1 Sets and Maps Starring: keySet Co-Starring: Collections

2 Purpose: In this lecture we discus two interfaces, the Set and the Map and sets up our discussion of HashSet, HashMap, TreeMap & TreeSet

3 Resources: Barrons Chapter 11 p.368 (Only Collections, Sets & Maps p & p.377 & p.382) Java Essentials Study Guide Chapter 17 p.303 Java Fundanentals, Lambert Chapter 17 p.567

4 Handouts: 1.CreateMySet.java 2.Map-Key-Value.java 3.Sets Maps and the ADTs.doc 4.set map interfaces.Doc

5 Intro: Two ADT’s Set and Map provide rules for creating Data Structures that conform to specific behaviors

6 We are required to understand the requirements of these two interfaces and then, in the next few lectures, we will discuss and work with the following implementations of these interfaces: HashSet HashMap TreeMap TreeSet

7 In this Lecture we will discuss: The Collections API Set Interface Map Interface

8 Collections: Collections are simply a group of objects There are collections that permit duplicate objects while others do not Some collections order the objects while others do not

9 A collection data type has the following behaviors: insert elements remove elements iterate over the elements in the collection

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11 Set: Set is a collection Set is not ordered Set does NOT allow duplicate elements Set may have a null element

12 Insert a unique object / element into the Set Remove an object / element from the Set Determine if and object / element is in the Set Use the Iterator to traverse the elements in the Set

13 Hashset (hash table) and TreeSet (BST) implement the Set Interface interface java.util.Set Required methods boolean add(Object x) adds element if unique otherwise leaves set unchanged

14 boolean contains(Object x) determines if a given object is an element of the set boolean remove(Object x) removes the element from the set or leaves set unchanged int size( ) number of elements in the set Iterator iterator( ) allows for set traversal

15 Map: Map is not a real collection, they Produce Collections Maps keys to values Map does NOT allow duplicate elements as each Key in a Map has only one (a unique) Value However, different Keys can map to the same object (value)

16 The Key and the Value can be any object Insert a key / value pair into a Map Obtain a value thru its Key Determine if a Target Key is in the Map

17 Traverse the elements of the Map using the keySet method Iterate thru the Map elements (iterate using the Keys) The TreeMap and the HashMap implement the Map Interface interface java.util.Map (AB only)

18 Required methods: Object put(Object key, Object value) Associates a Value with a Key and places this pair into the Map REPLACES a prior value if the Key already is Mapped to a value Returns the PREVIOUS Key associated value or NULL if no prior mapping exists

19 Object get(Object key) Returns the value associated with a Key OR NULL if no map exists or the Key does map to a NULL Object remove (Object key) Removes the map to this Key and returns its associated value OR returns NULL if no map existed or mapping was to NULL

20 boolean containsKey(Object key) True if there is a key / value map otherwise false int size( ) Returns the number key / value mappings Set keySet( ) Retuns the Set of keys in the map

21 You can map: Names to phone numbers College friends to the school they attend Animals to animal sounds Coin name to its value Car model to its make Log in IDs to Passwords

22 The keySet produces a Set of keys from which we can visit all of the elements of a HashMap or a TreeMap We can visit all of the values (elements) by iterating over the key Set that is returned from the call to the Map’s keySet method

23 The following class contains Key / Value elements Student ID is the Key & the Name is the Value

24 public class Student { Integer id; String name; public Student i, String n) { id = new Integer(i); name = n; } public Integer getId() { return id; } public String getName() { return name; } public String toString() { return id.toString() + ", " + name; }

25 Illustration of a Map using the HashMap class: Map stuffMap = new HashMap(); myStuff[] ms = new Student[5]; ms[0] = new Student(21,"Farrell"); ms[1] = new Student (31,"Castro"); ms[2] = new Student (11,"Defazio"); ms[3] = new Student (61,"Zegers"); ms[4] = new Student (86,"Rogers"); for (int t=0; t < 5; t++) { stuffMap.put(ms[t].getId(), ms[t]); } for (int t=0; t < 5; t++) { myStuff x = (Student)stuffMap.get(ms[t].getId()); System.out.println(x.toString()); }

26 MAP accepts "elements" as 2 separate objects, a key and the data A MAP ADT must conform to the following: Put Associates the specified value with the specified key in this map If the map previously contained a mapping for this key, the old value is replaced.

27 Get -- Returns the value to which this map maps the specified key Remove -- Removes the mapping for this key from this map if present

28 Containskey - Returns true if this map contains a mapping for the specified key Keyset - Returns a set view of the keys contained in this map.

29 AP AB Subset Requirements: Understand the requirements, restrictions and behaviors of the Set and Map ADT’s You Will Not be required to implement the Set or Map Interface But you WILL be required to work with the HashSet, TreeSet, HashMap & TreeMap (Discussed in Next Lecture)

30 Project:Create Your Own Set and Map Classes

31 NO TEST FOR THIS LECTURE !!!