Table of Contents
Introduction
Maps are associative collections in Java. Every element in a map is treated as a value and the position of the value is denoted by a key. Therefore, each value key pair is treated as an entry in the map. Every key is unique in a map and it dictates the position of the element stored in it. All traversal techniques, searching, sorting, or updating elements in a map use the keys as references. The two map interfaces in Java are Map and Sorted Map. These interfaces combined have three classes namely HashMap, LinkedHashMap, and TreeMap.
Map Classes
| Class | Description |
| HashMap | In a HashMap structure, the map elements are not maintained in order. To traverse this type of a map, you will have to convert it into either a keySet() or entrySet(). |
| LinkedHashMap | A Linked HashMap maintains insertion order and is another implementation of the Map interface. |
| TreeMap | The TreeMap is a combination of both the Map as well as the SortedMap. In this map, the values are sorted in ascending order and the keys are thereafter ordered. |
Some Map Interface Methods
| Method | Description |
| put(Object key, Object value) | This method is used to insert new elements into the map |
| void putAll(Map map) | This method is used to insert a map (all elements) in another map. |
| putIfAbsent(K key, V value) | The value specified along with the key is inserted into the map. |
| remove(Object Key) | Used to remove an element from the specified key. |
Example
import java.util.*;
class DemoMap
{
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" );
//Elements can traverse in any order.
for (Map.Entry m:map.entrySet())
{
System.out.println( m.getKey()+" "+m.getValue() );
}
}
}
OUTPUT
102 Rahul
100 Amit
101 Vijay
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