java-core-examples/equals-hashcode/README.md

equals-hashcode

Companion code for the ankurm.com post "equals() and hashCode() in Java: The Contract, Real Bugs and How Records Change It." Module in java-core-examples, the Java-core series. There is no docs/ folder: the deeper material lives in the post itself.

Versions this was built and tested against

Component	Version	Notes
JDK	25.0.5+8 (Temurin, LTS)	Records (final since 16) generate equals/hashCode.
JUnit Jupiter	5.11.0
Maven	3.9.11

Quickstart

export JAVA_HOME=/path/to/jdk-25
mvn compile
scripts/run.sh MutableKeyDemo      # one demo
scripts/run-all.sh                 # regenerate every file in output/

What's in here

File	What it shows
ContractDemo.java	The five rules, each broken on purpose: symmetry (instanceof + extra field), transitivity (the "lenient" fix), consistency.
MissingHashCodeDemo.java	equals() without hashCode(); a constant hashCode() and the 2,002,999 equals() calls it costs for 2,000 lookups.
MutableKeyDemo.java	A key mutated after insertion: get returns null, remove fails, iteration still sees it.
SymmetryDemo.java	java.util.Date vs java.sql.Timestamp: the JDK breaks symmetry itself.
RecordsDemo.java	What records generate; array components, mutable lists, NaN and -0.0; overriding equals.
EntityIdDemo.java	The JPA entity trap simulated without a database (id null until "saved"). No Hibernate involved.
HashCodeRecipeDemo.java	What String, List, Set, Map, Objects.hash, Arrays.hashCode actually compute.
EqualsHashCodeTest.java	Pins every claim above as an assertion.
output/01-08	Captured runs of everything above.

License

MIT - see LICENSE.