What consequences can arise from using a mutable object as a key in a collection?

Using a mutable object as a key in collections, such as in HashMap or HashSet, can lead to serious problems:

• Breaking collection integrity: If the fields of the object involved in computing hashCode() or equals() are changed after placing the object in the collection, the object may become "invisible" for search. For example, map.get(key) will return null, even though the key exists in the collection.

• Data loss: Changing the key can make it impossible to delete or update the element, as the collection cannot find it by the modified key.

• Unpredictable behavior: Iteration and operations on the collection may work incorrectly, complicating debugging.

Therefore, it is recommended to use immutable objects as keys or ensure that the fields involved in hashCode() and equals() are not changed after adding to the collection.