Collection Framework in JAVA 5

Collection Framework in JAVA 5

Map Interface

A Map stores data in key and value association. Both key and values are objects. The key must be unique but the values can be duplicate. Although Maps are a part of Collection Framework, they can not actually be called as collections because of some properties that they posses. However we can obtain a collection-view of maps.

Map is useful if you have to search, update or delete elements on the basis of key.

Interface	Description
Map	Maps unique key to value
Map.Entry	Describe an element in key and value pair in a map. This is an inner class of map
NavigableMap	Extends SortedMap to handle the retrienal of entries based on closest match searches
SortedMap	Extends Map so that key are maintained in an ascending order

The Map Interface is divided into two parts:

1) SortedMap Interface

• Extends Map Interface

• Ensures that the entries are in ascending order based on the keys.

2) NavigableMap Interface

• Extends SortedMap Inteface

• Declares behaviour that supports retrieval of entries based on closest match.

Commonly used Methods defined by Map

Methods	Description
boolean containsKey(Object k)	returns true if map contain k as key, otherwise false
Object get(Object k)	returns values associated with the key k
Object put(Object k, Object v)	stores an entry in map
Object putAll(Map m)	put all entries from m in this map
Set keySet()	returns Set that contains the key in a map
Set entrySet()	returns Set that contains the entries in a map

HashMap class

1. HashMap class extends AbstractMap and implements Map interface.

2. It uses a hashtable to store the map. This allows the execution time of get() and put() to remain same.

3. HashMap has four constructor.

HashMap()

HashMap(Map< ? extends k, ? extends V> m)

HashMap(int capacity)

HashMap(int capacity, float fillratio)

4. HashMap does not maintain order of its element.

Example of HashMap Class

import java.util.*;
class HashMapDemo
{
 public static void main(String args[])
 {
  HashMap< String,Integer> hm = new HashMap< String,Integer>();
  hm.put("a",new Integer(100));
  hm.put("b",new Integer(200));
  hm.put("c",new Integer(300));
  hm.put("d",new Integer(400));

  Set< Map.Entry< String,Integer> > st = hm.entrySet(); //returns Set view
  for(Map.Entry< String,Integer> me:st)
  {
   System.out.print(me.getKey()+":");
   System.out.println(me.getValue());
  }
 }
}
Output :
c:300
a:100
d:400
b:200

 

LinkedHashMap class

1. LinkedHashMap extends HashMap class.

2. It maintains a linked list of entries in map in order in which they are inserted.

3. LinkedHashMap defines the following constructor.

LinkedHashMap()

LinkedHashMap(Map< ? extends k, ? extends V> m)

LinkedHashMap(int capacity)

LinkedHashMap(int capacity, float fillratio)

LinkedHashMap(int capacity, float fillratio, boolean order)

4. It adds one new method removeEldestEntry(). This method is called by put() and putAll() By default this method does nothing. However we can override this method to remove oldest element in the map. Syntax

protected boolean removeEldestEntry(Map.Entry e)

 

TreeMap class

1. TreeMap class extends AbstractMap and implements NavigableMap interface.

2. It creates Map, stored in a tree structure.

3. A TreeMap provides an efficient means of storing key/value pair in efficient order.

4. It provides key/value pairs in sorted order and allows rapid retrieval.

Example of TreeMap Class

import java.util.*;
class TreeMapDemo
{
 public static void main(String args[])
 {
  TreeMap< String,Integer> tm = new TreeMap< String,Integer>();
  tm.put("a",new Integer(100));
  tm.put("b",new Integer(200));
  tm.put("c",new Integer(300));
  tm.put("d",new Integer(400));

  Set< Map.Entry< String,Integer> > st = tm.entrySet();
  for(Map.Entry me:st) 
  {
   System.out.print(me.getKey()+":");
   System.out.println(me.getValue());
  }
 }
}
Output :
a 100
b 200
c 300
d 400

 

EnumMap class

1. EnumMap extends AbstractMap and implements Map interface.

2. It is used for key as enum