Differences between hashtable and hashmap

Hashtable is widely used. hashmap is a class used to replace hashtable in the new framework. In other words, hashmap is recommended instead of hashtable. Maybe you think hashtable is very useful. Why not? Here we will briefly analyze their differences.

1. The hashtable method is synchronous, And the hashmap is not synchronized. Therefore, the difference between manual synchronization of hashmap in multithreading is like that of vector and arraylist.

2. hashtable does not allow null values (neither key nor value). hashmap allows null values (both key and value ).

3. hashtable has a contains (object value), which has the same functions as containsvalue (object value.

4. hashtable uses enumeration and hashmap uses iterator. The above is just a difference on the surface, and their implementation is also very different.

5. The default size of the hash array in hashtable is 11, and the increase is in the old * 2 + 1 mode. The default size of the hash array in hashmap is 16, and it must be an index of 2.

6. For different hash values, hashtable directly uses the hashcode of the object,CodeYes:

Int hash = key. hashcode ();

Int Index = (hash & 0x7fffffff) % tab. length;

Hashmap recalculates the hash value and replaces the modulo:

Int hash = hash (k );
Int I = indexfor (hash, table. Length );

Static int Hash (Object X ){
Int H = x. hashcode ();
H + = ~ (H <9 );
H ^ = (H >>> 14 );
H + = (H <4 );
H ^ = (H >>> 10 );
Return h;
}Static int indexfor (int h, int length ){
Return H & (length-1 );
}
The above are just some of the outstanding differences. Of course, there are still many different implementations, such as hashmap's operations on null.

Differences Between hashtable and hashmap:
1. hashtable is a subclass of dictionary, and hashmap is an implementation class of the map interface;
2. methods In hashtable are synchronized, while those in hashmap are not synchronized by default. That is to say, in the multi-threaded application Program , hashtable can be safely used without special operations. For hashmap, an additional synchronization mechanism is required. However, the synchronization problem of hashmap can be solved through a static method of collections:
map collections. synchronizedmap (MAP m)
This method returns a synchronous map, which encapsulates all the underlying hashmap methods, this ensures that the underlying hashmap is secure even in a multi-threaded environment.
3. In hashmap, null can be used as a key. There is only one such key. The value corresponding to one or more keys can be null. When the get () method returns a null value, it can indicate that the key does not exist in hashmap or that the corresponding value of the key is null. Therefore, the get () method cannot be used to determine whether a key exists in the hashmap. Instead, the containskey () method should be used to determine whether a key exists in the hashmap.

Hashtable inherits from the dictionary class, while hashmap is an implementation of the map interface introduced by java1.2.

Hashmap allows null as the key or value of an entry, while hashtable does not.

In addition, hashmap removes the hashtable contains method and changes it to containsvalue and containskey. The contains method is easy to misunderstand.

The biggest difference is that the hashtable method is synchronize, but hashmap is not.
When multiple threads access hashtable, they do not need to implement synchronization for their methods, but hashmap
You must provide external synchronization for it.

Hash/rehash used by hashtable and hashmapAlgorithmSo the performance will not be significantly different.