What is a Hashtable in Java?

A Hashtable is an array of a list. Each list is known as a bucket. The position of the bucket is identified by calling the hashcode() method. A Hashtable contains values based on the key. Java Hashtable class contains unique elements.

What is Hashtable and how it works?

A hash table is a data structure that is used to store keys/value pairs. It uses a hash function to compute an index into an array in which an element will be inserted or searched. By using a good hash function, hashing can work well.

What is a Hashtable used for?

A hash table uses a hash function to compute an index, also called a hash code, into an array of buckets or slots, from which the desired value can be found. During lookup, the key is hashed and the resulting hash indicates where the corresponding value is stored.


What is Hashtable and HashMap in Java?

Hashmap vs Hashtable

It is not thread-safe and can't be shared between many threads without proper synchronization code whereas Hashtable is synchronized. It is thread-safe and can be shared with many threads. HashMap allows one null key and multiple null values whereas Hashtable doesn't allow any null key or value.

What is meant by Hashtable?

A lookup table that is designed to efficiently store non-contiguous keys (account numbers, part numbers, etc.) that may have wide gaps in their alphabetic or numeric sequences. Hash tables are created with an algorithm that stores the keys into hash buckets, which contain key-value pairs.


14.11 HashMap and HashTable in Java



What are the advantages of hash tables?

The main advantage of hash tables over other data structures is speed . The access time of an element is on average O(1), therefore lookup could be performed very fast. Hash tables are particularly efficient when the maximum number of entries can be predicted in advance.

What are the properties of hash table in Java?

Java Hashtable class
  • A Hashtable is an array of a list. Each list is known as a bucket. ...
  • Java Hashtable class contains unique elements.
  • Java Hashtable class doesn't allow null key or value.
  • Java Hashtable class is synchronized.
  • The initial default capacity of Hashtable class is 11 whereas loadFactor is 0.75.


Is hash table same as Map?

Though both Hashtable and HashMap are data-structure based upon hashing and implementation of Map interface, the main difference between them is that HashMap is not thread-safe but Hashtable is thread-safe. This means you cannot use HashMap in a multi-threaded Java application without external synchronization.

What is difference between HashSet and Hashtable?

Hashtable and HashMap both implement Map , HashSet implements Set , and they all use hash codes for keys/objects contained in the sets to improve performance. Hashtable is a legacy class that almost always should be avoided in favor of HashMap .


Is Map a hash table?

Data Structures hash map Hashing

Hash map is a widely used efficient data structure that used to store data which can be searched in constant time O(1) . It is also referred as hash table, unordered map, dictionary, hash set and others. This data structure is implemented over an array that maps keys to values.

How is hash table better than array?

In arrays, you have to loop over all items before you find what you are looking for while in a hash table you go directly to the location of the item. Inserting an item is also faster in Hash tables since you just hash the key and insert it.

Why is it called a hash table?

A hash function is any function that can be used to map data of arbitrary size to fixed-size values. The values returned by a hash function are called hash values, hash codes, digests, or simply hashes. The values are usually used to index a fixed-size table called a hash table.

Where is Hashtable used in real time?

Hash tables are used as disk-based data structures and database indexing. They can be used to implement caches mainly used to that are used to speed up the access to data. Several dynamic programming languages like Python, JavaScript, and Ruby use hash tables to implement objects.


How is data stored in a hash table?

In a hash table, data is stored in an array format, where each data value has its own unique index value. Access of data becomes very fast if we know the index of the desired data. Thus, it becomes a data structure in which insertion and search operations are very fast irrespective of the size of the data.

What is the load factor of a hash table?

Load factor is defined as (m/n) where n is the total size of the hash table and m is the preferred number of entries that can be inserted before an increment in the size of the underlying data structure is required.

Can hash tables have duplicate keys?

Yes a hash can have duplicate keys as I demonstrate below...

Why HashMap is faster than hash table?

HashMap is not synchronized, therefore it's faster and uses less memory than Hashtable. Generally, unsynchronized objects are faster than synchronized ones in a single threaded application.


Why HashMap is faster than HashSet?

HashMap is faster than HashSet because the values are associated to a unique key. In HashSet , member object is used for calculating hashcode value which can be same for two objects so equals() method is used to check for equality.

What is the difference between Hashtable and ConcurrentHashMap?

Hashtable is belongs to the Collection framework; ConcurrentHashMap belongs to the Executor framework. Hashtable uses single lock for whole data. ConcurrentHashMap uses multiple locks on segment level (16 by default) instead of object level i.e. whole Map . ConcurrentHashMap locking is applied only for updates.

Why null key is allowed in HashMap?

In Hashmap one null key is allowed and multiple null values are allowed. Since hashcode of null is 0 so null key actually stored at index 0 in hashmap. Hashmap in java actsually created as Array of linkedlist where index in array is find out by hashing on key.

What is the difference between a Hashtable and Properties?

Properties is a subclass of Hashtable. It is used to maintain lists of values in which the key is a String and the value is also a String. The Properties class is used by many other Java classes. For example, it is the type of object returned by System.


Why hashing is used in Java?

hashCode in Java helps the program to run faster. For example, comparing two objects by their hashcodes will give the result 20 times faster than comparing them using the equals() function. This is so because hash data structures like HashMaps, internally organize the elements in an array-based data structure.

What is the time complexity of a hash table?

Like arrays, hash tables provide constant-time O(1) lookup on average, regardless of the number of items in the table. The (hopefully rare) worst-case lookup time in most hash table schemes is O(n).

Can Hashtable store multiple values?

You're looking for a Lookup, which can natively store multiple values for each key. As pointed out this only works for a fixed list since you cannot add entries to a lookup once you have created it. Lookup is immutable. That can be nice, but might not work in the OPs scenario.