A hash value is an integer returned by hashCode() and used by hash-based collections. Two practical mistakes make it unreliable: misparenthesized expressions can damage distribution, while mutating a stored key can make it unreachable. section 2.1.1 section 8.6
Avoid these failure modes:
- Parenthesize expressions that combine addition with bitwise shifts. Addition has higher precedence, so lo << 16 + hi means lo << (16 + hi), not (lo << 16) + hi. section 2.1.1
- Do not modify an object after storing it as a HashMap key or HashSet element when the modification changes its hash value. Lookup uses the new bucket while the object remains in the old one. section 8.6
- Treat static-analysis advice about parentheses as a prompt to verify intended semantics, not as proof that the existing expression is wrong. section 2.1.1
A damaged hash distribution
In a manual hashCode() implementation such as xmin + ymin << 8 + xmax << 16 + ymax << 24, shifts occur after the additions. The final left shift can erase 24 low-order bits, producing poor distribution and possible HashMap performance problems. Explicit parentheses isolate the intended operations. section 2.1.1
A key that disappears
Adding a List to a HashSet and then changing the list changes its hash value. set.contains(list) searches the bucket for the updated value instead of the bucket selected during insertion. section 8.6
