Why must hashCode be consistent with equals, and what breaks if you override one but not the other?

The contract requires that equal objects have equal hash codes; if you override equals to compare by fields but leave the identity hashCode, two logically equal objects land in different buckets and the map fails to find what you inserted.

Conversely overriding only hashCode without equals means lookups fall back to identity equality and still miss. The production symptom is a put followed by a get returning null, or duplicate entries in a HashSet, which is why you always override both together, typically via Objects.hash and Objects.equals.