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:
Java //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 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:
- HashMap()
- HashMap(int initialCapacity)
- HashMap(int initialCapacity, float loadFactor)
- 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 // 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); } } OutputMappings 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 // 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); } } OutputMappings 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 // 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); } } OutputMappings 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); } } OutputMappings 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 // 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); } } OutputMappings 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 // 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); } } OutputInitial 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 // 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); } } OutputMappings 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 // 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()); } } OutputKey: 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.
Methods | Time Complexity | Space Complexity |
|---|
Adding Elements in HashMap | O(1) | O(N) |
|---|
Removing Element from HashMap | O(1) | O(N) |
|---|
Extracting Element from Java | O(1) | O(N) |
|---|
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
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:
- int hash
- K key
- V value
- 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:
- 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).
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.
Method | Description |
|---|
| clear() | Removes all of the mappings from this map. |
| clone() | Returns a shallow copy of this HashMap instance: the keys and values themselves are not cloned. |
| 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 (or null if there is no current mapping). |
| computeIfAbsent(K key, Function<?super K,? extends V> mappingFunction) | If the specified key is not already associated with a value (or is mapped to null), attempts to compute its value using the given mapping function and enters it into this map unless 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 |
|---|
equals() | Compares the specified object with this map for equality. |
hashCode() | Returns the hash code value for this map. |
toString() | 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. |
forEach(BiConsumer<? super K, ? super V> action) | 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. |
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.
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