How to Use a Java HashSet by Example

What is a HashSet

A HashSet is an unordered collection containing unique elements. It has the standard collection operations Add, Remove, Contains, but since it uses a hash-based implementation, these operations are O(1)

Learn with a Programming Question

Given an integer array with even length, where different numbers in this array represent different kinds of candies. Each number means one candy of the corresponding kind. You need to distribute these candies equally in number to brother and sister. Return the maximum number of kinds of candies the sister could gain.

Example 1:

Input: candies = [1,1,2,2,3,3]
Output: 3
There are three different kinds of candies (1, 2 and 3), and two candies for each kind.
Optimal distribution: The sister has candies [1,2,3] and the brother has candies [1,2,3], too. 
The sister has three different kinds of candies. 

Example 2:

Input: candies = [1,1,2,3]
Output: 2
Explanation: For example, the sister has candies [2,3] and the brother has candies [1,1]. 
The sister has two different kinds of candies, the brother has only one kind of candies. 


  1. The length of the given array is in range [2, 10,000], and will be even.
  2. The number in given array is in range [-100,000, 100,000].

The Solution using a HashSet

// Our class
class Solution {
    // Our method call
    public int distributeCandies(int[] candies) {
        // Use a HashSet to store all the intergers
        HashSet<Integer> set = new HashSet<>();
        // for each candy
        for (int candy: candies) {
            // add it to the set
        // Return the minimum between the size and half the length
        return Math.min(set.size(), candies.length / 2);