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What are the differences between a HashMap and a Hashtable in Java?

Gautam Ramesh8121-Jul-2023

What are the differences between a HashMap and a Hashtable in Java?

javajava hashtable 
Updated on 17-Nov-2023
Gautam Ramesh

Gautam Ramesh

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I am Gautam Ramesh from Uttar Pradesh. Have completed my B. Tech. course from MNNIT campus Prayagraj in 2022. I have good knowledge of computer networking.

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Aryan Kumar

Aryan Kumar

17-Nov-2023


In Java, both HashMap and Hashtable are implementations of the Map interface, and they provide key-value pair storage and retrieval. Despite their similar functionalities, there are several differences between HashMap and Hashtable:

Synchronization:

  • Hashtable is synchronized: All methods in Hashtable are synchronized, making it thread-safe. This means that multiple threads can safely access a Hashtable concurrently without the need for external synchronization. While this ensures thread safety, it can also introduce performance overhead.
  • HashMap is not synchronized by default:HashMap is not synchronized by default. If multiple threads access a HashMap concurrently and at least one of the threads modifies it structurally (i.e., adding or removing elements), it must be synchronized externally. However, you can make a HashMap synchronized using Collections.synchronizedMap().

Null Values:

  • Hashtable does not allow null keys or values: Neither keys nor values in a Hashtable can be null. Attempting to store or retrieve null values or keys results in a NullPointerException.
  • HashMap allows one null key and multiple null values: In contrast, HashMap allows one null key and multiple null values. This flexibility can be useful in certain scenarios.

Performance:

  • HashMap generally performs better: Due to its unsynchronized nature, HashMap generally performs better than Hashtable in a non-multithreaded environment. If thread safety is not a requirement, using HashMap is often preferred for its better performance.

Iterator Fail-Fast vs Fail-Safe:

  • Hashtable uses Enumeration and is fail-fast:Hashtable uses the Enumeration interface, and if the underlying collection is modified while an Enumeration is in progress, it throws a ConcurrentModificationException.
  • HashMap uses Iterator and is fail-fast (if not synchronized):HashMap uses the Iterator interface, and if the underlying collection is modified while an Iterator is in progress (and the HashMap is not synchronized), it throws a ConcurrentModificationException.

Inheritance:

  • Hashtable is a legacy class:Hashtable is a legacy class, and its use is discouraged in favor of HashMap, which is part of the Java Collections Framework introduced later. However, Hashtable is still supported for backward compatibility.
  • HashMap is part of the Java Collections Framework:HashMap is part of the Java Collections Framework and provides additional methods introduced with Java 2.

In summary, the key differences between HashMap and Hashtable in Java lie in their synchronization, handling of null values, performance characteristics, and the fact that Hashtable is a legacy class. If thread safety is not a concern and you are working in a non-multithreaded environment, HashMap is generally the preferred choice due to its better performance.

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