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

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  • Sets and MapsStarring: keySet Co-Starring: Collections

  • Purpose:

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

  • Resources:

    Barrons Chapter 11 p.368 (Only Collections, Sets & Maps p.368-369 & p.377 & p.382)

    Java Essentials Study Guide Chapter 17 p.303Java Fundanentals, Lambert Chapter 17 p.567

  • Handouts:



    3.Sets Maps and the ADTs.doc

    4.set map interfaces.Doc

  • Intro:

    Two ADTs Set and Map provide rules for creating Data Structures that conform to specific behaviors

  • 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:HashSetHashMapTreeMapTreeSet

  • In this Lecture we will discuss:

    The Collections APISet InterfaceMap Interface

  • 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

  • A collection data type has the following behaviors:

    insert elementsremove elementsiterate over the elements in the collection

  • Set:

    Set is a collection

    Set is not ordered

    Set does NOT allow duplicate elements

    Set may have a null element

  • 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

  • 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

  • boolean contains(Object x) determines if a given object is an element of the setboolean remove(Object x) removes the element from the set or leaves set unchangedint size( ) number of elements in the setIterator iterator( ) allows for set traversal

  • 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)

  • 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

  • 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)

  • Required methods:

    Object put(Object key, Object value) Associates a Value with a Key and places this pair into the MapREPLACES 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

  • Object get(Object key) Returns the value associated with a Key OR NULL if no map exists or the Key does map to a NULLObject 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

  • 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

  • You can map:

    Names to phone numbersCollege friends to the school they attend

    Animals to animal soundsCoin name to its value

    Car model to its makeLog in IDs to Passwords

  • 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 Maps keySet method

  • The following class contains Key / Value elements

    Student ID is the Key & the Name is the Value

  • 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;}}

  • 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());}

  • 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.

  • Get -- Returns the value to which this map maps the specified key

    Remove -- Removes the mapping for this key from this map if present

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

  • AP AB Subset Requirements:Understand the requirements, restrictions and behaviors of the Set and Map ADTsYou 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)

  • Project:Create Your Own Set and Map Classes