Lecture 11

Lecture-11
Instructor Name:
Object Oriented Programming

Today’s Lecture
 The Concept of Map
 HashMap
 HashSet
2

MAP
The Concept of Map
 A map is a collection of key/value pairs of objects.
 As with the ArrayList, a map can store a flexible number of entries.
 One difference between the ArrayList and a Map is that with a Map
 each entry is not an object, but a pair of objects.
 This pair consists of a key object and a value object.
 Instead of looking up entries in this collection using an integer index
(as we did with the ArrayList), we use the key object to look up the
value object.
3

MAP
Everyday Example of Map
 An everyday example of a map is a telephone directory.
 A telephone directory contains entries, and each entry is a pair: a name and a
phone number.
 You use a phone book by looking up a name and getting a phone number.
 We do not use an index—the position of the entry in the phone book—to find
it.
 A map can be organized in such a way that looking up a value for a key is
easy. In the case of a phone book, this is done using alphabetical sorting.
 By storing the entries in the alphabetical order of their keys, finding the key
and looking up the value is easy
4

HashMap
Using HashMap
 HashMap is a particular implementation of Map.
 The HashMap class uses a hashtable to implement the Map interface.
 The most important methods of the HashMap class are put and get.
 The put method inserts an entry into the map
 The get method retrieves the value for a given key.
 Important thing is that NULL is not allowed in the map.
 Object must be compatible with the elements in the map.
5

HashMap
Using HashMap
 The following code fragment creates a HashMap and inserts three entries into
it. Each entry is a key/value pair consisting of a name and a telephone
number.
HashMap<String, String> phoneBook = new HashMap<String, String>();
phoneBook.put(“Muhammad Safyan", “(300) 9999 999");
phoneBook.put(“Atif Ishaq", "(333) 8888 888");
phoneBook.put(“Muhammad Asif", "(321) 7777 777");
 The following code will find the phone number for Atif Ishaq and print it.
String number = phoneBook.get(“Atif Ishaq");
System.out.println(number); 6

HashMap
Address Book Example
import java.util.HashMap;
public class PhoneBook
{
HashMap<String,String> phonebook=new HashMap<String,String>();
public void enterNumber(String name, String number){
phonebook.put(name,number);
}
public String lookUpNumber(String name){
String number = phonebook.get(name);
return number;
}
}
7

HashMap
HashMapTester Class
import java.util.Scanner;
public class HashMapTester
{
public static void main(String[] args){
PhoneBook phonebook=new PhoneBook();
Scanner sc=new Scanner(System.in);
String opt="Y";
do{
System.out.print("Enter Name : ");
String name=sc.next();
System.out.print("Enter Number : ");
String number=sc.next();
phonebook.enterNumber(name,number);
System.out.print ("Do you want to add more Numbers : ");
opt=sc.next();
}while(opt.equals("Y"));
}
} 8

HashMap
What Happens if you add an entry to map
with a key that already exists in the map?
 Duplicate Key values are not allowed in the HashMap.
 If a Key already exists, it will replace its value with new value.
 It will replace the phone number 7777 777 with 6666 777 against the key
Muhammad Asif. No New will be created in the HashMap.
9

HashMap
How a value can be removed from HashMap?
public void removeEntry(String name){
if(phonebook.containsKey(name))
phonebook.remove(name);
else
System.out.println("Invalid Entry");
}
10

HashMap
How you can check the total number of
entries in the HashMap?
public int totalNumber(){
return phonebook.size();
}
11

Lecture 11

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Lecture 11