Object Model Violation: Just One of Equals and Hashcode Defined

In Java, the `equals()` and `hashCode()` methods work as a pair to define object identity for collections like `HashMap` and `HashSet`. When you override only one of these methods, you violate a core rule: if two objects are considered equal by the `equals()` method, they must return the same `hashCode()` value. Failing to uphold this contract causes unpredictable behavior in hash-based collections, leading to objects that are 'equal' being stored separately, becoming impossible to retrieve, or causing duplicate entries. To fix this, always override both methods together, ensuring their logic is based on the same set of object attributes. Use your IDE's generator or a library like Lombok to maintain consistency. This ensures your objects behave correctly in all standard Java collections and prevents subtle, hard-to-debug errors in your application's data handling.