Detailed explanation of the difference between java HashMap and HashTable

The difference between HashMap and HashTable is often asked by others. Let’s summarize it here today.

(1) The history of inheritance is different

public class Hashtable extends Dictionary implements Map
public class HashMap extends AbstractMap implements Map

## Hashtable is inherited from the Dictionary class, while HashMap was introduced in Java 1.2 An implementation of the Map interface.

(2) Different security

HashMap is non-synchronized, while HashTable is synchronized by default, which means that HashTable is thread-safe and multiple threads can share a HashTable; Without proper synchronization, multiple threads cannot share a HashMap. Java 5 and later provides ConcurrentHashMap, which is a replacement for HashTable and has better scalability than HashTable. Of course, we can synchronize HashMap through the following methods:

Map m = Collections.synchronizeMap(hashMap);

(3) Similarities and differences of whether the value can be null

HashMap allows you to use null values ​​as a table The key or value of the entry. Only one record in a HashMap can be an empty key, but any number of entries can be an empty value. That is to say, if the search key is not found in the table, or if the search key is found, but it is an empty value, then get() will return null; but not in HashTable, null values ​​are not allowed in key and value. .

(4) The internal implementation of the traversal methods between the two is different

Both Hashtable and HashMap use Iterator, and HashMap’s Iterator (Iterator) is a fail-fast iterator, while HashTable's enumerator iterator is not fail-fast. Due to historical reasons, Hashtable also uses Enumeration.

(5) The use of hash values ​​is different

HashTable directly uses the hashCode of the object, while HashMap needs to recalculate the hash value.

(6) The initial size of the array and the expansion method of the internal implementation of the two are different

The default size of the hash array in HashTable is 11, and the increasing method is old*2+1; HashMap The default size of the hash array is 16, and it must be an exponent of 2.

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