Which of the following is NOT a characteristic of a Set in Java?

A fundamental characteristic of a Set in Java is that it does not permit duplicate elements. This means that every element in a Set must be unique, ensuring that only one instance of each item can exist within the collection. Therefore, the correct understanding aligns with the statement that Sets do not allow duplicate elements.

Sets are defined to maintain a collection of distinct values, and this uniqueness is central to their functionality. As a result, when working with Sets in Java, if you attempt to add a duplicate value, the Set will ignore the request or will not change its state.

The other listed features accurately describe Sets in Java. Sets are unordered collections, meaning the elements do not maintain any specific order after insertion. They also permit null values, allowing for the inclusion of null as an element within the collection. Thus, the assertion that Sets allow duplicate elements is not aligned with the core definition of a Set.