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Java / Map and its implementations

List the differences between HashTable and HashMap in Java collections.

HashMap. HashTable.
HashMap is not synchronized and therefore it is not thread safe. HashTable is internally synchronized and therefore it is thread safe.
HashMap allows maximum one null key and any number of null values.HashTable does not allow null keys and null values.
Iterators returned by the HashMap are fail-fast in nature.Enumeration returned by the HashTable are fail-safe in nature.
HashMap extends AbstractMap class.HashTable extends Dictionary class.
HashMap returns only iterator to traverse.HashTable returns both Iterator as well as Enumeration for traversal.
HashMap is fast.HashTable is slow.
HashMap is not a legacy class.HashTable is a legacy class.
HashMap is preferred in single threaded applications. If you want to use HashMap in multi threaded application, wrap it using Collections. synchronizedMap() method.Although HashTable is there to use in multi threaded applications, now a days it is not at all preferred. Because, ConcurrentHashMap is better option than HashTable.

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Set and its implementations

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