1. Sets and Maps Chris Nevison

2. 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

3. 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

4. Set, other methods boolean isEmpty() void clear() addAll(Collection stuff) containsAll(Collection stuff) removeAll(Collection stuff) retainAll(Collection stuff)

5. Set example- method intersection using 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; }

6. Set example- method union using 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; }

7. Set example- method intersection using other methods public static Set intersect2(Set setA, Set setB) { Set aSet = new HashSet(setA); aSet.retainAll(setB); return aSet; }

8. Set example- method union using other methods public static Set union2(Set setA, Set setB) { Set aSet = new HashSet(setA); aSet.addAll(setB); return aSet; }

9. 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 )

10. 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

11. 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

12. 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.)

13. 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

14. 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()

15. 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

16. 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 )

17. Example Sets 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

18. Example Sets 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

19. 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()); }

20. Example public void printState(String state) { Set citySet = (Set)myMap.get(state); Iterator itr = citySet.iterator(); System.out.print(state + ": "); while(itr.hasNext()) System.out.print(itr.next() + ", "); System.out.println(); }

21. Example public void printAllStates() { Set stateSet = myMap.keySet(); Iterator itr = stateSet.iterator(); while(itr.hasNext()) printState(itr.next()); }

22. Example Change HashMap to TreeMap (in constructor) to see the difference Change HashSet to TreeSet (in addCity method) to see the difference

23. Exercise - Environment for Marine 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