Slide 1 (of 21)

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TraitsEmerson R. Murphy-Hill, Philip J. Quitslund, Andrew P.

BlackMaseeh College of Engineering and Computer Science

Portland State University

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Introduction

• Duplication: widespread and problematic

• Here we demonstrate it• Look through some well-weathered

code• Try to factor out encountered

duplication with traits

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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

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Duplication: Problem

ZooDweller

Mammal Bird

Platypus

Insect ...

Bear void reproduce(){ copulate(getMate()); if(this.isMale()) return; layEggs(); }

void reproduce(){ copulate(getMate()); if(this.isMale()) return; layEggs(); }

Trait TEggLaying void reproduce(){ copulate(getMate()); if(this.isMale()) return; layEggs(); } abstract void layEggs();

and Solution

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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

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java.ioOutputStream

FileOutputStream ByteArrayOutputStream FilterOutputStream ObjectOutputStream PipedOutputStream

BufferedOutputStream DataOutputStream PrintSteam

InputStream

ByteArrayInputStream FileInputStream FilterInputStream

BufferedInputStream DataInputStream LineNumberInputStream PushbackInputStream

ObjectOutputStream PeekInputStream PipedInputStream PipedInputStream SequenceInputStream StringBufferInputStream

Reader

CharArrayReader FilterReader

PushbackReader

BufferedReader InputStreamReader

FileReader

PipedReader StringReader

Writer

BufferedWriter CharArrayWriter FilterWriter OutputStreamWriter

FileWriter

PipedWriter PrintWriter StringWriter

LineNumberReader

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The Plan

9 AMFind duplication

9:15 AMRemove duplication using Traits

11:15 AMPat selves on back; go home early

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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…

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Interesting Detected Duplication

• Other throwers of IndexOutOfBoundsException?

} else if (b == null) { throw new NullPointerException();} else if ((off < 0) || (off > b.length) || (len < 0) || ((off + len) > b.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException();} else if (len == 0) { return 0;}

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Detection Effectiveness

3

26

Matchesfoundby CPD

Matchesfound

by hand

Differences in:

•Semantics

•Structure

•Efficiency

•Miscellaneous

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Semantic differences

(StringReader>>read(...))if ((off < 0) || (off > cbuf.length) || (len < 0) || ((off + len) > cbuf.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException();} else if (len == 0) { return;}

(StringWriter>>write(...)) if ((off < 0) || (off > cbuf.length) || (len < 0) || ((off + len) > cbuf.length) || ((off + len) < 0)) { throw new IndexOutOfBoundsException();} else if (len == 0) { return 0;}

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Structural Differences(ByteArrayOutputStream>>write(...)) if ((off < 0) || (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 < 0) || (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;}

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Exception Handling Differences(ByteArrayOutputStream>>write(...)) if ((off < 0) || (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 < 0) || (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

?

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Efficiency Differences

(ByteArrayOutputStream>>write(...)) if ((off < 0) || (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!

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Miscellaneous Differences

(PushbackReader>>read(...)) try { if (len <= 0) { if (len < 0) { throw new IndexOutOfBoundsException(); } else if ((off < 0) || (off > cbuf.length)) { throw new IndexOutOfBoundsException(); } return 0; } //...actual work...} catch (ArrayIndexOutOfBoundsException e) { throw new IndexOutOfBoundsException();}

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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

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Our Refactoring

InputStream

ByteArrayInputStream StringBufferInputStreamFileInputStreamFileOutputStream PipedInputStream

LineNumberInputStream

FilterInputStream

TSkip

Provideslong skip(long)

Requiresint read(byte,int,int)

int getSKIP_BUFFER_SIZE()byte[] getSkipBuffer

void setSkipBuffer(byte[])

TInputStream

Provides-

Requires-

TChannelClose

Providesvoid close()

Requiresvoid close0()

FileChannel getChannel()

To TClosing

TArrayBufferInputStream

Provideslong skip(long)int available()

Requiresint getPos()

int getCount()void setPos(long)

TReadProvides

int read(byte, int, int)Requiresint read()

Legend

Class A extends Class B

Class or Trait A uses Trait T

T

A

A

B

TBoundsCheck

Providesboolean boundsCheck(byte,off,len)

Requires-

PipedReader

Reader

LineNumberReader

BufferedReader

TSkip2Provides

long skip(long)Requires

int read(char,int,int)Object getLock

char[] getSkipBuffervoid setSkipBuffer(char[])

int getMaxSkipBufferSize()

TClosing

Provides-

Requires-

TRecievedLastProvides

void receivedLast()Requires

void setClosedByWriter(boolean)

TCloseProvides

void close()Requires

void setIn()void setClosedByReader(boolean)

From PipedInputStream

PrintStream PrintWriter

TPrinting

Provides-

Requires-

TPrintln

Providesvoid println()

void println(boolean)void println(int)

void println(long)void println(double)

void println(float)void println(char[])void println(String)void println(Object)

Requiresvoid newLine()

Object getLock()void print(boolean)

void print(int)void print(long)

void print(double)void print(float)

void print(char[])void print(String)void print(Object)

TPrint

Providesvoid print(boolean)

void print(int)void print(long)

void print(double)void print(float)

void print(char[])void print(String)void print(Object)

Requireswrite(char[])write(String)

TIOOperationProvides

void ensureOpen()boolean checkError()

RequiresObject getOut()

void flush()boolean getTrouble()

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The Plan

9 AMFind duplication

9:15 AMRemove duplication using Traits

11:15 AMPat selves on back; go home early

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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

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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?

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Questions?