Core answer
Start here. This is the direct spoken answer to practice first.
Overview
Hash-based collections use both methods, so an equality definition that ignores hashing can make a logically present key unreachable.
If two objects are equal according to equals, they must return the same hashCode. Hash-based collections use the hash to choose a bucket and then use equality to find the matching entry inside that bucket. If equal objects produce different hashes, a HashMap lookup or HashSet membership check may search the wrong bucket and fail. The reverse is not required: different objects may share a hash because collisions are allowed and are resolved with equality checks.