Sets and Maps

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Sets and Maps

• Chris Nevison

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Set Interface

• Models collection with no repetitions
• subinterface of Collection
• has all collection methods
• has a subinterface SortedSet
• items stored must implement Comparable OR
• must have Comparator supplied at creation
• same as set except iterator will traverse items
  in increasing order
• has extra methods based on order

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Set, basic methods

• boolean add(Object obj)
• adds obj to set, returns false if duplicate
• boolean contains(Object obj)
• true if obj is in this set, else false
• boolean remove(Object obj)
• if obj is in this set removes it and returns true
  , else returns false
• int size()
• Iterator iterator()
• returns an iterator for this set

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Set, other methods

• boolean isEmpty()
• void clear()
• addAll(Collection stuff)
• containsAll(Collection stuff)
• removeAll(Collection stuff)
• retainAll(Collection stuff)

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Set example- method intersectionusing basic
methods, iterator
public static Set intersect(Set setA, Set
setB) Set aSet new HashSet()
Iterator itr setA.iterator()
while(itr.hasNext()) String str
(String)itr.next() if(setB.contains(str))
aSet.add(str) return aSet
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Set example- method unionusing basic methods,
iterator
public static Set union(Set setA, Set setB)
Set aSet new HashSet() Iterator itr
setA.iterator() while(itr.hasNext())
aSet.add(itr.next()) itr
setB.iterator() while(itr.hasNext())
aSet.add(itr.next()) return aSet
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Set example- method intersectionusing other
methods
public static Set intersect2(Set setA, Set
setB) Set aSet new HashSet(setA)
aSet.retainAll(setB) return aSet
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Set example- method unionusing other methods
public static Set union2(Set setA, Set setB)
Set aSet new HashSet(setA)
aSet.addAll(setB) return aSet
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Set - implementations

• HashSet
• requires method hashCode for objects stored
• inherited from Object, but should be overridden
• must be consistent with equals method
• uses hash table
• expected time for add, remove, contains all O(1)
• depends on "good" hashing function (hashCode)

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Set - implementations

• TreeSet
• implements SortedSet
• items must implement Comparable OR
• must have Comparator supplied when constructed
• uses a balanced binary search tree
• time for add, remove, contains is O(log N) for
  set containing N items
• iterator traverses elements in increasing order
• other order-based methods available

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Set - implementations

• Compare results of sample code for HashSet and
  TreeSet
• sets are printed in arbitrary order for HashSet
• sets are printed in order for TreeSet

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Set - exercises

• in Marine Biology Simulation
• Rewrite an Environment implementation (any one
  will do) so that the method allObjects returns a
  set of Locatable objects. Then rewrite the
  Simulation method step to use a set with
  iterator. (Note, this will break the given
  implementations of EnvDisplay -- you must use a
  special version.)

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Map interface

• Not a sub-interface of collection
• Defines a map or directory
• Store a pair of a key and an item
• example key is name, item is address and/or
  phone number
• Look up items by using key

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Map interface - basic methods

• Object put(Object key, Object value)
• stores value under this key, returns previous
  value stored
• Object get(Object key)
• gets the value stored under key
• boolean containsKey(Object key)
• returns true if some value is stored under this
  key
• Set keySet()
• returns a Set containing all keys
• size()

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Map interface - other methods

• boolean isEmpty()
• Object remove(Object key)
• removes the key-value pair for this key and
  returns the value stored, or null
• void clear()
• Collection values()
• returns a collection with all values from this
  map, including duplicates

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implementations of Map

• HashMap
• same requirements on keys as for HashSet elements
• hashCode method appropriate
• uses hash table
• O(1) average time for look-ups, puts, removes
• TreeMap
• same requirements on keys as for TreeSet elements
• needs elements Comparable or Comparator to
  constructor
• uses balanced binary search tree
• O(log N) time for look-ups, puts, removes
• keySet() method returns a TreeSet (SortedSet)

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ExampleSets of cities associated with states

• class CitiesByState encapsulates the map and
  operations
• keys are names of states, as Strings
• values are sets
• each set contains one or more cities as Strings
• uses CityInfo type (interface)
• String city() -- returns name of city
• String state() -- returns name of state

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ExampleSets of cities associated with states

• class CitiesByState has private instance variable
  myMap to store information
• operations
• constructor takes BufferedReader to create
  initial map
• void addCity(CityInfo info)
• adds city with associated state from info
• void printState()
• prints state name followed by all city names
  under this state
• void printAllStates()
• prints information for all states

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Example
public void addCity(CityInfo info) String
state info.state() Set citySet
if(myMap.containsKey(state)) citySet
(Set)myMap.get(state) else citySet
new HashSet() myMap.put(state, citySet)
citySet.add(info.city())
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Example
public void printState(String state)
Set citySet (Set)myMap.get(state) Iterator
itr citySet.iterator() System.out.print(st
ate " ") while(itr.hasNext())
System.out.print(itr.next() ", ")
System.out.println()
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Example
public void printAllStates() Set
stateSet myMap.keySet() Iterator itr
stateSet.iterator() while(itr.hasNext())
printState(itr.next())
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Example

• Change HashMap to TreeMap (in constructor) to see
  the difference
• Change HashSet to TreeSet (in addCity method) to
  see the difference

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Exercise - Environment forMarine Biology
Simulation

• Store Locatable Objects using a Map
• key is location
• value is Fish, or other Locatable object
• Reimplement methods that access the storage data
  structure