HashMap vs HashSet Java

A HashMap stores key-value pairs, where each key maps to an associated value. A HashSet stores just unique elements, with no values attached. The detail that ties them together, and that interviewers like to hear, is that a HashSet is actually backed by a HashMap internally. When you add an element to a HashSet, that element becomes a key in an internal HashMap, and it's paired with a single shared dummy object, often called PRESENT, as the value. So the uniqueness guarantee of a set is really just the key uniqueness of a map. That's a neat way to show I understand how the collections relate rather than treating them as totally separate things. In terms of when I use each: I use a HashSet whenever the only thing I care about is membership. Things like detecting duplicates, deduplicating a list, or asking have I seen this value before, where contains is the operation. I use a HashMap when I need to store something for each key, like a frequency count, an index, or a cached result. A good rule of thumb is that if I find myself creating a HashMap where all the values are dummy placeholders, I should just use a HashSet, because it expresses the intent more clearly. Both give me average constant-time add and contains, since the set just delegates to the map. The one behavioral difference worth noting is duplicates: adding a key that already exists in a map overwrites the value, while adding a duplicate to a set is simply ignored and returns false.