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Java HashSet  - Java Tutorials

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Java Collections Framework- HashSet Class

A HashSet is a collection set that neither allows duplicate elements nor order or position its elements.

This class implements the Set interface and extends AbstractSet. It creates a collection that uses a hash table for storage. Hash table stores information by using a mechanism called hashing. In hashing, the informational content of a key is used to determine a unique value, called its hash code. The hash code is then used as an index at which the data associated with the key is stored. The transformation of key into its hash code is performed automatically.HashSet is not synchronized. It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element. 

The HashSet class supports four constructors. The first form constructs a default hash set:

HashSet( )  
HashSet(Collection c)
HashSet(int capacity)
HashSet(int capacity, float fillRatio)

Here is an example that demonstrates a HashSet:- HashSetDemo.java

package devmanuals.com;

import java.util.*;

public class HashSetDemo {
	public static void main(String args[]) {

		HashSet HSet = new HashSet();

		HSet.add("C");
		HSet.add("A");
		HSet.add("B");
		HSet.add("E");
		HSet.add("F");
		HSet.add("D");
		System.out.println("The HashSet elements are: " + HSet);
	}
}

Output:

The HashSet elements are: [D, E, F, A, B, C]
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Posted on: April 18, 2011

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Comments
Sudhakar
September 2, 2011
Details

A HashSet is a collection set that neither allows duplicate elements nor order or position its elements. This class implements the Set interface and extends AbstractSet. It creates a collection that uses a hash table for storage. Hash table stores information by using a mechanism called hashing. In hashing, the informational content of a key is used to determine a unique value, called its hash code. The hash code is then used as an index at which the data associated with the key is stored. The transformation of key into its hash code is performed automatically.HashSet is not synchronized. It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
sarath
October 28, 2011
hash set

In the printing of "hash list" .. why the list is in reverse order? based on what it is displaying????
Jiyan
August 16, 2012
select the elements without angular-clammier

how can select the elements without angular-clammier []? for Exaple: not so output : [A,B,C] but so output : A,B,C
Ed Shihabeddin
November 5, 2012
Runtime of Collection interface

What is the runtime for a Hashset add method? get method? What about for a TreeMap?
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