Collections Framework

HashSet

• The HashSet class of the Java Collections framework provides the functionalities of the hash table data structure.
• There are 2 ways to create HashSet
• Non-generic
Syntax:
HashSet arraydeque=new HashSet();
• Generic
Syntax:
HashSet<className> hashset=new HashSet<className>();

• Non-generic HashSet can store Any type of object like Integer, String, Date…etc
• Generic HashSet can store only specify the ClassName.

HashSet

• It creates a collection that uses hash table for storage. A hash table stores information by using a mechanism called hashing.
• HashSet does not maintain any order of elements.
• HashSet contains only unique elements.
• It allows to store null value.
• It is the best approach for search operations.
• The initial default capacity of HashSet is 16.
• It implements the Set interface.

HashSet Methods

• void add(value)
• inserts the specified element to the set.
• void addAll(collection)
• inserts all the elements of the specified collection to the set.
• void remove(value)
• removes the specified element from the set.
• void removeAll()
• removes all the elements from the set.

HashSet Set Operations

• The various methods of the HashSet class can also be used to perform various set operations.
• Union of Sets: To perform the union between two sets, we can use the addAll() method.

import java.util.*;
class demo
{
  public static void main(String args[])
  {
    HashSet setA=new HashSet();
    setA.add("C");
    setA.add("C++");
    setA.add("Java");
    setA.add("MySQL");
    setA.add("HTML");
    HashSet setB=new HashSet();
    setB.add("Python");
    setB.add("C");
    setB.add("Java");
    System.out.println("SetA is "+setA);
    System.out.println("SetB is "+setB);
    setA.addAll(setB);
    System.out.println("Union SetA and SetB is "+setA);
  }
}

SetA is [Java, C++, C, MySQL, HTML] 
SetB is [Java, C, Python] 
Union SetA and SetB is [Java, C++, C, MySQL, HTML, Python]

HashSet Set Operations

• The various methods of the HashSet class can also be used to perform various set operations.
• Intersection of SetsTo perform the intersection between two sets, we can use the retainAll() method.

import java.util.*;
class demo
{
  public static void main(String args[])
  {
    HashSet setA=new HashSet();
    setA.add("C");
    setA.add("C++");
    setA.add("Java");
    setA.add("MySQL");
    setA.add("HTML");
    HashSet setB=new HashSet();
    setB.add("Python");
    setB.add("C");
    setB.add("Java");
    System.out.println("SetA is "+setA);
    System.out.println("SetB is "+setB);
    setA. retainAll(setB);
    System.out.println("Union SetA and SetB is "+setA);
  }
} 
  

SetA is [Java, C++, C, MySQL, HTML] 
SetB is [Java, C, Python] 
Union SetA and SetB is [Java, C]

HashSet Set Operations

• The various methods of the HashSet class can also be used to perform various set operations.
• Difference of SetsTo calculate the difference between the two sets, we can use the removeAll().

import java.util.*;
class demo
{
  public static void main(String args[])
  {
    HashSet setA=new HashSet();
    setA.add("C");
    setA.add("C++");
    setA.add("Java");
    setA.add("MySQL");
    setA.add("HTML");
    HashSet setB=new HashSet();
    setB.add("Python");
    setB.add("C");
    setB.add("Java");
    System.out.println("SetA is "+setA);
    System.out.println("SetB is "+setB);
    setA.removeAll(setB);
    System.out.println("Union SetA and SetB is "+setA);
  }
}

SetA is [Java, C++, C, MySQL, HTML] 
SetB is [Java, C, Python] 
Union SetA and SetB is [C++, MySQL, HTML]