HashMap in Java

In Java, HashMap is part of the Java Collections Framework and is found in the java.util package. It provides the basic implementation of the Map interface in Java. HashMap stores data in (key, value) pairs. Each key is associated with a value, and you can access the value by using the corresponding key.

• Internally uses Hashing (similar to Hashtable in Java).
• Not synchronized (unlike Hashtable in Java) and hence faster for most of the cases.
• 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.
• Duplicate keys are not allowed in HashMap, if you try to insert the duplicate key, it will replace the existing value of the corresponding key. 
• HashMap uses keys in the same way as an Array uses an index.
• HashMap allows for efficient key-based retrieval, insertion, and removal with an average O(1) time complexity.

Example:

//Driver Code Starts
// Java Program to Create 
// HashMap in Java
import java.util.HashMap;

// Driver Class
public class ExampleHashMap {
      // main function
      public static void main(String[] args) {
//Driver Code Ends

      
      // Create a HashMap
      HashMap<String, Integer> hashMap = new HashMap<>();
      
      // Add elements to the HashMap
      hashMap.put("John", 25);
      hashMap.put("Jane", 30);
      hashMap.put("Jim", 35);
      

//Driver Code Starts
      // Access elements in the HashMap
      System.out.println(hashMap.get("John")); 
      // Output: 25
      
      // Remove an element from the HashMap
      hashMap.remove("Jim");
      
      // Check if an element is present in the HashMap
      System.out.println(hashMap.containsKey("Jim")); 
      // Output: false
      
      // Get the size of the HashMap
      System.out.println(hashMap.size()); 
      // Output: 2
   }
}
//Driver Code Ends

25
false
2

HashMap Declaration

public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable

It takes two parameters namely as follows:

• The type of keys maintained by this map
• The type of mapped values

Note: Keys and value can't be primitive datatype. Key in Hashmap is valid if it implements hashCode() and equals() method , it should also be immutable (immutable custom object ) so that hashcode and equality remains constant. Value in hashmap can be any wrapper class, custom objects, arrays, any reference type or even null . For example, Hashmap can have array as value but not as key.

HashMap in Java implements Serializable, Cloneable, Map<K, V> interfaces.Java HashMap extends AbstractMap<K, V> class. The direct subclasses are LinkedHashMap and PrinterStateReasons.

Hierarchy of HashMap in Java

Characteristics of HashMap

A HashMap is a data structure that is used to store and retrieve values based on keys. Some of the key characteristics of a hashmap include:

• Not ordered: HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved. However, LinkedHashMap is a variation of HashMap that preserves the insertion order.
• Thread-unsafe: HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies. If thread safety is required, ConcurrentHashMap can be used.
• Capacity and load factor: HashMaps have a capacity, which is the number of elements that it can hold, and a load factor, which is the measure of how full the hashmap can be before it is resized.

Java HashMap Constructors

HashMap provides 4 constructors and the access modifier of each is public which are listed as follows:

1. HashMap()
2. HashMap(int initialCapacity)
3. HashMap(int initialCapacity, float loadFactor)
4. HashMap(Map map)

Now discuss the above constructors one by one alongside implementing the same with the help of clean Java programs.

1. HashMap()

It is the default constructor which creates an instance of HashMap with an initial capacity of 16 and a load factor of 0.75.

Syntax:

HashMap<K, V> hm = new HashMap<K, V>();

Example:

// Java program to Demonstrate the HashMap() constructor

// Importing basic required classes
import java.io.*;
import java.util.*;

// Main class
// To add elements to HashMap
class 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}

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 program to Demonstrate
// HashMap(int initialCapacity) Constructor

// Importing basic classes
import java.io.*;
import java.util.*;

// Main class
// To add elements to HashMap
class 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}

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, float loadFactor);

Example:

// Java program to Demonstrate
// HashMap(int initialCapacity,float loadFactor) Constructor

// Importing basic classes
import java.io.*;
import java.util.*;

// Main class
// To add elements to HashMap
class 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 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}

Performing Various Operations on HashMap

1. Adding Elements in HashMap in Java

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 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 in HashMap in Java

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 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 from Java HashMap

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 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 Java 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 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

Time and Space Complexity

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 the number of key-value pairs. 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.

DSA Problems On HashMap

1. Count Frequencies in an Array
2. Most Frequent Element
3. Count distinct elements in every window of size K
4. Check if two arrays are equal or not
5. 2 Sum - Count Pairs with target sum
6. Count all pairs with absolute difference equal to K
7. Check If Array Pair Sums Divisible by K
8. Max distance between two occurrences in array
9. Subarray with Given Sum – Handles Negative Numbers
10. Remove minimum elements such that no common elements exist in two arrays
11. 3 Sum - Count all triplets with target sum
12. Longest Subarray with Sum Divisible by K
13. Longest Subarray having Majority Elements Greater than K

Important Features

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 a small String that represents the same String. A shorter value helps in indexing and faster searches. HashSet also uses HashMap internally.
A 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 the Map interface.
• It also implements Cloneable and Serializable interfaces. 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 a 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.

Internal Structure of HashMap

Internally HashMap contains an array of Node and a node is represented as a class that contains 4 fields: 

1. int hash
2. K key
3. V value
4. Node next

It can be seen that the node contains a reference to its object. So it's a linked list. 

HashMap: 

Node: 

Performance of HashMap

The performance of HashMap depends on 2 parameters which are named as follows:

1. Initial Capacity
2. 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). 

Synchronized HashMap

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 the iterator, in any way except through the iterator's remove method. In a failure of an iterator, it will throw ConcurrentModificationException.

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.

Methods in HashMap

• K – The type of the keys in the map.
• V – The type of values mapped in the map.

 Methods inherited from class java.util.AbstractMap

Methods inherited from interface java.util.Map

Advantages of HashMap

• Fast retrieval: HashMaps provide constant time access to elements, which means that retrieval and insertion of elements is very fast.
• Efficient storage: HashMaps use a hashing function to map keys to indices in an array. This allows for quick lookup of values based on keys, and efficient storage of data.
• Flexibility: HashMaps allow for null keys and values, and can store key-value pairs of any data type.
• Easy to use: HashMaps have a simple interface and can be easily implemented in Java.
• Suitable for large data sets: HashMaps can handle large data sets without slowing down.

Disadvantages of HashMap

• Unordered: HashMaps are not ordered, which means that the order in which elements are added to the map is not preserved.
• Not thread-safe: HashMaps are not thread-safe, which means that if multiple threads access the same hashmap simultaneously, it can lead to data inconsistencies.
• Performance can degrade: In some cases, if the hashing function is not properly implemented or if the load factor is too high, the performance of a HashMap can degrade.
• More complex than arrays or lists: HashMaps can be more complex to understand and use than simple arrays or lists, especially for beginners.
• Higher memory usage: Since HashMaps use an underlying array, they can use more memory than other data structures like arrays or lists. This can be a disadvantage if memory usage is a concern.