Collection Considerations Data Structures, ADTs, class Object, Generic Collections

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Collection ConsiderationsData Structures, ADTs,
class Object, Generic Collections

• Rick Mercer

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Some Definitions

• Abstract Data Type (ADT) A set of data values and
  associated operations that are precisely
  specified independent of any particular
  implementation.
• Bag, Set, List, Stack, Queue, Map
• Collection Class A Java language construct for
  encapsulating the data and operations
• ArrayList, LinkedList, Stack, TreeSet, HashMap
• Data Structure An organization of information
  usually in memory
• arrays, linked structure, binary trees, hash
  tables

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Why Collection classes?

• Need data structures to store data to model real
  world entities as a collection of objects
• Things recognized by a jet fighter's radar system
• 35,267 students at UofA
• All courses taken by one student
• A set of Poker Hands
• An appointment book
• Phone numbers on your cell phone

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Common Methods

• Collection classes often have methods for
  performing operations such as these
• Adding an object to the collection of objects
• Removing an object from the collection
• Getting a reference to a particular object find
• then you can send messages to the object while it
  is till in the collection. Program do this a lot
• Retrieving certain objects such as the most
  recently pushed (Stack) or least recently added
  (Queue)
• Arranging objects in a certain order sorting

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Storage Mechanisms

• We have used a 1-D array for storage
• elements in a set
• We have used a 2-D array for storage
• represent a society of cells in Artificial Life
• Can have linked structures to store elements
• Elements are not located in contiguous (next to
  each other) memory as with arrays

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Structures to store elements

• Collection classes can store data in contiguous
  memory (arrays)
• or in a linked structure (linked lists, trees)

2 8 9 11 14 14 22 24 27 31
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- or in tree structures
50
75
25
12
35
66
90
28
41
81
95
54
91
100
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- or in hash tables

• Maps associate a key with a value
• Elements could be stored in a hash table

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Generic Collections

• Collection classes a big part of C Sc 227
• Next up Implement one class that can store a
  collection of any element as a list
• There are three approaches
• Use Java's Collection Framework (not yet)
• Use an array of Object (next)
• Use Java 5 Generics (soon)
• First learn about class Object

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The Object class

• Java has a class named Object
• It communicates with the operating system to
  allocate memory at runtime
• Object is the root of all other classes
• All classes extend Object or they extend a class
  that extends object, or a class that extends a
  class that extends Object

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EmptyClass inherits the methods of Object

• public class EmptyClass extends Object
• // This class inherits Object's 11 methods
• // Send some messages that Object has already
  implemented
• EmptyClass one new EmptyClass()
• EmptyClass two new EmptyClass()
• System.out.println(one.toString())
• System.out.println(one.hashCode())
• System.out.println(one.getClass())
• System.out.println(two.toString())
• System.out.println(two.hashCode())
• System.out.println(one.equals(two))
• one two
• System.out.println("\n"one.equals(two))

Output EmptyClass_at_ffb8f763 -4655261 class
EmptyClass EmptyClass_at_ffbcf763 -4393117 false tru
e
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One way assignment

• Can assign any reference to an Object object
• Object obj1 new String("a string")
• Integer obj2 new Integer(123)
• System.out.println(obj1 " " obj2)
• Output
• a string 123
• But not the other way compiletime error
• String str obj1 // incompatible types
• 1 error

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Cast

• Sometimes an explicit cast is needed
• Consider retrieving an object with get
• Enclose the class name with what you know the
  class to be in parentheses (String) and place it
  before the reference to the Object object.
• str (String)obj1

A reference to an Object object
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Cast

• Alternatively, you can write this more clearly in
  two statements 
• // ArrayList's get methods returns a
• // reference to an Object object
• Object object stringList.get(0)
• // Now cast object from Object to String with
  String str (String)object

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A collection of Anything

• Code Demo
• Implement a collection class that can store any
  type
• // Add any Object to this collection as the next
  public void add(Object elementToAdd)
• // Return the number of meaningful elements added
  public int size()
• // Return a reference to the element at the index
• // Precondition index gt 0 index lt size()
• public Object get(int index)

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Primitives as ObjectsObjects as Primitives

• To allow collections of primitives Java has
  wrapper classes for each primitive type
• Integer Double Character Boolean Long Float
• In Java 1.4 we need lots of extra code that some
  consider
• ListOfAnything list new ListOfAnything()
• list.add(new Integer(50))
• int firstVal ((Integer)list.get(0)).intValue()
  
• Switch Eclipse to 1.4 compliance and see the
  above code work, but not this
• list.add(50)
• int firstVal get(0)

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List of Primitives

• In Java 1.4, ListOfAnything could not store
  primitives
• ListOfAnything list new ListOfAnything()
• list.add(50) // Error in Java 1.4
• Since Java 5, objects can appear to be primitives
• Integer i 3 // assign an int to an Integer
• int j new Integer(4) // Integer to int
• int k j new Integer(7) 3 i // mix
• Is this legal in Java 5?
• Object obj 3
• int j (Integer)obj

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Boxing / Unboxing

• Autoboxing is the process of treating a primitive
  as if it were an object. When the compiler sees
  this
• Integer anInt 3
• the code transforms into this
• Integer anInt new Integer(3)
• Switch Eclipse to 5.0 Compliance and see the code
  on the previous slide works

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A Problem with Object Parameters and Returns

• Java does not check the type of element being
  added
• list.add(2)
• list.add(new Double(3.4))
• list.add(new BankAccount("Pat", 2.00))
• list.add(new Integer(2))
• So what do you cast list.get(0) to?
• So what do you cast list.get(1) to?
• So what do you cast list.get(2) to?
• Program could throw ClassCastException

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Generics

• Can use Java 5 Generics
• New class heading needed with ltEgt
• public class ListOfAnythingltEgt
• Then, in the class, the compiler sees E as the
  same type as the type use to construct it
• ListOfAnythingltStringgt strings
• new ListOfAnythingltStringgt()
• ListOfAnything ltIntegergt ints
• new ListOfAnythingltIntegergt()
• ListOfAnything ltBankAccountgt accounts
• new ListOfAnythingltBankAccountgt()

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Can't add the wrong type

• Java 5 generics checks the type being added at
  compiletime. These are errors
• strings.add(1)
• accounts.add("Strings")
• ints.add(new BankAccount("Pat", 12))
• Convert ListOfAnyType to a generic collection
  with a type parameter ltEgt

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Code Demo Implement a Generic Collection with
ltEgt

• Here is a beginning
• public class ArrayBagltEgt implements BagltEgt
• private Object elements
• private int n
• public ArrayBag()
• elements new Object1000
• n 0