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Maps

Maps

Map is a relation between set of keys and set ofvalues

Map ADT

Java provides a Map interface
Some implementing classes
Hashmap^
Hashtable implementation of a map interface
Hashtable^
Extended dictionary class Map Hashmap Hashtable

Example of Maps

Taurus;Kesden, Greg
RX-8;Slater, Don
Accord;Mouse, Mickey
Camry;Roberts, Jim
Dakota;Pattis, Rich
F-150;Stehlik, Mark
I-300;McElfresh, Scott
big yellow bus;Cortina, Tom

Java Dictionary Class - abstract

The Dictionary class is the abstract parent ofHashtable, which maps keys to values.

A Dictionary object, every key is associated with atmost one value

Given a Dictionary and a key, the associatedelement can be looked up

Any non-null object can be used as a key and as avalue.

Some Methods :
get ( Object key)
put ( Object key, Object value)
remove ( Object key)

Java Tools - HashMap Class

This implements the Map interface
HashMap permits null values and null keys.
Constant time performance for get and put operations
HashMap has two parameters that affect its performance: initial capacity and load factor
Capacity – number of buckets in the Hash Table
Load factor – How full the Hash Table is allowed to get beforecapacity is automatically increased using rehash function.

More on Java HashMaps
HashMapHashMapHashMapHashMap (int initialCapacity)
putputputput

Object key, Object value)
getgetgetget

Object key) – returns Object
keySetkeySetkeySetkeySet () Returns a setsetsetset view of the keys contained in this map.
valuesvaluesvaluesvalues () Returns a collectioncollectioncollectioncollection view of the values contained in this map.

Sets

Classes that implements set interface^

public class HashSet
Implements Set interface using HashTable
Offers constant time performance
public class TreeSet
Implements Set interface using TreeMap
Sorted set will be ascending order according to thenatural order

Exercise

Write a method
public static Set intersect(Set A, Set B)
That takes two sets A and B and return theintersection of the two sets
Write a method
public static Set union(Set A, Set B)
That takes two sets A and B and return the union ofthe two sets