Why Doesn't the Set Interface Allow Duplicate Elements in Java?

       If you have 3+ years of Java experience, an interviewer may ask this question to test your understanding of how a Set works internally in Java.

Internally, a HashSet stores its elements using a HashMap. A HashMap stores data as key-value pairs, whereas a Set contains only unique elements. When an element is added to a HashSet, it is stored as the key in the underlying HashMap, and a constant dummy object is used as the value for every entry.

Since HashMap does not allow duplicate keys, HashSet also does not allow duplicate elements. If you attempt to add a duplicate element, the add() method returns false, and the element is not added to the Set.

When a new element is added successfully, the add() method returns true. If the element already exists, it returns false. Therefore, adding a duplicate element does not result in a compile-time or runtime error—it is simply ignored.

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