Java HashSet

The HashSet is used to create a collection of unique values. In order to use it, we need to import the package java.util.

Using HashSet to store the collection of values is the best approach for searching the values since it uses the mechanism of hashing. HashSet cannot contain any duplicate values.

Create HashSet

import java.util.HashSet;
HashSet<String> userName= new HashSet<String>();

Add values to HashSet

The HashSet class provides a method add() to add values to the HashSet.

Example to add values to HashSet

import java.util.HashSet;

public class Main {
  public static void main(String[] args) {
    HashSet<String> users = new HashSet<String>();
    users.add("Andrew");
    users.add("Rayan");
    users.add("John");
    users.add("Stephen");
    users.add("Mark");
    System.out.println(users);
  }
}

Output

[Andrew, Rayan, John, Mark, Stephen]

Check Exists

HashSet class has a method contains() to check whether a particular value is present or not. It will return true if it exists, and false is not available in the HashSet.

Example to check whether the value is present

users.add("Michael");

Removing values from HashSet

HashSet class provides the remove() method to remove a particular value from the HashSet.

Example to remove a value from HashSet

users.remove("Stephen");

We can use the clear() method to clear all the values in the HashSet.

users.clear();

Looping through HashSet

import java.util.HashSet;

public class Main {
  public static void main(String[] args) {
    HashSet<String> users = new HashSet<String>();
    users.add("Andrew");
    users.add("Rayan");
    users.add("John");
    users.add("Stephen");
    users.add("Mark");
    System.out.println("Total users: "+ users.size());
    for(String user: users){
        System.out.println(user);
    }
  }
}

Output

Total users:5
Andrew
Rayan
John
Mark
Stephen

In the above program, we have created a HashSet to store the list of users and used the following syntax to loop through the values of the HashSet.

for(String user: users){
    System.out.println(user);
}

To find out the number of items in the HashSet we use size() method.

Example for HashSet

import java.util.*;  

class Employee {  
int employeeId;  
String name;  
String designation;  
public Employee(int employeeId, String name, String designation) {  
    this.employeeId = employeeId;  
    this.name = name;  
    this.designation = designation;  
}  
}  

public class Main {  
public static void main(String[] args) {  
    HashSet<Employee> employees=new HashSet<Employee>();  
    //Creating employees  
    Employee emp1=new Employee(101,"Andrew","CTO");  
    Employee emp2=new Employee(102,"Beslin","Business Analyst");  
    Employee emp3=new Employee(103,"Joanna","Architect");  
    //Adding employees to HashSet  
    employees.add(emp1);  
    employees.add(emp2);  
    employees.add(emp3);  
    //Traversing HashSet  
    for(Employee employee:employees){  
        System.out.println(employee.employeeId+" "+employee.name+" "+employee.designation);  
    }  
}  
}  

Output

101 Andrew CTO
103 Joanna Architect
102 Beslin Business Analyst