Hashmap objects use an iterator for traversing or iterating and HashTable uses both iterator and enumerator for iterating the object values. Hashtable is synchronized. Map interface, it is added to the Collection Framework. Csharp Programming Server Side Programming. HashTable is thread-safe while HashMap is not thread-safe. Performance is slow in HashTable, while fast in HashMap. JavaTpoint offers college campus training on Core Java, Advance Java, .Net, Android, Hadoop, PHP, Web Technology and Python. A Map is used to map keys to objects (one object set to another). HashTable is a legacy class but HashMap is introduced only from JDK 1.2 and it is a new class. Both HashTable and HashMap uses the hashing mechanism for working. These data structures help to sort the particular data according to its identifiers and the associated values. Hashtable is internally synchronized and can't be unsynchronized. For thread-safe operations, HashTable can be used as all the methods of HashTable are synchronized as it’s a legacy class. AbstractMap is the superclass of HashMap and Dictionary is the superclass of HashTable. Or in other words, a Hashtable is used to create a collection which uses a hash table for storage. Implementation in C. Live Demo. A Hashtable is a collection of key/value pairs that are arranged based on the hash code of the key. Duplicate values are allowed in both HashMap and HashTable. Both these classes, HashMap and HashTable implements the same Map interface. In hash table, the data is stored in an array format where each data value has its own unique index value. Additionally, since hash tables use the key to find the index that will store the value, an insert or lookup can be done in amortised O(1) time (assuming few collisions in the hash table). It is thread-safe and can be shared with many threads. Hashtable never allows any null values and null keys in its objects, while Hashmap allows multiple null values and only a single null key value. Access of data becomes very fast, if we know the index of the desired data. Hash Table is a data structure which stores data in an associative manner. 5) We can make the HashMap as synchronized by calling this code Map m = Collections.synchronizedMap(hashMap); Hashtable is internally synchronized and can't be unsynchronized. The members in a Hashtable are thread safe. Initially, HashTable did not belong to Collection Framework but after getting retrofitted for implementing java.util. 2) The second important difference between Hashtable and HashMap is performance since HashMap is not synchronized it perform better than Hashtable. In Hashtable, key objects must be immutable as long as they are used as keys in the Hashtable. If you want the order of objects to remain constant with time, use LinkedHashMap. Hashtable vs Hashmap Hashtable and the hashmaps are the data structures in great use these days for most of the web-based applications and many other applications as well. There are two interfaces namely Map and Set in Java. HashMap supports any number of null values and one null key value whereas HashTable has no permission for null values and null keys. Hashtable is slow. Method level synchronizations cannot be removed from HashTable (until you break the code) and thus the HashMap was developed in the future. A Set is just a collection of objects and has no other structures. Iterator type in HashMap is fail-fast whereas it is fail-safe in HashTable. HashMap and Hashtable both are used to store data in key and value form. HashTable is thread-safe while HashMap is not thread-safe. The certification names are the trademarks of their respective owners. Let us consider a Java program to understand HashMap and HashTable better. 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