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This article is to share with you about the difference between HashMap and HashTable. The editor thinks it is very practical, so share it with you as a reference and follow the editor to have a look.

HashMap is not thread safe.

When HashMap does the put operation, two threads enter the addEntry operation at the same time and calculate the same hash value. After A thread writes to the header node, B thread also writes to the header node, then the An operation will be overwritten, resulting in the loss of A write operation.

Vector and HashTable,concurrentHashMap,copyOnwriteArrayList are thread safe.

Inheritance is different. HashMap inherits from the AbstractMap class, while HashTable inherits from the Dictionary class.

The methods in HashTable are synchronous, and the HashMap is asynchronous. Synchronized keyword is added to the implementation of HashTable to ensure synchronization. If you use HashMap, you have to add your own synchronization processing. Synchronization method Map m = Collections.synchronizeMap (hashMap)

In HashTable, KQuery V is not allowed to appear in null;HashMap, null can be used as a key, there is only one such key, and there can be one or more values for null. (get () cannot be used in HashMap to determine whether a key exists. Instead, containsKey () should be used.)

Traversing the way. Both HashMap and HashTable use Iterator. HashTable also uses the Enumeration approach for historical reasons. The iterator of Iterator is the fail-fast iterator, while Enumeration is not.

The use of hash values is different. HashTable uses the object's Hashcode,HashMap directly to recalculate the hash value.

The initial size and expansion mode of the array are different. The default size of the array of HashTable is 11, and the expansion method is old * 2 + 1. The default size of the array in HashMap is 16, and the expansion mode is an exponential power of 2.

The array subscript is found in different ways. HashTable takes the remainder by dividing by length, and hashMap performs bitwise sum operations with length-1.

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