Comparison of Hashtable and HashMap

	Hashtable	HashMap
Concurrent operations	Using the synchronization mechanism, the actual application, only the synchronization of the Hashtable itself does not guarantee the correctness of the program under concurrent operation, and requires high-level concurrency protection.   The following code attempts to modify value to x+1{value = Hashtable.get (key) with the value of value equal to x corresponding to key;   if (value.intvalue () = = x) {hashtable.put (Key, New Integer (Value.intvalue () +1)); }} If 2 threads execute the above code at the same time, it may be put into a x+2 instead of x+1.	There is no synchronization mechanism, requiring users to make concurrent access control
How data is traversed	Iterator and enumeration	Iterator
Whether to support Fast-fail	With iterator traversal, support for Fast-fail with enumeration does not support Fast-fail.	Support Fast-fail
Do you want to accept a key or value that is NULL?	Do not accept	Accept
An algorithm for calculating array subscript based on hash value	When the length of the array is small, and the hash value of key is unevenly dispersed, the probability of the different hash values to get the same subscript value is higher. hash = Key.hashcode (); index= (hash&0x7fffffff)% Tab.length;	Better than Hashtable, the hash of the key is done by shifting operations and bits and operations, so that it can be more widely dispersed to the different positions of the array hash = Hash (k); index = 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 length of the entry array	ø         default initial length is one, ø         can specify initial capacity when initializing	Ø The default initial length is 2,Ø The length of the n-th square O-time can be specified when initializing initial capacity, if not 2, HashMap will select the first greater than I Nitial capacity's 2n square value as its initial length
Loadfactor load factor	0.75
When the load exceeds (Loadfactor * array length), the internal data is adjusted	Extended array: +1 of the original array length	Extended array: Original array length * 2
	Both will re-place the new position in the array based on the hash value of the key. Similar algorithms, same time and space efficiency

In general, HashMap can work better and faster than Hashtable, largely thanks to its hashing algorithm, and without synchronization. Instead of relying on the synchronization of these underlying data structures, applications generally implement a protection mechanism at a higher level, so HashMap is able to meet the needs in most applications. It is recommended to use HashMap, and if synchronization is required, you can use the Sync tool class to convert it to a hashmap that supports synchronization.

Comparison of Hashtable and HashMap