Dinesh on Java

HashMap class in collection framework

HashMap class extends AbstractMap and implements Map interface.
It uses a hashtable to store the map. This allows the execution time of get() and put() to remain same.
HashMap has four constructor.

HashMap()
HashMap(Map< ? extends k, ? extends V> m)
HashMap(int capacity)
HashMap(int capacity, float fillratio)

HashMap does not maintain order of its element.

Hierarchy of HashMap class:

SN	Methods with Description
1	void clear() Removes all mappings from this map.
2	Object clone() Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned.
3	boolean containsKey(Object key) Returns true if this map contains a mapping for the specified key.
4	boolean containsValue(Object value) Returns true if this map maps one or more keys to the specified value.
5	Set entrySet() Returns a collection view of the mappings contained in this map.
6	Object get(Object key) Returns the value to which the specified key is mapped in this identity hash map, or null if the map contains no mapping for this key.
7	boolean isEmpty() Returns true if this map contains no key-value mappings.
8	Set keySet() Returns a set view of the keys contained in this map.
9	Object put(Object key, Object value) Associates the specified value with the specified key in this map.
10	putAll(Map m) Copies all of the mappings from the specified map to this map These mappings will replace any mappings that this map had for any of the keys currently in the specified map.
11	Object remove(Object key) Removes the mapping for this key from this map if present.
12	int size() Returns the number of key-value mappings in this map.
13	Collection values() Returns a collection view of the values contained in this map.

Example of HashMap class:

import java.util.*;
class Simple{
 public static void main(String args[]){
 
  HashMap hm=new HashMap();

  hm.put(100,"Dinesh");
  hm.put(101,"Sweety");
  hm.put(102,"Nimmo");

  Set set=hm.entrySet();
  Iterator itr=set.iterator();

  while(itr.hasNext()){
   Map.Entry m=(Map.Entry)itr.next();
   System.out.println(m.getKey()+" "+m.getValue());
  }
 }
}

Output:
102 Dinesh
100 Sweety
101 Nimmo

Java Collections Tutorial

ArrayList class
LinkedList class
ListIterator interface
HashSet class
LinkedHashSet class
TreeSet class
Difference between TreeSet & HashSet
Map interface
HashMap class
How does work HashMap?
Difference between TreeMap vs HashMap
LinkedHashMap class
TreeMap class
Hashtable class
Difference between HashMap and HashTable in Java
Sorting
Comparable interface

Java Collections Tutorial

ArrayList class
LinkedList class
ListIterator interface
HashSet class
LinkedHashSet class
TreeSet class
Difference between TreeSet & HashSet
Map interface
How does work HashMap?
Difference between TreeMap vs HashMap
LinkedHashMap class
TreeMap class
Hashtable class
Difference between HashMap and HashTable in Java
Sorting
Comparable interface

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Dinesh Rajput

Dinesh Rajput is the chief editor of a website Dineshonjava, a technical blog dedicated to the Spring and Java technologies. It has a series of articles related to Java technologies. Dinesh has been a Spring enthusiast since 2008 and is a Pivotal Certified Spring Professional, an author of a book Spring 5 Design Pattern, and a blogger. He has more than 10 years of experience with different aspects of Spring and Java design and development. His core expertise lies in the latest version of Spring Framework, Spring Boot, Spring Security, creating REST APIs, Microservice Architecture, Reactive Pattern, Spring AOP, Design Patterns, Struts, Hibernate, Web Services, Spring Batch, Cassandra, MongoDB, and Web Application Design and Architecture. He is currently working as a technology manager at a leading product and web development company. He worked as a developer and tech lead at the Bennett, Coleman & Co. Ltd and was the first developer in his previous company, Paytm. Dinesh is passionate about the latest Java technologies and loves to write technical blogs related to it. He is a very active member of the Java and Spring community on different forums. When it comes to the Spring Framework and Java, Dinesh tops the list!