HashMap in Java with Examples

08 Jul, 2022
HashMap<K, V> is a part of Java’s collection since Java 1.2. This class is found in java.util package. It provides the basic implementation of the Map interface of Java. It stores the data in (Key, Value) pairs, and you can access them by an index of another type (e.g. an Integer). One object is used as a key (index) to another object (value). If you try to insert the duplicate key, it will replace the element of the corresponding key.
HashMap is similar to HashTable, but it is unsynchronized. It allows to store the null keys as well, but there should be only one null key object and there can be any number of null values.  This class makes no guarantees as to the order of the map. To use this class and its methods, you need to import java.util.HashMap package or its superclass.
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Java
// Java program to illustrate HashMap class of java.util// package  // Importing HashMap classimport java.util.HashMap;  // Main classpublic class GFG {      // Main driver method    public static void main(String[] args)    {        // Create an empty hash map by declaring object        // of string and integer type        HashMap<String, Integer> map = new HashMap<>();          // Adding elements to the Map        // using standard put() method        map.put("vishal", 10);        map.put("sachin", 30);        map.put("vaibhav", 20);          // Print size and content of the Map        System.out.println("Size of map is:- "                           + map.size());          // Printing elements in object of Map        System.out.println(map);          // Checking if a key is present and if  &nbnbsp;    // present, print value by passing        // random element        if (map.containsKey("vishal")) {              // Mapping            Integer a = map.get("vishal");              // Printing value fr the corresponding key            System.out.println("value for key"                               + " \"vishal\" is:- " + a);        }    }}
Size of map is:- 3
{vaibhav=20, vishal=10, sachin=30}
value for key "vishal" is:- 10
 
The Hierarchy of HashMap is as follows:
 Syntax: Declaration
public class HashMap<K,V> extends AbstractMap<K,V>
                          implements Map<K,V>, Cloneable, Serializable
Parameters: It takes two parameters namely as follows:
The type of keys maintained by this map
The type of mapped values
HashMap implements Serializable, Cloneable, Map<K, V> interfaces. HashMap extends AbstractMap<K, V> class. The direct subclasses are LinkedHashMap, PrinterStateReasons.
HashMap provides 4 constructors and the access modifier of each is public which are listed as follows:
HashMap()HashMap(int initialCapacity)HashMap(int initialCapacity, float loadFactor)HashMap(Map map)
HashMap()
HashMap(int initialCapacity)
HashMap(int initialCapacity, float loadFactor)
HashMap(Map map)
Now discussing above constructors one by one alongside implementing the same with help of clean java programs.
Constructor 1: HashMap()
It is the default constructor which creates an instance of HashMap with an initial capacity of 16 and load factor of 0.75.
Syntax:
HashMap<K, V> hm = new HashMap<K, V>();
Example
Java
// Java program to Demonstrate the HashMap() constructor  // Importing basic required classesimport java.io.*;import java.util.*;  // Main class// To add elements to HashMapclass GFG {      // Main driver method    public static void main(String args[])    {        // No need to mention the        // Generic type twice        HashMap<Integer, String> hm1 = new HashMap<>();          // Initialization of a HashMap using Generics        HashMap<Integer, String> hm2            = new HashMap<Integer, String>();          // Adding elements using put method        // Custom input elements        hm1.put(1, "one");        hm1.put(2, "two");        hm1.put(3, "three");          hm2.put(4, "four");        hm2.put(5, "five");        hm2.put(6, "six");          // Print and display mapping of HashMap 1        System.out.println("Mappings of HashMap hm1 are : "                           + hm1);          // Print and display mapping of HashMap 2        System.out.println("Mapping of HashMap hm2 are : "                           + hm2);    }}
Mappings of HashMap hm1 are : {1=one, 2=two, 3=three}
Mapping of HashMap hm2 are : {4=four, 5=five, 6=six}
Constructor 2: HashMap(int initialCapacity)
It creates a HashMap instance with a specified initial capacity and load factor of 0.75.
Syntax:
HashMap<K, V> hm = new HashMap<K, V>(int initialCapacity);
Example
Java
// Java program to Demonstrate// HashMap(int initialCapacity) Constructor  // Importing basic classesimport java.io.*;import java.util.*;  // Main class// To add elements to HashMapclass AddElementsToHashMap {      // Main driver method    public static void main(String args[])    {        // No need to mention the        // Generic type twice        HashMap<Integer, String> hm1 = new HashMap<>(10);          // Initialization of a HashMap using Generics        HashMap<Integer, String> hm2            = new HashMap<Integer, String>(2);          // Adding elements to object of HashMap        // using put method          // HashMap 1        hm1.put(1, "one");        hm1.put(2, "two");        hm1.put(3, "three");          // HashMap 2        hm2.put(4, "four");        hm2.put(5, "five");        hm2.put(6, "six");          // Printing elements of HashMap 1        System.out.println("Mappings of HashMap hm1 are : "                           + hm1);          // Printing elements of HashMap 2        System.out.println("Mapping of HashMap hm2 are : "                           + hm2);    }}
Mappings of HashMap hm1 are : {1=one, 2=two, 3=three}
Mapping of HashMap hm2 are : {4=four, 5=five, 6=six}
Constructor 3: HashMap(int initialCapacity, float loadFactor)
It creates a HashMap instance with a specified initial capacity and specified load factor.
Syntax:
HashMap<K, V> hm = new HashMap<K, V>(int initialCapacity, int  loadFactor);
Example
Java
// Java program to Demonstrate// HashMap(int initialCapacity,float loadFactor) Constructor  // Importing basic classesimport java.io.*;import java.util.*;  // Main class// To add elements to HashMapclass GFG {      // Main driver method    public static void main(String args[])    {        // No need to mention the generic type twice        HashMap<Integer, String> hm1            = new HashMap<>(5, 0.75f);          // Initialization of a HashMap using Generics        HashMap<Integer, String> hm2            = new HashMap<Integer, String>(3, 0.5f);          // Add Elements using put() method        // Custom input elements        hm1.put(1, "one");        hm1.put(2, "two");        hm1.put(3, "three");          hm2.put(4, "four");        hm2.put(5, "five");        hm2.put(6, "six");          // Print and display elements in object of hashMap 1        System.out.println("Mappings of HashMap hm1 are : "                           + hm1);          // Print and display elements in object of hashMap 2        System.out.println("Mapping of HashMap hm2 are : "                           + hm2);    }}
Mappings of HashMap hm1 are : {1=one, 2=two, 3=three}
Mapping of HashMap hm2 are : {4=four, 5=five, 6=six}
 4. HashMap(Map map): It creates an instance of HashMap with the same mappings as the specified map.
HashMap<K, V> hm = new HashMap<K, V>(Map map);
Java
// Java program to demonstrate the // HashMap(Map map) Constructor  import java.io.*;import java.util.*;  class AddElementsToHashMap {    public static void main(String args[])    {        // No need to mention the        // Generic type twice        Map<Integer, String> hm1 = new HashMap<>();          // Add Elements using put method        hm1.put(1, "one");        hm1.put(2, "two");        hm1.put(3, "three");          // Initialization of a HashMap        // using Generics        HashMap<Integer, String> hm2            = new HashMap<Integer, String>(hm1);          System.out.println("Mappings of HashMap hm1 are : "                           + hm1);                System.out.println("Mapping of HashMap hm2 are : "                           + hm2);    }}
Mappings of HashMap hm1 are : {1=one, 2=two, 3=three}
Mapping of HashMap hm2 are : {1=one, 2=two, 3=three}
 
1. Adding Elements: In order to add an element to the map, we can use the put() method. However, the insertion order is not retained in the Hashmap. Internally, for every element, a separate hash is generated and the elements are indexed based on this hash to make it more efficient.
Java
// Java program to add elements// to the HashMap  import java.io.*;import java.util.*;  class AddElementsToHashMap {    public static void main(String args[])    {        // No need to mention the        // Generic type twice        HashMap<Integer, String> hm1 = new HashMap<>();          // Initialization of a HashMap        // using Generics        HashMap<Integer, String> hm2            = new HashMap<Integer, String>();          // Add Elements using put method        hm1.put(1, "Geeks");        hm1.put(2, "For");        hm1.put(3, "Geeks");          hm2.put(1, "Geeks");        hm2.put(2, "For");        hm2.put(3, "Geeks");          System.out.println("Mappings of HashMap hm1 are : "                           + hm1);        System.out.println("Mapping of HashMap hm2 are : "                           + hm2);    }}
Mappings of HashMap hm1 are : {1=Geeks, 2=For, 3=Geeks}
Mapping of HashMap hm2 are : {1=Geeks, 2=For, 3=Geeks}
2. Changing Elements: After adding the elements if we wish to change the element, it can be done by again adding the element with the put() method. Since the elements in the map are indexed using the keys, the value of the key can be changed by simply inserting the updated value for the key for which we wish to change.
Java
// Java program to change// elements of HashMap  import java.io.*;import java.util.*;class ChangeElementsOfHashMap {    public static void main(String args[])    {          // Initialization of a HashMap        HashMap<Integer, String> hm            = new HashMap<Integer, String>();          // Change Value using put method        hm.put(1, "Geeks");        hm.put(2, "Geeks");        hm.put(3, "Geeks");          System.out.println("Initial Map " + hm);          hm.put(2, "For");          System.out.println("Updated Map " + hm);    }}
Initial Map {1=Geeks, 2=Geeks, 3=Geeks}
Updated Map {1=Geeks, 2=For, 3=Geeks}
3. Removing Element: In order to remove an element from the Map, we can use the remove() method. This method takes the key value and removes the mapping for a key from this map if it is present in the map.
Java
// Java program to remove// elements from HashMap  import java.io.*;import java.util.*;class RemoveElementsOfHashMap{    public static void main(String args[])    {        // Initialization of a HashMap        Map<Integer, String> hm            = new HashMap<Integer, String>();          // Add elements using put method        hm.put(1, "Geeks");        hm.put(2, "For");        hm.put(3, "Geeks");        hm.put(4, "For");          // Initial HashMap        System.out.println("Mappings of HashMap are : "                           + hm);          // remove element with a key        // using remove method        hm.remove(4);          // Final HashMap        System.out.println("Mappings after removal are : "                           + hm);    }}
  
Mappings of HashMap are : {1=Geeks, 2=For, 3=Geeks, 4=For}
Mappings after removal are : {1=Geeks, 2=For, 3=Geeks}
4. Traversal of HashMap
We can use the Iterator interface to traverse over any structure of the Collection Framework. Since Iterators work with one type of data we use Entry< ? , ? > to resolve the two separate types into a compatible format. Then using the next() method we print the entries of HashMap.
Java
// Java program to traversal a// Java.util.HashMap  import java.util.HashMap;import java.util.Map;  public class TraversalTheHashMap {    public static void main(String[] args)    {        // initialize a HashMap        HashMap<String, Integer> map = new HashMap<>();          // Add elements using put method        map.put("vishal", 10);        map.put("sachin", 30);        map.put("vaibhav", 20);          // Iterate the map using        // for-each loop        for (Map.Entry<String, Integer> e : map.entrySet())            System.out.println("Key: " + e.getKey()                               + " Value: " + e.getValue());    }}
  
Key: vaibhav Value: 20
Key: vishal Value: 10
Key: sachin Value: 30
To access a value one must know its key. HashMap is known as HashMap because it uses a technique called Hashing. Hashing is a technique of converting a large String to small String that represents the same String. A shorter value helps in indexing and faster searches. HashSet also uses HashMap internally.Few important features of HashMap are: 
HashMap is a part of java.util package.
HashMap extends an abstract class AbstractMap which also provides an incomplete implementation of Map interface.
It also implements Cloneable and Serializable interface. K and V in the above definition represent Key and Value respectively.
HashMap doesn’t allow duplicate keys but allows duplicate values. That means A single key can’t contain more than 1 value but more than 1 key can contain a single value.
HashMap allows null key also but only once and multiple null values.
This class makes no guarantees as to the order of the map; in particular, it does not guarantee that the order will remain constant over time. It is roughly similar to HashTable but is unsynchronized.
Internally HashMap contains an array of Node and a node is represented as a class that contains 4 fields: 
int hashK keyV valueNode next
int hash
K key
V value
Node next
It can be seen that the node is containing a reference to its own object. So it’s a linked list. 
HashMap:  
Node:  
 
Performance of HashMap depends on 2 parameters which are named as follows:
Initial CapacityLoad Factor
Initial Capacity
Load Factor
1. Initial Capacity – It is the capacity of HashMap at the time of its creation (It is the number of buckets a HashMap can hold when the HashMap is instantiated). In java, it is 2^4=16 initially, meaning it can hold 16 key-value pairs.
2. Load Factor – It is the percent value of the capacity after which the capacity of Hashmap is to be increased (It is the percentage fill of buckets after which Rehashing takes place). In java, it is 0.75f by default, meaning the rehashing takes place after filling 75% of the capacity.
3. Threshold – It is the product of Load Factor and Initial Capacity. In java, by default, it is (16 * 0.75 = 12). That is, Rehashing takes place after inserting 12 key-value pairs into the HashMap.
4. Rehashing – It is the process of doubling the capacity of the HashMap after it reaches its Threshold. In java, HashMap continues to rehash(by default) in the following sequence – 2^4, 2^5, 2^6, 2^7, .... so on.  
If the initial capacity is kept higher then rehashing will never be done. But by keeping it higher increases the time complexity of iteration. So it should be chosen very cleverly to increase performance. The expected number of values should be taken into account to set the initial capacity. The most generally preferred load factor value is 0.75 which provides a good deal between time and space costs. The load factor’s value varies between 0 and 1. 
Note: From Java 8 onward, Java has started using Self Balancing BST instead of a linked list for chaining. The advantage of self-balancing bst is, we get the worst case (when every key maps to the same slot) search time is O(Log n).  
As it is told that HashMap is unsynchronized i.e. multiple threads can access it simultaneously. If multiple threads access this class simultaneously and at least one thread manipulates it structurally then it is necessary to make it synchronized externally. It is done by synchronizing some object which encapsulates the map. If No such object exists then it can be wrapped around Collections.synchronizedMap() to make HashMap synchronized and avoid accidental unsynchronized access. As in the following example:  
Map m = Collections.synchronizedMap(new HashMap(...));
Now the Map m is synchronized.  Iterators of this class are fail-fast if any structure modification is done after the creation of iterator, in any way except through the iterator’s remove method. In a failure of iterator, it will throw ConcurrentModificationException. Time complexity of HashMap: HashMap provides constant time complexity for basic operations, get and put if the hash function is properly written and it disperses the elements properly among the buckets. Iteration over HashMap depends on the capacity of HashMap and a number of key-value pairs. Basically, it is directly proportional to the capacity + size. Capacity is the number of buckets in HashMap. So it is not a good idea to keep a high number of buckets in HashMap initially. Applications of HashMap: HashMap is mainly the implementation of hashing. It is useful when we need efficient implementation of search, insert and delete operations. Please refer to the applications of hashing for details.
K – The type of the keys in the map.
V – The type of values mapped in the map.
METHOD
DESCRIPTION
METHOD
DESCRIPTION
METHOD
DESCRIPTION
forEach(BiConsumer<? super K,
? super V> action)
replaceAll(BiFunction<? super K,
? super V,? extends V> function)
Must Read:
Hashmap vs Treemap
Hashmap vs HashTable
Recent articles on Java HashMap!
This article is contributed by Vishal Garg. If you like GeeksforGeeks and would like to contribute, you can also write an article using write.geeksforgeeks.org or mail your article to review-team@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks. Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
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