What is the difference between HashMap and Hashtable in Java?

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In Java, both the Hashtable and HashMap are data structures based on hashing and implementation of the Map interface the primary distinction is that HashMap is not thread-safe, whereas Hashtable is. As a result, HashMap cannot be used in a multi-threaded Java application without external synchronization. Another distinction is that HashMap supports one null key and null values, whereas Hashtable does not. Furthermore, the hash table achieves thread safety by internal synchronization, which makes it slower than HashMap.

By the way, the distinction between HashMap and Hashtable in Java is frequently addressed in core Java interviews to determine whether the candidate understands the correct usage of collection classes and is aware of alternate alternatives.

This is one of the oldest questions from the Java Collection framework, along with How HashMap Internally Works and ArrayList vs Vector. Hashtable is a legacy Collection class that has been in the Java API for a long time. Still, in Java 4 it was refactored to implement the Map interface, and Hashtable became part of the Java Collection framework.

The subject of Hashtable vs HashMap in Java is so common that it can be found at the top of any list of Java Collection Interview Questions. Before attending any Java programming interview, you cannot afford to study HashMap versus Hashtable.

Difference between HashMap and Hashtable in Java

In Java, both HashMap and Hashtable are used to store and retrieve key-value pairs. However, there are several differences between them:

  1. Synchronization: The most significant difference is that Hashtable is synchronized, which means it is thread-safe and can be accessed by multiple threads concurrently without causing data corruption. In contrast, HashMap is not synchronized and is not thread-safe by default. If you need thread safety with HashMap, you can use the Collections.synchronizedMap() method to create a synchronized version.
  2. Null values and keys: Hashtable does not allow null keys or values. If you try to store a null key or value, it will throw a NullPointerException. On the other hand, HashMap allows a single null key and multiple null values.
  3. Performance: HashMap is generally faster than Hashtable because it does not have the overhead of synchronization. If you do not require thread safety, HashMap is usually the preferred choice for better performance.
  4. Iterating over elements: Hashtable‘s enumerator returns the elements in random order while HashMap providing no guarantee regarding the order of elements. If you need a specific iteration order, you can use LinkedHashMap, which maintains the insertion order.
  5. Inheritance: Hashtable is a legacy class and part of the original Java Collections Framework. It extends the obsolete Dictionary class. On the other hand, HashMap is a newer class introduced in Java 1.2 as part of the Collections Framework and implements the Map interface.

Some important terms related to HashMap and Hashtable

Certainly! Here are some important terms related to HashMap and Hashtable in Java:

  1. Key-Value Pair: Both HashMap and Hashtable store data in the form of key-value pairs. The key is used to retrieve the corresponding value.
  2. Hashing: Hashing is the process of converting an object into an integer value (hash code) using a hash function. HashMap and Hashtable use hashing internally to determine the index or bucket where the key-value pairs are stored.
  3. Hash Code: A hash code is an integer value generated by the hash function for a given object. It is used to calculate the index of the bucket where the key-value pair will be stored in the underlying data structure.
  4. Bucket: A bucket is a container within the underlying data structure (such as an array or linked list) where the key-value pairs are stored. The index of the bucket is determined by the hash code of the key.
  5. Collision: Collision occurs when two different keys produce the same hash code, resulting in them being assigned to the same bucket. HashMap and Hashtable handle collisions differently.
  6. Chaining: Chaining is a collision resolution technique where each bucket contains a linked list of key-value pairs. If a collision occurs, the new key-value pair is added to the linked list of the corresponding bucket.
  7. Load Factor: The load factor is a measure of how full the underlying data structure is allowed to get before its capacity is automatically increased. Both HashMap and Hashtable have a default load factor of 0.75.
  8. Capacity: The capacity of a hash-based collection is the number of buckets it can hold. It is automatically increased as needed when the load factor is reached. The initial capacity can be specified during the creation of HashMap or Hashtable.
  9. Iterator: Both HashMap and Hashtable provide an iterator that allows you to iterate over the key-value pairs stored in the collection. The iterator provides methods like next(), hasNext(), and remove().
  10. Concurrent Modification Exception: If you modify the underlying collection (e.g., add or remove elements) while iterating over it using an iterator, a ConcurrentModificationException may be thrown.

Understanding these terms will help you work effectively with HashMap and Hashtable in Java.

Conclusion

In summary, if you need thread safety, require strict non-null values and keys, or are working with legacy code, Hashtable can be used. However, if you don’t need thread safety, want better performance, and prefer more flexibility, HashMap is the recommended choice.

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