Java Collections - Map Interface

Java Collection – MAP

•          It is an Interface
•          It stores values in  key-value pair
•          Each key-value is known as ENTRY
•          Map is an Object that maps key to Value
• ·     It does not contain duplicate Key and each key can map to at most one value

For Understanding, We can say that, Map is a combination of set and list

 

Let’s investigate the above statement which is said above.

•          SET only allows unique element as same as MAP’s key allow unique element
•          LIST allows duplicate elements as same as MAP’s values also allow the duplicate element. 

Map interface include methods 

Operations	Method Name
Basic Operations	put, get, remove, containsKey, containsValue, size, and empty
Bulk Operations	putall and clear
collection Operations	KeySet,entryset and values

 

Analysis MAP declaration and its implemented class

I am using java 6 for below example:

1. Map <Integer,String> mySampleMap = new HashMap <Integer,String> ();
2. System.out.println("1");

I am using HashMap implementation for Map interface, which accept Integer value as Key and String value as Map value.

When debug point comes to second line (2.) and I inspect the map reference (mySampleMap).

 

 

Now I am adding a value in map and let see what happen in map reference.

              3.   mySampleMap.put(1, "One");
 

Let understand the Map reference properties

     EntrySet

As we know that, Entry is KEY-VALUE pair. Set of the collection view is called EntrySet. entrySet is the method of MAP interface which returns a collection view of the map.

 

 package com.sample.collections;

import java.util.HashMap;
import java.util.Map;
import java.util.Map.Entry;
import java.util.Set;

public class EntrySetMapExample {

    public static void main(String[] args) {

        Map<Integer,String> hashMap = new HashMap<Integer,String>();
      
        hashMap.put(1, "hello");
        hashMap.put(2, "hello world");
      
        Set<Entry<Integer,String>> setEntry = hashMap.entrySet();
      
        for(Entry<Integer,String> setEntryObj : setEntry) {
            System.out.println("Key ::::---> "+setEntryObj.getKey()+" and  Value ::::---> "+setEntryObj.getValue());
        }
    }
}


Output:

Key ::::---> 1 and Value ::::---> hello

Key ::::---> 2 and Value ::::---> hello world

     KeySet

It returns the set of keys

public Set<K> keySet()

Example:

Set<Integer> setKeys = hashMap.keySet();

                               

                                for(Integer key : setKeys) {

                                                System.out.println("Key "+key);

                                }

Map implementations are grouped in three part as mentioned below. (Each map implementation has their own features, we will cover it in next post /article.)

Group Name	Map Implementation
General-Purpose Map	HashMap , TreeMap and LinkedHashMap
Special-Purpose Map	EnumMap, WeakHashMap and IdentityHashMap
Concurrent Map	ConcurrentHashMap

 

You may also like to read below article related to MAP  interface.

http://www.jigarnagar.com/2015/07/hash-map-initial-capacity-and-load.html