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Mastering HashMap in Java: A Comprehensive Guide to its Methods

Introduction to HashMap

In Java, HashMap is a part of the Collections Framework, which allows storing items in key-value pairs. Keys are unique in the map. It provides efficient retrieval, insertion, and deletion operations. Before diving into specific methods, it’s essential to understand that HashMap relies on hashing, where it uses the key’s hash code to find where the value is stored.

Basic Operations

1. put(K key, V value)

This method is used to insert a new key-value pair into the map. If the map previously contained a mapping for the key, the old value is replaced.

2. get(Object key)

Retrieve the value associated with the given key. Returns null if the map contains no mapping for the key.

3. remove(Object key)

Removes the mapping for a key from this map if it is present.

4. containsKey(Object key)

Checks if the map contains a mapping for the specified key.

5. containsValue(Object value)

Verifies if the map maps one or more keys to the specified value.

6. size()

Returns the number of key-value mappings in this map.

7. isEmpty()

Checks if the map contains no key-value mappings.

Complex Operations

8. putAll(Map m)

Copies all of the mappings from the specified map to this map.

9. clear()

Removes all of the mappings from this map.

10. merge(K key, V value, BiFunction remappingFunction)

This method is a bit more advanced. It is used to combine the value already associated with the given key with the new value using the given remapping function. If the key is not already present or the associated value is null, the new value is simply put into the map.

Understanding the merge Method

The merge method signature looks like this:

V merge(K key, V value, BiFunction<? super V, ? super V, V> remappingFunction)


11. keySet()

Returns a Set view of the keys contained in this map.

12. values()

Provides a Collection view of the values contained in this map.

13. entrySet()

Returns a Set view of the mappings contained in this map. This is particularly useful for iterating over the map’s contents.


The keySet(), values(), and entrySet() methods provide views of the map, which are very powerful for iterating over a HashMap, allowing the developer to access keys, values, or key-value pairs respectively.


HashMap in Java provides a wide array of methods catering to different needs, from basic operations like inserting and retrieving data to more complex manipulations like merging values based on keys. Understanding these methods can greatly enhance your ability to work efficiently with data structures in Java. Remember, while HashMap is incredibly useful, it’s also important to be aware of its limitations, such as being unsynchronized and allowing at most one null key (but multiple null values).

For specific use cases, consider other implementations like Hashtable, LinkedHashMap, or TreeMap depending on your requirements regarding order, concurrency, or sorting.

This blog post provides a foundational overview of HashMap methods, and there’s always more to explore with practical examples and specific use cases. Happy coding!



About Author

As Neelabh Singh, I am a Senior Software Engineer with 6.6 years of experience, specializing in Java technologies, Microservices, AWS, Algorithms, and Data Structures. I am also a technology blogger and an active participant in several online coding communities.

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