HashSet and LinkedHashSet are both implementations of the Set interface in Java, and they share certain similarities:

1. Set Interface: Both HashSet and LinkedHashSet implement the Set interface, which means they provide a collection of unique elements with no duplicate values.
2. Unique Elements: Both HashSet and LinkedHashSet enforce uniqueness of elements. They do not allow duplicate elements to be stored in the set. If you try to add an element that already exists in the set, it will be ignored.
3. No Defined Order: Neither HashSet nor LinkedHashSet guarantees the order of elements. The elements are not stored in any particular order based on their insertion sequence or any other criteria.
4. Null Values: Both HashSet and LinkedHashSet can store null values. They allow storing a single null element.

Despite these similarities, there is a key difference between HashSet and LinkedHashSet:

LinkedHashSet maintains insertion order: LinkedHashSet, unlike HashSet, maintains the order in which elements were inserted. It uses a doubly linked list to preserve the insertion order alongside a hash table for fast element lookup. This means that when you iterate over a LinkedHashSet, the elements will be returned in the same order they were added.

In contrast, HashSet uses a hash table for storing elements, which allows for fast retrieval but does not guarantee any specific order of elements during iteration.

In summary, both HashSet and LinkedHashSet provide the uniqueness of elements and allow null values. The main difference lies in the iteration order: HashSet has no specific order, while LinkedHashSet maintains the insertion order of elements.