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Map is an interface and though its implementation classes, we can create an object which is used to store data in the form of key-value pairs, where key and value are objects. Each key can be used to extract a value linked with it i.e. if you know the key, you can retrieve the value with it. Each key-value pair may also referred to as a map entry. A Map object is a type of class which has implemented Map interface.

Some important classes that implement Map interface are -:

Classes	Description
AbstractMap	An abstract class but still provides most of the implementation of Map interface, to minimize the effort required by other classes to implement it.
HashMap	This class extends AbstractMap and uses Hash table to store the key-value pairs/map entries. Ordering of its elements may appear out-of-order.
TreeMap	This class extends AbstractMap and it sorts its elements according to the natural ordering of its keys.
LinkedHashMap	This class extends AbstractMap and it uses the insertion order of it's elements to provide a consistent ordering.

Some important points about Map

• Each key must be unique i.e. no duplicate keys are allowed in classes that implement Map interface.
• In some Map classes, null keys and null values are allowed, while in some it's not.
• Unlike collection classes, Map classes don't implement Iterable interface, hence you can't iterate over its entries by simply using for-each loop, nor does it have an iterator() method.
• In order to iterate over each element in a Map, we need to use its method entrySet() which gives us a Set(collection class)-view of a map & thereby we can access each element of a Map in a for-each loop.

A generic Map is declared as

interface Map<K,V>

where K stands for type of keys and V stands for types of values we are going to store in a Map.

Some method of Map interface.

Methods	Description
void clear()	Removes all the key/value pairs from the HashMap.
int size()	Returns the total number of key-value pairs in a HashMap.
boolean isEmpty()	Returns true if HashMap has no key-value pair in it.
V get(Object key)	Returns the value associated with a specified key.
put(K key, V value)	Puts the specified key and its associated value in the HashMap.
boolean containsKey(Object key)	Returns true if HashMap contains a particular key, otherwise false.
Set<Map.Entry<K,V>> entrySet()	Returns the Set containing Map.Entry objects.

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How values are stored in a Map?

We put values in a Map by passing a key-value pair to put() method of Map.

Map.Entry interface

We read at the beginning of this article that method entrySet() of Map interface returns a Set(collection class) view of a Map. This Set contains objecs of type Map.Entry, to call some highly userful methods of Map.Entry interface in order to iterate over each element(key-value pair) of a Map.

A few very important methods of Map.Entry interface -:

Methods	Description
K getKey()	This method returns the key of the current map entry or current key-value pair.
V getValue()	This method returns the value of a current map entry or current key-value pair.
int hashCode()	This method returns the hash code of current key-value pair.

Understanding the working of Map's interface methods -:

• In this code, we will create an object of HashMap which implements Map inteface.
• We will show how to enter entries in this Map & how to iterate over each entry using the entrySet() method, which returns a Set containing objects of type Map.Entry.

import java.util.*;

class HashMapDemo
{
public static void main(String... ar)
{
Map <String,Integer> hm = new HashMap <String,Integer>();

hm.put("Max", 1);
hm.put("John", 4);
hm.put("Tom", 2);
hm.put("Ana", 6);
hm.put("Rick", 5);

System.out.println("Iterating TreeMap using Map.Entry and for-each loop");
Set<Map.Entry<String,Integer>> set = tm.entrySet();

//for-each loop to iterate each element of Map, using Map.Entry objects and methods
for(Map.Entry <String,Integer> me : set)
System.out.print(getKey() +":");
System.out.println(getValue());

}

}

Output is :

Iterating HashMap using Map.Entry in a for-each loop
Max:1
Tom:2
John:4
Ana:6
Rick:5

Program Analysis

• We have created an object of HashMap to store objects in the form of key-value pair, where String objects are keys and Integer objects are values.
• Method put() is used to put the values of Key-Value pair by specifying the Key and its Value.
• Method entrySet() returns a Set of the entries in the Map, in the form of objects of type Map.Entry.
• Inside for-each loop we have called methods of Map.Entry like getKey() and getValue() to iterate over each map entry which is in the form of Key-Value pair stored in the HashMap.

We will see read much more about these concrete subclasses of Map interface in the next few articles. Stay connected.

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