Question

What is the output? Hashset set \(=\) new HashSet () set. \(\operatorname{add}(1)\); set. \(\operatorname{add}(2)\); set \(\cdot\) add \((1)\); System.out.println(set): a. \([1,2,1]\) b. \([1,2]\) c. \([1]\) d. IllegalArgumentException

Short answer

b. [1,2]

Step-by-step solution

- Understanding HashSet

A HashSet is a collection that does not allow duplicate elements. Elements are stored in no particular order.

- Adding Elements to HashSet

First, the number 1 is added to the set. Then, the number 2 is added to the set. Lastly, the number 1 is attempted to be added again.

- Handling Duplicates

Since HashSet does not allow duplicate elements, the second attempt to add 1 will be ignored. Thus, only unique elements are retained.

- Output the HashSet

When the set is printed, it will output the unique elements it contains, which are [1, 2]. The specific order is not guaranteed, but the unique elements are.

Key concepts

Java Collections Framework

Java Collections Framework (JCF) is a set of classes and interfaces that help manage groups of objects, also known as collections. It's a powerful tool included in Java's standard library for handling data structures.
The JCF provides several types of collections such as Lists, Sets, Maps, and Queues.
These collections are designed to be easily interchangeable and to make coding more efficient and less error-prone. With collections, common tasks such as sorting, searching, and manipulating data become much easier.
The HashSet class, which we explore here, is one such type provided by JCF.

Unique Elements

One of the key characteristics of a HashSet is that it maintains only unique elements. This means that any attempt to add a duplicate element will be ignored by the HashSet.
When you add an element to the HashSet, like in the exercise example where 1, 2, and another 1 are added, only the unique ones are retained.
This is extremely useful for scenarios where you need to ensure that each item appears only once, such as in checking for duplicate entries or in filtering results.
Internally, HashSet uses a hashing mechanism, which is both efficient and fast, to ensure that each element is unique.

Handling Duplicates

Handling duplicates is a common task that many programs need to address. With a HashSet, this task is simplified as it automatically takes care of duplicates for you.
In the given example, adding the elements 1, 2, and then attempting to add 1 again, demonstrates this perfectly.
When the duplicate 1 is added for the second time, the HashSet simply ignores it. This is because HashSet follows the property that no two elements can be the same.
Consequently, when you print the HashSet, only the unique elements are displayed: [1, 2].
Hence, using a HashSet can be a very effective way to manage lists where duplicates should not be allowed, making your code cleaner and more efficient.