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Map Interface in Java

Last Updated : 10 Apr, 2026

In Java, the Map Interface is part of the java.util package and represents a collection of key-value pairs, where Keys should be unique, but values can be duplicated.

  • It provides efficient retrieval, insertion, and deletion operations based on keys.
  • HashMap and LinkedHashMap allow one null key, and TreeMap does NOT allow null keys (if natural ordering is used).
  • Use ConcurrentHashMap for thread-safe operations, or Collections.synchronizedMap() to make an existing map synchronized.

Declaration of the Map interface

public interface Map<K, V>

  • K -> Type of keys maintained by the map
  • V -> Type of mapped values

Creating Map Objects

Since a Map is an interface, we cannot create its object directly. We must use a class that implements it (e.g., HashMap).

Map<String, Integer> hm = new HashMap<>();


Output
Map elements: {Geek3=3, Geek2=2, Geek1=1}

Hierarchy of Map

It is part of the Java Collections Framework, and its key implementation classes include HashMap, LinkedHashMap, TreeMap, and Hashtable.

👁 TreeMap

Note: Map does not extend the Collection interface and is used to efficiently store and retrieve data using keys.

Implemented Classes of Map Interface

  • HashMap: Stores key-value pairs using hashing for fast access, insertion, and deletion.
  • LinkedHashMap: Similar to HashMap but maintains the insertion order of key-value pairs.
  • TreeMap: Stores key-value pairs in sorted order using natural ordering or a custom comparator.
  • Hashtable: A synchronized Map implementation that doesn’t allow null keys or values.

Operations on Map using HashMap

Now, let’s see how to perform a few frequently used operations on a Map using the widely used HashMap class.

1. Adding Elements

Use the put() method to add elements to a Map. In HashMap, insertion order isn’t preserved, each element is stored based on its hash for faster access.


Output
{1=Geeks, 2=For, 3=Geeks}
{1=Geeks, 2=For, 3=Geeks}

2. Changing Element

To update a value, use the put() method with the same key. The new value replaces the old one for that key.


Output
Initial Map: {1=Geeks, 2=Geeks, 3=Geeks}
Updated Map: {1=Geeks, 2=For, 3=Geeks}

3. Removing Elements

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.


Output
{1=Geeks, 2=For, 3=Geeks, 4=For}
{1=Geeks, 2=For, 3=Geeks}

4. Iterating through the Map

There are multiple ways to iterate through the Map. The most famous way is to use a for-each loop and get the keys. The value of the key is found by using the getValue() method. 


Output
1 : Geeks
2 : For
3 : Geeks

Methods in Java Map Interface

MethodsAction Performed 
clear()This method is used in Java Map Interface to clear and remove all of the elements or mappings from a specified Map collection.
containsKey(Object)Checks if a key exists in the map.
containsValue(Object)Checks if a value exists in the map.
entrySet()Returns a set view of the map’s key-value pairs.
equals(Object)Compares two maps for equality.
get(Object)Returns the value for the given key, or null if not found.
hashCode()This method is used in Map Interface to generate a hashCode for the given map containing keys and values.
isEmpty()This method is used to check if a map is having any entry for key and value pairs. If no mapping exists, then this returns true.
keySet()Returns a set view of the keys in the map.
put(Object, Object)This method is used in Java Map Interface to associate the specified value with the specified key in this map.
putAll(Map)This method is used in Map Interface in Java to copy all of the mappings from the specified map to this map.
remove(Object)This method is used in Map Interface to remove the mapping for a key from this map if it is present in the map.
size()This method is used to return the number of key/value pairs available in the map.
values()Returns a collection view of the map’s values.
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.
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.
putIfAbsent(K key, V value)Adds a mapping only if the key is not already mapped.
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