Java example dictionary with "default" HashSet of integers

DictionaryExample.java

import java.util.*;

public class Main {
    public static void main(String[] args) {
        int[] numbers = {1, 2, 3, 2, 4, 5, 3, 6, 7, 4};
        HashMap<Integer, HashSet<Integer>> dictionary = new HashMap<>();

        for (int i = 0; i < numbers.length; i++) {
            int number = numbers[i];
            if (!dictionary.containsKey(number)) {
                dictionary.put(number, new HashSet<Integer>());
            }
            HashSet<Integer> indexSet = dictionary.get(number);
            indexSet.add(i);
        }

        // Print the dictionary
        for (Map.Entry<Integer, HashSet<Integer>> entry : dictionary.entrySet()) {
            int number = entry.getKey();
            HashSet<Integer> indexSet = entry.getValue();
            System.out.println("Number: " + number + ", Index Set: " + indexSet.toString());
        }
    }
}

/*
Run example:

Number: 1, Index Set: [0]
Number: 2, Index Set: [1, 3]
Number: 3, Index Set: [2, 6]
Number: 4, Index Set: [4, 9]
Number: 5, Index Set: [5]
Number: 6, Index Set: [7]
Number: 7, Index Set: [8]
*/