HashSet, thesarfo

Key points about the HashSet class

Unordered and unsorted — elements in a HashSet are not ordered or sorted, and there’s no guarantee the order stays constant over time.

Unique elements — HashSet does not allow duplicate values; adding the same value again doesn’t replace the old one.

Null elements — HashSet allows one null value.

Non-synchronized — HashSet is not thread-safe; multiple threads can access it simultaneously.

Underlying data structure — HashSet is implemented in terms of a hash table, and internally uses a HashMap to store elements.

Performance — add, remove, and contains all have O(1) time complexity.

Implements Set — HashSet implements the Set interface, inheriting all its methods.

Creating a HashSet

HashSet<String> set = new HashSet<>();

Adding elements to a HashSet

This is done using the add method.

HashSet<String> set = new HashSet<>();

// add elements
set.add("Java");
set.add("Python");
set.add("JavaScript");

// display the elements
for (String language : set) {
    System.out.println(language);
}

Creating a HashSet from another collection

List<Integer> list = new ArrayList<>();
list.add(5);
list.add(10);
list.add(15);
list.add(20);
list.add(25);

List<Integer> list2 = new ArrayList<>();
list2.add(3);
list2.add(6);
list2.add(9);
list2.add(12);
list2.add(15);

// Creating a HashSet from another collection (ArrayList)
Set<Integer> set = new HashSet<>(list);

// Adding all the elements from an existing collection to a HashSet
set.addAll(list2);

System.out.println(set);

// Output = [3, 5, 6, 9, 10, 12, 15, 20, 25]

Accessing elements

HashSet doesn’t have a get() method to access elements, since it’s not an indexed collection. You can iterate through it to access its elements.

public class Main {
    public static void main(String[] args) {
        // Creating a HashSet
        HashSet<String> set = new HashSet<>();

        // Adding new elements to the HashSet
        set.add("Java");
        set.add("Python");
        set.add("JavaScript");

        // Accessing elements using an iterator
        Iterator<String> iterator = set.iterator();
        while (iterator.hasNext()) {
            String language = iterator.next();
            System.out.println(language);
        }
    }
}

Removing elements

You can remove elements from a HashSet using:

remove(Object o) — removes a single specified element from the set.
removeAll(Collection<?> c) — removes all elements from the set that are contained in the specified collection.
removeIf(Predicate<? super E> filter) — removes all elements from the set that satisfy the given predicate.

public class Main {
    public static void main(String[] args) {
        // Creating a HashSet
        HashSet<String> set = new HashSet<>();
        set.addAll(Arrays.asList("Apple", "Banana", "Cherry", "Date", "Elderberry"));

        System.out.println("Original HashSet: " + set);

        // Using remove(Object o)
        set.remove("Cherry");
        System.out.println("After remove('Cherry'): " + set);

        // Using removeAll(Collection c)
        set.removeAll(Arrays.asList("Apple", "Banana"));
        System.out.println("After removeAll(['Apple', 'Banana']): " + set);

        // Using removeIf(Predicate<? super E> filter)
        set.removeIf(fruit -> fruit.startsWith("E"));
        System.out.println("After removeIf(fruit -> fruit.startsWith('E')): " + set);
    }
}