At an entry-level, one of the most frequently asked interview questions is about Java HashMap vs Hashtable. So you must be fully prepared to answer anything related to HashMap or Hashtable. Java makes use of HashMap and Hashtable to store data in the form of key and values. So, this article will help you know the major differences between these two.
I’ll be discussing the topics in the following order:
Let’s begin!
What is HashMap?
HashMap is a Map-based collection class in Java which is used to store data in Key and Value pairs. It helps in implementing the Map interface in Java. It is basically a part of Java’s collection since Java version 1.2 and provides the basic implementation of the Map interface in Java. To access a value within the HashMap, one must know its Key.
It is termed as HashMap because it uses a technique called Hashing. Hashing is a process of converting a large String to a smaller one by keeping the value of the String as constant. The resulting compressed value helps in indexing and faster searches.
What is a Hashtable?
A Hashtable is a data structure that is used to store keys/ value pairs. In a Hashtable, data is stored in an array format, where each data value has its own unique index value. You can access data really fast if you know the index of the desired data.
Java Hashtable class implements a hashtable, that maps keys to values. It inherits the Dictionary class and implements the Map interface.
Hashtable Declaration
public class Hashtable<K,V> extends Dictionary<K,V> implements Map<K,V>, Cloneable, Serializable
K: It is the type of keys that by the map.
V: This is the type of mapped values.
Now that you guys have understood how HashMap and Hashtable in Java works, let’s take a look at the parameters to understand the differences between HashMap and Hashtable.
Now let’s point out the major differences between HashMap and Hashtable.
Java HashMap vs Hashtable
| Parameters | HashMap | Hashtable |
Synchronization | Non-synchronized meaning that it is not thread-safe and cannot be shared between many threads without a proper synchronization code. | Synchronized and can be shared with many threads |
Null keys | Allows only one null key and multiple null values | Does not allow null key or its value |
Legacy System | This is a part of Java Collections | Hashtable is a legacy class was not part of the initial Java Collections |
Iterator | Iterator is fail-fast and it throws a concurrentModificationException if any other thread tries to modify the map | The enumerator is not fail-fast |
Inheriting class | Inherits AbstractMap class | Inherits Dictionary class |
Now, when can you use Java HashMap and Hashtable?
When to use HashMap and Hashtable?
- Synchronization is the major difference between Java HashMap and Hashtable. But if there is a need for a thread-safe operation, then Hashtable can be used as all its methods are synchronized. But, this is a legacy class and they have to be avoided. This is not possible by HashMap.
- For a multi-thread environment, you can use ConcurrentHashMap which is almost similar to Hashtable. Here you can even make the HashMap synchronized explicitly
- The synchronized operation results in a poor performance so it should be avoided until and unless it is required. Hence for the non-thread environment, HashMap is definitely used without any doubt.
This brings us to the end of this article where we have learned the differences between Java HashMap and Hashtable. Hope you guys are clear with this topic.
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