Why does changing the equals method without updating hashCode violate the correctness of hash tables?

In Java, the contract between the equals() and hashCode() methods requires that if two objects are considered equal (equals returns true), then their hashCode must be the same. If equals is changed without updating hashCode, objects that are equal according to equals may have different hashCodes. As a result, hash-based collections (such as HashMap or HashSet) may not be able to correctly find objects, because they first search by hashCode, then by equals. This can lead to errors when storing and retrieving objects in hash-based collections.