What is HashSet ?
java.util.HashSet implements Set interface interface
and it use hash table to store data in memory. Hash Table uses hashing
mechanism to store data in memory. It makes no guarantees as to the iteration order of the set;
in particular, it is unordered collection. It does not guarantee that the order will remain constant over
time. Later on we will discuss hashing mechanism using by HashSet.
Use Of HashSet
1. HashSet does not allow duplicate elements, So whenever we want to store unique elements we use Hash Set. A common use for a set is to remember which items in a list have 'already been seen' so that they are not processed more than once.
2. Another advantage of hashing is that it allows the execution time of basic operation, such as add(), contains(), remove(), and size() to remain constant even for large sets.
Different declaration of HashSet
1. Simple Declaration
2. Declare HashSet of Specific class
3. Declare HashSet with approximate capacity or size
4. Declare HashSet with initial capacity and load factor
Here load factor means when the size of hashset will increase automatically. Suppose initial capacity is 100 and load factor is 0.75. So when set contains 75 elements then size of set will increase, internally new set of more capacity will generate.
5. Declare HashSet with existing collection
Important methods of HashSet
1. boolean add(Object obj)
It adds object to set if it is not exist
2. boolean contains(Object obj)
It return true if object exists in set
3. boolean remove(Object obj)
It remove object if exists and return true, if obj is not exist in set it return false.
4. int size()
It returns the number of objects in the set.
5. boolean isEmpty()
It returns true if set is empty otherwise returns false.
6. void clear()
This method remove all elements exist in set.
7. Iterator iterator()
This method returns iterator over the elements in the set.
What happens when you try to add duplicate element?
On every add operation hash set interanally call equals() method and hashCode() method. If it find same element already exist in the set. it will not add duplicate.
Can we add null value into set?
This question is mostly asked by beginners. your answer is Yes you can add null, but as per rule you can add only one null value.
Complete code :
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| package com.javasuitor.demo;
import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
public class HashSetDemo {
public static void main(String[] args) {
// Declare simple hashset, its default capacity is 16.
HashSet hashSet1 = new HashSet();
// Declare hash set of Particular class
HashSet<String> hashSet2 = new HashSet<String>();
// Declare hash set using initial capacity
HashSet<String> hashSet3 = new HashSet<String>(20);
// Declare hash set using initial capacity and Load factor
HashSet<String> hashSet4 = new HashSet<String>(20, 0.75f);
ArrayList <String> countryList = new ArrayList<String>();
countryList.add("India");
countryList.add("Australia");
countryList.add("South Africa");
//Declare hash set using pre built Array List
HashSet<String> hashSet5 = new HashSet<String>(countryList);
//Add values in set
hashSet3.add("Ashok");
hashSet3.add("Akanksha");
hashSet3.add("Sumit");
hashSet3.add("Shivi");
hashSet3.add("Bhanu");
// Iterator of Set
Iterator<String> iterator = hashSet3.iterator();
System.out.println("Elements of set");
while(iterator.hasNext()){
System.out.println(iterator.next());
}
// Try to add duplicate element
hashSet3.add("Ashok");
System.out.println("\nSet elements after adding duplicate object\n"+hashSet3);
// HashSet allow to add null value
hashSet3.add(null);
System.out.println("\nSet elements after adding null\n"+hashSet3);
// Check whether the element present in set or not
System.out.println("\nIs Ashok present in HashSet: "+hashSet3.contains("Ashok"));
System.out.println("Is Riddhi present in HashSet: "+hashSet3.contains("Riddhi"));
// Remove particular element from set
hashSet3.remove(null);
System.out.println("\nSet elements after removing null \n"+hashSet3);
// Get the number of element in hash set
System.out.println("\nNumber of elements in hashSet "+hashSet3.size());
// Check whether the HashSet is empty or not
System.out.println("\nIs hashSet3 is empty : "+hashSet3.isEmpty());
System.out.println("Is hashSet1 is empty : "+hashSet1.isEmpty());
// Remove all elements from set
hashSet3.clear();
System.out.println("\n After Clear method values in hashset : "+ hashSet3);
}
}
|
Output :
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