Collections - Maps

Core Java Training
Collections - Maps

Classification: Restricted
3-February-
2003
cse403-10-Collections © 2003 University of Washington 1
Agenda
• Collections – Maps
• Map Interface
• Map methods
• Mapuse
• Hashmap
• Treemap
• Utilities

3-February-
2003
Map Interface Context
Map

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2003
Map Interface
• Stores key/value pairs
• Maps from the key to the value
• Keys are unique
• a single key only appears once in the Map
• a key can map to only one value
• Values do not have to be unique

3-February-
2003
Map methods

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2003
Map views
• A means of iterating over the keys and values in a Map
• Set keySet()
• returns the Set of keys contained in the Map
• Collection values()
• returns the Collection of values contained in the Map. This Collection
is not a Set, as multiple keys can map to the same value.
• Set entrySet()
• returns the Set of key-value pairs contained in the Map. The Map
interface provides a small nested interface called Map.Entry that is
the type of the elements in this Set.

Map.Entry interface Example
import java.util.*;
class MapInterfaceExample{
public static void main(String args[]){
Map<Integer,String> map=new HashMap<Integer,String>();
map.put(100,"Amit");
map.put(101,"Vijay");
map.put(102,"Rahul");
for(Map.Entry m:map.entrySet()){
System.out.println(m.getKey()+" "+m.getValue());
}
}
}

Exercise..
• Create a Map with 5 values…
• Iterate through and print
• the keys,
• the values,
• both the keys and the values of the Map

3-February-
2003
HashMap and TreeMap Context
HashMap TreeMap
Map

HashMap and TreeMap
• HashMap
• The keys are a set - unique, unordered
• Fast
• TreeMap
• The keys are a set - unique, ordered
• Same options for ordering as a TreeSet
• Natural order (Comparable, compareTo(Object))
• Special order (Comparator, compare(Object, Object))

HashMap
• A HashMap contains values based on the key.
• It contains only unique elements.
• It may have one null key and multiple null values.
• It maintains no order.

HashMap Constructors

HashMap methods

HashMap Example: remove()
import java.util.*;
public class HashMapExample {
public static void main(String args[]) {
// create and populate hash map
HashMap<Integer, String> map = new HashMap<Integer, String>();
map.put(101,"Let us C");
map.put(102, "Operating System");
map.put(103, "Data Communication and Networking");
System.out.println("Values before remove: "+ map);
// Remove value for key 102
map.remove(102);
System.out.println("Values after remove: "+ map);
}
}

Difference between HashSet and HashMap
• Think…

HashMap Example: Book
import java.util.*;
class Book {
int id;
String name,author,publisher;
int quantity;
public Book(int id, String name, String author,
String publisher, int quantity) {
this.id = id;
this.name = name;
this.author = author;
this.publisher = publisher;
this.quantity = quantity;
}
}
public class MapExample {
public static void main(String[] args) {
//Creating map of Books
Map<Integer,Book> map=new HashMap<Integer,Book>();
//Creating Books
Book b1=new Book(101,"Let us C","Yashwant Kanetkar","BPB",8);
Book b2=new Book(102,"Data Communications &
Networking","Forouzan","Mc Graw Hill",4);
Book b3=new Book(103,"Operating System","Galvin","Wiley",6);
//Adding Books to map
map.put(1,b1);
map.put(2,b2);
map.put(3,b3);
//Traversing map
for(Map.Entry<Integer, Book> entry:map.entrySet()){
int key=entry.getKey();
Book b=entry.getValue();
System.out.println(key+" Details:");
System.out.println(b.id+" "+b.name+" "+b.author+" "+b.publisher+
"+b.quantity);
}
}
}

Exercise..
• What is a TreeMap? Implement the book example for TreeMap.
• What is a LinkedHashMap?
• What is a HashTable? How is it different from a HashMap?
• What is Comparable and Comparator interfaces in Java? How are they
different? Write programs utilizing both to test their usage.

3-February-
2003
Bulk Operations
• In addition to the basic operations, a Collection may provide “bulk” operations
boolean containsAll(Collection c);
boolean addAll(Collection c); // Optional
boolean removeAll(Collection c); // Optional
boolean retainAll(Collection c); // Optional
void clear(); // Optional
Object[] toArray();
Object[] toArray(Object a[]);
A = {1,2,3,4} B={2,3,4,5}
addAll() -> A U B = {1,2,3,4,5} UNION
A = STUDENTS LEARNING JAVA; B = STUDENTS LEARNING .NET
A U B = ALL STUDENTS LEARNING EITHER JAVA OR .NET OR BOTH
A INTERSECTION B = ALL STUDENTS LEARNING BOTH JAVA AND .NET
removeAll() -> A-B = {1}, B-A={5}..
A IS ALL EMPLOYEES, B IS EMPLOYEES SERVING NOTICE PERIOD
A-B = EMPLOYEES NOT SERVING NOTICE PERIOD
retainAll() -> A INTERSECTION B = {2,3,4}

HashMap vs TreeMap
• HashMap can contain one null key. TreeMap cannot contain a null key.
• HashMap does not maintain any order, whereas TreeMap maintains
ascending order.

Utilities Context

3-February-
2003
Utilities
• The Collections class provides a number of static methods for fundamental
algorithms
• Most operate on Lists, some on all Collections
• Sort, Search, Shuffle
• Reverse, fill, copy
• Min, max
• Wrappers
• synchronized Collections, Lists, Sets, etc
• unmodifiable Collections, Lists, Sets, etc

3-February-
2003
Properties class
• Located in java.util package
• Special case of Hashtable
• Keys and values are Strings
• Tables can be saved to/loaded from file

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2003
Topics to be covered in next session
• Inner Classes
• Method-local Inner Class
• Anonymous Inner Class
• Static Nested Inner Class

3-February-
2003
Thank you!

Collections - Maps

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