How HashSet behave while adding a duplicate value?

This is one of the tricky interview questions. Interviewer asked when a duplicate object is inserted into HashSet will it

a.   Replace the old value with new one (or)

b.   The new item is not inserted

 

Answer to the above question

Item isn't inserted to the HashSet. As per the Java documentation 'public boolean add(E e)' method, adds the specified element to this set if it is not already present. If this set already contains the element, the call leaves the set unchanged and returns false.

 

Let’s experiment with an example.

 

HashSetDuplicateValue.java

package com.sample.app.collections;

import java.util.HashSet;
import java.util.Objects;
import java.util.Set;

public class HashSetDuplicateValue {

	private static class Employee {
		private int id;
		private String name;

		public Employee(int id, String name) {
			this.id = id;
			this.name = name;
		}

		@Override
		public int hashCode() {
			return Objects.hash(id);
		}

		@Override
		public boolean equals(Object obj) {
			if (this == obj)
				return true;
			if (obj == null)
				return false;
			if (getClass() != obj.getClass())
				return false;
			Employee other = (Employee) obj;
			return id == other.id;
		}

		@Override
		public String toString() {
			return "Employee [id=" + id + ", name=" + name + "]";
		}

	}

	public static void main(String args[]) {

		Set<Employee> empSet = new HashSet<>();

		System.out.println("Adding emp1 with id 1");
		Employee emp1 = new Employee(1, "Krishna");
		empSet.add(emp1);
		
		System.out.println(empSet);
		
		System.out.println("\nAdding emp2 with id 1");
		Employee emp2 = new Employee(1, "Ram");
		empSet.add(emp2);
		
		System.out.println(empSet);

	}

}

 

Output

Adding emp1 with id 1
[Employee [id=1, name=Krishna]]

Adding emp2 with id 1
[Employee [id=1, name=Krishna]]

 

In the above snippet,

a.   I defined Employee class where id is used as hash code. So two employee objects are equal when their id’s are same.

b.   I defined two employee objects emp1 and emp2 with same id.

c.    When I inserted both emp1 and emp2 to HashSet, emp2 is not inserted since as per the hashCode and equals contract employee with id 1 is already present in HashSet.

 

 

 

Reference

https://docs.oracle.com/javase/8/docs/api/java/util/HashSet.html

https://self-learning-java-tutorial.blogspot.com/2015/12/how-hashmap-works-in-java.html

 

 

 

  

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