HashMap Class

HashMap class extends AbstractMap and it implements Map interface. HashMap stores the data in the form of key-value pairs, where key and value are objects. Each key-value pair may also referred to as a map-entry.

Use of HashMap

A HashMap is used to find a value based on its key information and using the key information, we can modify/access its value.
HashMap provides constant-time performance for the basic operations like insertion or retrieval of values.
Using HashMap, we can store null key and null values.

How values are stored in a HashMap?

In HashMap, these pairs(key-value) of objects are stored in a hash table, which stores its elements using hash code.
Hash code of each key is computed internally and this value of hash code is used as an index at which the value is stored in the hash table.
As the hash code of each key is computed internally, hence the order of elements may appear different to the insertion order, hence the order of elements cannot be guaranteed.

A simple constructor of HashMap

HashMap()
This constructor creates an empty HashMap to hold a particular type of key-value pair objects. Example -

HashMap <String,Integer> hm = new HashMap <String,Integer>();

This constructor example creates a HashMap to hold key-value pairs of String and Integer objects.

Some methods in a HashMap.

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.

Creating a HashMap

In this example we will create a HashMap to store object in the pair of key and value, where key is a String object and value is an Integer object. As you know that given a key, we can find its value, hence, we will obtain a value on the basis of a particular key.

import java.util.*;

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

hm.put("Max", 1000);
hm.put("John", 4000);
hm.put("Tom", 2000);
hm.put("Ana", 6000);
hm.put("Rick", 5000);

System.out.println(hm);

System.out.println("Value at the key, Tom is "+ hm.get("Tom"));
System.out.println("Value at the key, Ana is "+ hm.get("Ana"));

}

}

Output is :

{Max=1000, Tom=2000, John=4000, Rick=5000, Ana=6000}
Value at the key Tom is 2000
Value at the key Ana is 6000

Program Analysis

In the code above, we have created a generic HashMap to store objects in the form of String and Integer objects.
Method put() is used to put the values of Key-Value pair by specifying the Key and its Value.
Method get() is used to obtain a Value associated with a particular Key in a HashMap.
On the first call, method get() has returned the Value(2000) associated with the Key "Tom".
On the second call, method get() has returned Value(6000) associated with the Key "Ana".

Iterating over each key-value pair in a HashMap using entrySet() method

We will create a HashMap to store objects in the pair of key and value, where key is a String object and value is an Integer object. We will use entrySet() method to get a Set view of this HashMap. We will iterate over each map entry(key-value pair) using for-each loop.

import java.util.*;

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

hm.put("Max", 1000);
hm.put("John", 4000);
hm.put("Tom", 2000);
hm.put("Ana", 6000);
hm.put("Rick", 5000);

System.out.println("Iterating HashMap using Map.Entry in a for-each loop");
Set<Map.Entry<String,Integer>> set = hm.entrySet();


for(Map.Entry<String,Integer> mapE : set)
{
	System.out.print(mapE.getKey() + " : ");
	System.out.println(mapE.getValue());
}

}
}

Output is :

Iterating HashMap using Map.Entry in a for-each loop
Max : 1000
Tom : 2000
John : 4000
Rick : 5000
Ana : 6000

Program Analysis

We have created a generic HashMap to store objects in the form of String and Integer objects.
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 read each map entry which is in pair of key and value.
Method getKey() of Map.Entry returns the key information of a particular may entry, while the method getValue() returns the value associated with this key.

Please share this article -

Please Subscribe

Please subscribe to our social media channels for daily updates.

Notifications

Please check our latest addition

C#, PYTHON and DJANGO