Often Java HashMap is mistaken for HashTable by developers and server designers and vice-versa, the two terms are distinct and have an extensively broad meaning. Although the origin of HashMap is the same as that of HashTable, yet there is a wide chain of differences between the two.
First, let’s understand the meaning of the two terms and their implications individually, then we shall discuss their difference on various bases to get more clarity on Java HashMap vs HashTable.
What is HashMap?
HashMap is a Map-based collection class in Java that is used for storing data in key-value pairs. It assists us in implementing the Map interface in Java. It is an integral component of Java’s collection ever since Java version 1.2 was introduced. It comes with the simplest implementation of the Map interface in Java. For accessing any value stored inside the HashMap, you must be familiar with its Key.
It is called so, as it uses a technique known as Hashing. Hashing is a process of converting one large string to a smaller one by keeping its value constant. The resulting compressed value is used for indexing and running quick queries.
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What is HashTable?
Hashtable is a widely used data structure that can be used for storing data as (key, value) pairs. A hashtable stores data in an array format with every data value having its own index value which is unique. It allows very fast access to data if the index of the desired data is known.
In Java, a hashtable is implemented by the Hashtable class which maps the keys to the values. It inherits the Dictionary class and implements the Map interface.
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Declaration of a Hash Table:
public class Hashtable extends Dictionary implements Map, Cloneable, Serializable
K: It is the type of keys that are on the map.
V: This is the type of mapped values.
Similarities Between HashMap Vs Hashtable
1. Insertion Order: HashMap and Hashtable do not assure that the order of the map will remain constant at all instances. You can go for LinkedHashMap because the order remains constant over time.
2. Map interface: HashMap and Hashtable both implement Map interface .
3. Put and get method: HashMap and Hashtable both come with a constant time performance for put and get methods assuming that the objects are distributed uniformly across the bucket.
4. Internal working: HashMap and Hashtable are based on the Principle of Hashing.
HashMap Vs HashTable: A head-to-head comparison
Being a Java developer, you must distinctly understand the difference between the two widely used data structures: “HashMap” and “HashTable”. After reading the above-mentioned introduction, you must now go through the head-to-head comparison between the two through the difference table given below.
HashMap Vs HashTable : Difference Table
|Basis Of Difference||HashMap||HashTable|
|Synchronisation||Non-synchronised, this implies it isn’t thread-safe and cannot be shared across multiple threads without a well-written synchronisation code.||Synchronised and can be distributed across multiple threads|
|Null Keys||It is limited to one null key, but allows multiple null values||It doesn’t preserve any null key or its value|
|Legacy System||It comes under Java Collections.||Hashtable is a legacy class It was not included in the Java Collections initially.|
|Iterator||Iterator is fail-fast. It even throws a concurrent ModificationException in case a thread tries to alter the map.||The iterator isn’t fail-fast.|
|Inheriting class||It inherits the AbstractMap class.||It inherits Dictionary class.|
|Performance||HashMap is much faster and consumes less memory.||Better in performance in comparison to synchronised objects.|
Applications of HashMap Vs HashTable
1. Single-Threaded Application: HashMap is a better pick than Hashtable for non-threaded applications. In basic terms, use HashMap in unsynchronised or single threaded applications.
2. Multi-Threaded Application: Do not use Hashtable, because the class is now deprecated in the latest JDK 1.8. Oracle comes with a better replacement of Hashtable named ConcurrentHashMap. For multithreaded applications, you can use ConcurrentHashMap instead of Hashtable.
Frequently Asked Questions
Hashtable is a synchronised data structure, while the HashMap isn’t. This indicates that the HashMap is preferred for non-threaded applications because unsynchronised Objects usually perform better than synchronised ones. On top of this, the Hashtable does not allow null keys or values.
HashMap is faster than HashTable. This is because the Hashtable implicitly checks for synchronization on every method invocation even in a single thread environment. HashMap allows us to store null values, whereas the Hashtable doesn’t. HashMap can also be iterated by an Iterator which is considered as fail-fast, this adds even more to its swiftness.
A HashMap has no order at any given instance. It doesn’t aim at that specific purpose. The order might even alter when not rehashing. The aim of hashing strategy is to place objects in a pseudo random manner. That’s why it is unsynchronised.
While the HashMap will be slower at first and take more memory, it will be faster for large values of n. The reason the ArrayList has O(n) performance is that every item must be checked for every insertion to make sure it is not already in the list. We will do n insertions, so it is O(n^2) for the whole operation.
No, the HashMap is not-thread safe. It cannot be shared among multiple threads without a proper synchronisation code.
Finally, after understanding both these terms we can conclude that both HashMap and HashTable are efficient problem-solving Java data structures. A HashMap inherits the AbstractMap class whereas a HashTable Inherits Dictionary class.
If you are thinking of building a career in Java or Spring Boot you can learn about a few concepts including Hibernation, Class-object, JSON, this will help you in dealing with server designing better and devising the algorithms efficiently.
Before getting enrolled in any course understand the technical terms distinctly, so that you get to learn exactly what you have been looking for. You can check out our courses on Data Structures and Algorithms in Java if you wish to build a few projects on your own under the guidance of our Mentors.
By Vanshika Singolia