17 (of 21) Our Refactoring

18. MULTIVIE W Slide 18 (of 21) The Plan 9 AM Find duplication 9:15 AM Remove duplication using Traits 11:15 AM Pat selves on back; go home early

19. MULTIVIE W Slide 19 (of 21) Conclusion Finding duplication during the maintenance stage of software development is hard –Intentional and accidental differences Traits can factor out duplication in a commercial code base If coders use traits while programming, the need to duplicate is reduced

20. MULTIVIE W Slide 20 (of 21) Related Work Related Studies –“Applying traits to the smalltalk collection classes,” OOPSLA 2003. Related Approaches –Multiple Inheritance, Mixins, Delegation –Static Utility Classes –AOP AspectJ, CME(Hyper/J), etc. Any more?

21. MULTIVIE W Slide 21 (of 21) Questions?