Map and Sets

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Java's Collection Framework

• Map and Sets

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Collection Framework

• A collections framework is a unified architecture
  for representing and manipulating collections. It
  has
• Interfaces abstract data types representing
  collections
• Implementations concrete implementations of the
  collection interfaces
• Algorithms methods that perform useful
  computations, such as searching and sorting
• These algorithms are said to be polymorphic the
  same method can be used on different
  implementations

3
Interfaces

• An interface describes a set of methods
• no constructors or instance variables
• Interfaces must be implemented by classes
• 646 java classes implement gt 1 interfaces (02)
• 2 or more classes implement an interface
• Classes guaranteed to have the same methods
• Objects can be treated as the same type
• Can use different algorithms / instance variables

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Collection interfaces
Queuesince Java 5
5
Implementations

• A collection class
• implements an ADT as a Java class
• implements all methods of the interface
• selects appropriate instance variables
• can be instantiated
• Java implements interfaces with
• List ArrayList, LinkedList, Vector
• Map HashMap, TreeMap
• Set TreeSet, HashSet

6
Algorithms

• Java has polymorphic algorithms to provide
  functionality for different types of collections
• Sorting (e.g. sort)
• Shuffling (e.g. shuffle)
• Routine Data Manipulation (e.g. reverse, addAll)
• Searching (e.g. binarySearch)
• Composition (e.g. frequency)
• Finding Extreme Values (e.g. max)

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Two Useful ADTs

• List a collection with a first element, a last
  element, distinct predecessors and successors
• duplicates that "equals" each other are allowed
• Map maps keys to values
• Maps cannot contain duplicate keys
• Each key maps at most one value

8
Map and SortedMap

• The Map interface defines methods
• get, put, contains, keySet, values, entrySet
• TreeMap implements Map
• put, get, remove O(log n)
• HashMap implements Map
• put, get, remove O(1)

9
Set and SortedSet

• Some Map methods return Set
• The Set interface
• add, addAll, remove, size, but no get!
• Some implementations
• TreeSet values stored in order, O(log n)
• HashSet values in a hash table, no order, O(1)

10
Let's play

• Set up a small collection of mappings
• MapltString, BankAccountgt tm
• new TreeMapltString,
  BankAccountgt()
• tm.put("M", new BankAccount("Michel",
  111.11))
• tm.put("G", new BankAccount("Georgie",
  222.22))
• tm.put("S", new BankAccount("Sam", 333.33))
• tm.put("P", new BankAccount("Pei", 444.44))
• tm.put("Z", new BankAccount("Zac", 555.55))
• // Coming up Some Map methods

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keySet()

• // Get the set of keys in the Map.
• // With TreeMap, the keys are ordered.
• SetltStringgt keys tm.keySet()
• assertEquals("G, M, P, S, Z", keys.toString())

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values()

• // The values
• CollectionltBankAccountgt accountCollection
• tm.values()
• IteratorltBankAccountgt itr
• accountCollection.iterator()
• double allTheMoney 0.0
• while (itr.hasNext())
• allTheMoney itr.next().getBalance()
• assertEquals(1666.65, allTheMoney, 0.01)

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entrySet()

• // The mappings
• CollectionltEntryltString, BankAccountgtgt mapping
• tm.entrySet()
• IteratorltEntryltString, BankAccountgtgt itr2
• mapping.iterator()
• while (itr2.hasNext())
• System.out.print("lt" itr2.next().toString()
• "gt ")
• ltGGeorgie 222.22gt ltMMichel 111.11gt
• ltPPei 444.44gt ltSSam 333.33gt ltZZac 555.55gt

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Maps do not have an iterator

• Code Demo Sum up all balances in tm
• There is no iterator method in TreeMap

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Thank You