HashMap in Java
Last Updated :
27 Oct, 2025
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A HashMap is a part of Java’s Collection Framework and implements the Map interface. It stores elements in key-value pairs, where, Keys are unique. and Values can be duplicated.
- Internally uses Hashing, hence allows efficient key-based retrieval, insertion, and removal with an average of O(1) time.
- HashMap is not thread-safe, to make it synchronized, use Collections.synchronizedMap().
- Insertion order is not preserved in HashMap. To preserve the insertion order, LinkedHashMap is used and to maintain sorted order, TreeMap is used.
HashMap Declaration;
public class HashMap<K,V> extends AbstractMap<K,V> implements Map<K,V>, Cloneable, Serializable
It takes two parameters, namely as follows:
- K -> Type of keys maintained by this map
- V -> Type of mapped values
import java.util.HashMap;
import java.util.Map;
public class GFG{
public static void main(String[] args){
// 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);
// Iterate through the HashMap
for (Map.Entry<String, Integer> entry : hashMap.entrySet()) {
System.out.println(entry.getKey() + " -> " + entry.getValue());
}
}
}
Output
John -> 25 Jane -> 30 Jim -> 35
Hierarchy of HashMap in Java
It extends AbstractMap and implements the Map Interface.
Capacity of HashMap
The capacity of a HashMap is the number of buckets it can hold for storing entries.
new capacity=old capacity×2
- Default capacity: Default capacity of hashmap is 16.
- Load factor: 0.75 (default): when 75% of the capacity is filled, the capacity is doubled.
Constructors of HashMap
HashMap provides 4 constructors and the access modifier of each is public.
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.
HashMap<K, V> hm = new HashMap<K, V>();
2. HashMap(int initialCapacity)
It creates a HashMap instance with a specified initial capacity and load factor of 0.75.
HashMap<K, V> hm = new HashMap<K, V>(int initialCapacity);
3. HashMap(int initialCapacity, float loadFactor)
It creates a HashMap instance with a specified initial capacity and specified load factor.
HashMap<K, V> hm = new HashMap<K, V>(int initialCapacity, float loadFactor);
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);
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.
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);
}
}
Output
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
We can change a value in a HashMap by using the put() method with the same key, which replaces the old value with the new one.
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);
}
}
Output
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.
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);
}
}
Output
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 an Iterator with Map.Entry<?, ?> to traverse a HashMap and print its entries using the next() method.
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());
}
}
Output
Key: vaibhav Value: 20 Key: vishal Value: 10 Key: sachin Value: 30
Time and Space Complexity
HashMap provides constant time complexity for basic operations.
| Operation | Time Complexity | Space Complexity |
|---|---|---|
| Adding elements in HashMap | O(1) | O(N) |
| Removing elements from HashMap | O(1) | O(N) |
| Extracting elements from HashMap | O(1) | O(N) |
Methods of HashMap
- K – The type of the keys in the map.
- V – The type of values mapped in the map.
Method | Description |
|---|---|
| clear() | Removes all of the mappings from this map. |
| clone() | Returns a shallow copy of this HashMap instance. |
| compute(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) | Attempts to compute a mapping for the specified key and its current mapped value |
| computeIfAbsent(K key, Function<?super K,? extends V> mappingFunction) | Adds computed value if key absent/null. |
| computeIfPresent(K key, BiFunction<? super K,? super V,? extends V> remappingFunction) | If the value for the specified key is present and non-null, attempts to compute a new mapping given the key and its current mapped value. |
| containsKey(Object key) | Returns true if this map contains a mapping for the specified key. |
| containsValue(Object value) | Returns true if this map maps one or more keys to the specified value. |
| entrySet() | Returns a Set view of the mappings contained in this map. |
| get(Object key) | Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. |
| isEmpty() | Returns true if this map contains no key-value mappings. |
| keySet() | Returns a Set view of the keys contained in this map. |
| merge(K key, V value, BiFunction<? super V,? super V,? extends V> remappingFunction) | If the specified key is not already associated with a value or is associated with null, associate it with the given non-null value. |
| put(K key, V value) | Associates the specified value with the specified key in this map. |
| putAll(Map<? extends K,? extends V> m) | Copies all of the mappings from the specified map to this map. |
| remove(Object key) | Removes the mapping for the specified key from this map if present. |
| size() | Returns the number of key-value mappings in this map. |
| values() | Returns a Collection view of the values contained in this map. |
Methods inherited from class java.util.AbstractMap
Method | Description |
|---|---|
| Compares the specified object with this map for equality. | |
| Returns the hash code value for this map. | |
| Returns a string representation of this map. |
Methods inherited from interface java.util.Map
Method | Description |
|---|---|
| equals() | Compares the specified object with this map for equality. |
| Performs the given action for each entry in this map until all entries have been processed or the action throws an exception. | |
| getOrDefault(Object key, V defaultValue) | Returns the value to which the specified key is mapped, or defaultValue if this map contains no mapping for the key. |
| hashCode() | Returns the hash code value for this map. |
| putIfAbsent(K key, V value) | If the specified key is not already associated with a value (or is mapped to null) associates it with the given value and returns null, else returns the current value. |
| remove(Object key, Object value) | Removes the entry for the specified key only if it is currently mapped to the specified value. |
| replace(K key, V value) | Replaces the entry for the specified key only if it is currently mapped to some value. |
| replace(K key, V oldValue, V newValue) | Replaces the entry for the specified key only if currently mapped to the specified value. |
replaceAll(BiFunction<? super K,? super V,? extends V> function) | Replaces each entry's value with the result of invoking the given function on that entry until all entries have been processed or the function throws an exception. |
Recommended DSA Problems On HashMap
- Count Frequencies in an Array
- Most Frequent Element
- Count distinct elements in every window of size K
- Check if two arrays are equal or not
- 2 Sum - Count Pairs with target sum
- Count all pairs with absolute difference equal to K
- Check If Array Pair Sums Divisible by K
- Max distance between two occurrences in array
- Subarray with Given Sum – Handles Negative Numbers
- Remove minimum elements such that no common elements exist in two arrays
- 3 Sum - Count all triplets with target sum
- Longest Subarray with Sum Divisible by K
- Longest Subarray having Majority Elements Greater than K
HashMap with Java
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