CS201: Software Development Methods

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CS201 Software Development Methods
OO Design and the JavaCollections Framework Part
2

• Topics
• Comparable and Comparator interfaces in JCF
• Function objects
• Textbook readings
• More from MSD, Chapter 9
• Pages 641-647
• Pages 650-651 up to 9.4.1Pages 666-671.

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Overview

• In Part 1 of this Unit, we learned
• About the Collections framework
• Collection and List interfaces
• Iterator, ListIterator
• Also should now study static methods from the
  book or class notes
• Here in Part 2 of this Unit, we will see
• Defining order for sort(), max(), etc.
• Using an interface and a function-object

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Back to Java Comparable and Comparator

• In computing, we often want to order a set of
  items
• Find the max/best or min/worst
• Sort them in order
• Note how this is different than what is needed
  for search (where we just need equals)
• Need a way to compare two items
• Want to flexible way to compare using different
  criteria
• In Java, lets us use Collection(s) methods to
  meet our own special needs. (Powerful!)

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Check out the Collections class

• Class Collections
• utility methods for doing operations on
  Collections, Lists (that almost always contain
  homogenous elements)
• Note very similar Arrays class
• See MSD textbook, Section 9.5, pp. 666f
• Methods (static, mostly for Lists)
• search a list for an item binarySearch()
• sort(), max(), min() -- uses compareTo() or
  Comparator object
• reverse(), fill(), shuffle(), copy(),
  replaceAll()
• List list2 Collections.unmodifiableList(list1)
  // p. 668

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Comparable Interface

• First solution Can we ask an object how it
  compares to a second object?
• Sure string1.compareTo(string2)
• Programming convention Return value as follows
• zero if the same
• negative value if first item strictly less than
  second
• positive value if first item strictly greater
  than second
• Java provides a Comparable interface int
  compareTo(Object o)
• Note the parameter is an object
• Like equals() and for the same reason

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Writing compareTo for Your Classes

• If you ever want to put your own objects in
  Collections, and use sort(), max(), min(),
• Make your class implement Comparable
• Implement the compareTo() method in your class
• How to write compareTo()?
• Think about state-variables that determine
  natural order
• Compare them and return proper-value
• Note For number values, you can subtract.
• For object values, call compareTo() on them.

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Example Writing compareTo()

• Imagine something like an entry in a phonebook
• Order by last name, first name, then number
• int compareTo( Object o ) PhoneBookEntry item2
  (PhoneBookEntry ) oint retVal
  last.compareTo(item2.last)if ( retVal ! 0 )
  return retValretVal first.compareTo(item2.firs
  t)if ( retVal ! 0 ) return retValretVal
  phNum - item2.phNumreturn retVal

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Under the Hood for Sorting

• How might a sort() or any other method use this?
  Imagine
• Perhaps items stored as a list of Objects
  List theList
• Inside a loop, code might look like this
  Comparable item1 (Comparable) theList.get(i)
  Comparable item2 (Comparable) theList.get(j)
  int cmpResult item1.compareTo(item2)
• Such code will work when the list stores any
  class that implements Comparable!
• But, what happens if list-elements are of
  different classes (still Comparable, but
  different)? compareTo() fails!

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Flexible Design using Comparators

• Solution 1 Make classes Comparable
• Disadvantage just one way to compare is
  possible, because theres just one compareTo
  method per class
• Possible solutions
• Separate functions sortByName(), sortByNum(),
• We cant predict in advance how youll want to
  sort!
• Pass a parameter to indicate control
  sort(theList, byName) or sort(theList,
  byNum)
• Ugh. Same problem as before
• And the internals of sort() will grow to become
  very ugly

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Function Objects

• We need to somehow pass how to execute
  information as a parameter to sort()
• We pass objects as parameters
• Can we pass a method/operation as an object?
• Many languages support this, but in different
  ways
• C and C pointers to functions
• C delegates
• Java function objects that
• implement a specified interface, and
• the one method in that interface does the needed
  work

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Function Objects in Java

• Idea encapsulate a function inside a class
• Note not our usual idea of a class
• State? (None.)
• Identity? (Just need one instance.)
• Represents an entity? (Nope! Just a place to
  stash a function so it can be passed as a
  parameter.)
• Warning / caveat!
• This idea is contrary to many OO principles, but
• Useful if done in limited circumstances
• Use it when the libraries make it available
• Not often part of your own class-design
• But use it in libraries when its part of the
  framework

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Example Comparator objects

• We want to pass a function-object to a method
• Collections.sort(someList, function-object-goes-he
  re)
• But what type should this object be?
• Use an Interface
• Interface name can be used as a type in the
  parameter list
• Interface defines the method name itself!
• Javas Comparator interface int compare(
  Object o1, Object o2)
• Notes not compareTo()! Takes two parameters!
• Define a class for each kind of comparison you
  want. E.g. Classes CmpStudentByGpa,
  CmpStudentByGpaDesc Classes CmpDogByName,
  CmpDogByBreed

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Writing a Comparator Class

• Example on following slides like one from MSD
  text, p. 646-647
• We have a Dog class with name, breed and gender
• But see simpler example in Java on website
• list of Student objects
• sort by name, gpa
• Also sort in descending order too
• Look at testSorting1() in TestDriver class
• Look at compareTo() in Student
• Look at use of comparator classes

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Writing a Comparator Class

• Example like one from MSD text, p. 647
• We have a Dog class with name, breed and gender
• Compare two doggies by breed and then
  namepublic class CmpDogByBreedAndName
  implementsComparator
• public int compare(Object o1, Object o2)
• Dog d1 (Dog) o1 Dog d2 (Dog) o2
• int retVal d1.getBreed().compareTo(d2.getBre
  ed())
• if ( retVal ! 0 ) return retVal
• return d1.getName().compareTo( d2.getName()
  )

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Use of Comparator methods

• How to use with Collections.sort()
• ArrayList dogList
• Collections.sort( dogList, new CmpDogByName()
  )Collections.sort( dogList, new CmpDogByBreed()
  )
• (Do you understand what new does here?)
• Inside sort(), code looks something like this
• sort ( List theList, Comparator cmpObj )
• // in some loop Object item1
  list.get(i) Object item2 list.get(j)
  cmpResult cmpObj.compare(item1,item2)

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End of Comparable/Comparator topics

• Make sure you see slides on Abstraction
• They relate to this material and what follows

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(No Transcript)
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Java Aside Anonymous Classes

• Theres a Java technique called anonymous classes
• One of several types of nested class definition
• Youll very often see it in GUI programming
  (Swing) and with threads
• Situation
• Sometimes Javas design encourages us to create
  some thing that might be used just once
• That thing needs to be wrapped up in a class, say
  because we need a function object
• What couldnt we just declare it at the same
  place we use it? Why create a separate file?

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Creating and Using an Anonymous Class

• Example sort a list of Strings by their length
• Collections.sort ( stringList, new Comparator()
  public int compare( Object o1, Object o2 )
  return ((String) o1).length() - ((String)
  o2).length()
• )
• Weve created a new Comparator on the fly
• new creates a new instance, but what kind?
• Some object that implements Comparator
• Object not named, and its true class not named!
• What must a Comparator have? compare()
• We defined it right here, where its used!

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Anonymous Classes Comments

• Anonymous classes are unlike other classes
• They have no name
• Typically only implement methods in their
  interface or superclass. No new methods!
• Since they have no name, can only define and use
  them at one point in your code!
• Hard to understand at first? Sure!
• Naming an abstraction is important for human
  understanding!
• Sorting, a Collection, Comparing
• Advice
• Keep them very short (and simple)!
• Be ready to understand them when you see them in
  Swing and with threads

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Note on following slides

• The summary or Big Picture slides that follow
  may not make complete sense until you know
  material on these slides and
• Slides on abstraction
• Slides on Sets and Maps
• They are repeated at the end of those slide sets,
  too, but weve left them here just in case

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Big Picture on Java Collections Unit

• Java Collections Framework
• A large collections of interfaces and classes
• Includes things we can use as is (concrete
  classes) and building blocks for new things
  (abstract classes)
• Classes that exist to contain static methods,
  e.g. Collections class (see also Arrays class)
• Power of inheritance, interfaces, polymorphism
• See how type Collection and List are used in
  parameter lists and in Collections class methods
  (e.g. sort)
• Be aware of and start to use Java 5.0 generics
  with your collections

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Big Picture on Java Collections Unit (2)

• Iterators both concepts and the practice
• Example of procedural abstraction (use, methods)
  anddata abstraction (whats really in a iterator
  object?)
• Using these in Java for Collections, Lists
  (methods, cursor, etc.)
• Procedural abstraction the concepts, examples
• Function-objects and procedural abstraction
• Encapsulating functionality or control in an
  object
• Comparator objects (contrast to implementing
  Comparable interface)
• Java trick anonymous classes

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Big Picture on Java Collections Unit (3)

• Java skills Concrete collection classes and
  methods for you to use
• ArrayList and ListIterator
• HashSet
• Must over-ride hashCode()
• Collections static methods
• See JavaDoc or p 587 in MSD textbook
• E.g. sort, reverse, binarySearch, fill, shuffle,
  min, max,
• Implement Comparable for your collection items
• Define Comparator classes for more powerful,
  flexible use