Comparison of enumeration and iterator

Comparison of enumeration and iterator

	Enumeration interface	Iterator interface
Meaning of the parameter	Enum type	Iterator element type
Package	Java.util
Parent class	No
Sub-class	StringTokenizer	Beancontextsupport.bcsiterator, Eventreaderdelegate,scanner
Difference	An object that implements the enumeration interface, It generates a series of elements, one at a time A. Successive calls to the Nextelement method will Returns a series of contiguous elements.	Iterators
Method
Determine if there is a next element	hasMoreElements () tests whether this enumeration contains more elements.	Hasnext () If there are still elements that can iterate, Returns True.
Get element	Nextelement () If this enumeration object is at least There is also an element that can be supplied, then return this The next element of the enumeration.	Next () returns the next element of the iteration.
Removed from		Remove () from the collection that the iterator points to The last element returned by the iterator in the remove (optional operation).

Objective
In the code instance of the database connection pool analysis, you see that you use enumeration to traverse the vector collection. And then I found some information. See what methods you can use to iterate through the collection class, and find the following instances of traversing the collection class using enumeration and iterator on the web. However, this example mentions that enumeration is more efficient than iterator, in fact it is not the case, the time test is too one-sided, because the amount of data is too small. As the data increases, the efficiency of the two becomes more and more close, without a multiplier ratio. And now it is common to use iterator to iterate through the collection class, and only specifically explicitly declare that you must use enumeration to iterate through the collection with that class.

Package Edu.sjtu.erplab.hash;

Import java.util.Enumeration;
Import java.util.Hashtable;
Import Java.util.Iterator;
Import Java.util.Map.Entry;

A traversal Hashtable instance
public class Travesehashtable {
public static void Main (string[] args) {
Initialize Create Hashtable
Hashtable<string, string> ht = new hashtable<string, string> ();
for (int i = 0; i < 10000; i++) {
Ht.put ("key=" + I, "val=" + i);
}

1. Using enumeration
Long start = System.currenttimemillis ();
Enumeration<string> en = Ht.keys ();//Use enumerations to get key
while (En.hasmoreelements ()) {
En.nextelement ();
}
Long end = System.currenttimemillis ();
SYSTEM.OUT.PRINTLN ("Enumeration keys costs" + (End-start)
+ "milliseconds");

2. Using enumeration
Start = System.currenttimemillis ();
enumeration<string> en2 = ht.elements ();//Use enumerations to get this Key-value
while (En2.hasmoreelements ()) {
En2.nextelement ();
}
End = System.currenttimemillis ();
SYSTEM.OUT.PRINTLN ("enumeration elements costs" + (End-start)
+ "milliseconds");

3. Iterator
Start = System.currenttimemillis ();
Iterator<string> it = Ht.keyset (). Iterator ();//Use an iterator to get the key
while (It.hasnext ()) {
It.next ();
}
End = System.currenttimemillis ();
System.out.println ("Iterator keySet costs" + (End-start)
+ "milliseconds");

4. Iterator
Start = System.currenttimemillis ();
iterator<entry<string, string>> it2 = Ht.entryset (). Iterator ();//Use iterators to get this key-value
while (It2.hasnext ()) {
It2.next ();
}
End = System.currenttimemillis ();
System.out.println ("Iterator entryset costs" + (End-start)
+ "milliseconds");

}
}

Obsolete interface: Enumeration
The enumeration interface is JDK1.0 when it is the best iterative output interface, the earliest use of vectors (now recommended to use ArrayList) is to use the enumeration interface for output. Although enumeration is an old class, it expands the enumeration class after JDK1.5, adding generic operational applications.

Enumeration interface commonly used methods have hasMoreElements () (To determine whether there is a next value) and Nextelement () (take out the current element), these methods are similar to iterator, but there is a way to delete data in iterator, There is no delete operation for this interface.

Why continue to use the enumeration interface
Enumeration and iterator interface functions are similar, and iterator has more features than enumeration, so why use enumeration? This is because Java has evolved over a long period of time, and some older systems or methods in the class library are using the enumeration interface, so for compatibility, you still need to use enumeration.

Common subclasses of the list interface
The common subclasses of the list interface are ArrayList and vectors, and there are many similarities between the two, and the comparison between the two is given below.

Comparison of enumeration and iterator