In Java, when should you override both equals method and hashCode method in a custom class?

Introduction / Context:
In Java, every class inherits equals and hashCode from Object, but these default implementations provide identity based behavior, where two references are equal only if they point to the same object in memory. Many domain classes need a different notion of logical equality based on field values, such as two Customer objects with the same customerId being considered equal. When you change equals, you must also consider hashCode, especially if the class will be used as a key in hash based collections. This question asks when you should override both methods.

Given Data / Assumptions:

• We are writing a custom class with fields that define logical identity, such as id or code.
• We may store instances in collections like HashSet, HashMap, or HashTable.
• The equals and hashCode methods must obey the general Java contract.
• We want consistent behavior across different collection types.

Concept / Approach:
The equals method defines when two objects should be considered equal. The hashCode method returns an integer hash that must be consistent with equals: if two objects are equal according to equals, they must have the same hash code. Hash based collections use hashCode to decide which bucket to place an object in and then use equals to distinguish objects inside the bucket. If you override equals without updating hashCode, equal objects may end up in different buckets, causing lookups and containment checks to fail. Therefore, whenever you define a custom logical equality, especially for objects used in hash based collections, you must override both methods together.

Step-by-Step Solution:
Step 1: Identify the scenario where logical equality differs from reference equality. This happens whenever you decide equality should depend on field values. Step 2: Recognize that if such objects are added to HashSet or used as keys in HashMap, the collection will rely on hashCode and equals to manage them. Step 3: Option a states that you should override both methods whenever you define custom logical equality and plan to use the objects in hash based collections. This matches best practice. Step 4: Option b, c, d, and e describe situations that do not relate correctly to the contract between equals and hashCode. Step 5: Therefore, choose option a as the correct description.

Verification / Alternative check:
Official Java documentation states that if you override equals, you must also override hashCode so that equal objects produce the same hash code. Many bugs in real systems come from overriding equals only, leading to unexpected behavior in HashSet or HashMap where contains or get fails although an equal object seems to be present. This strong recommendation confirms that option a is the accurate guideline.

Why Other Options Are Wrong:
Option b proposes overriding methods only when there are no fields, which does not make sense. Option c claims equals and hashCode are final, but they are not; they are explicitly intended to be overridden. Option d suggests doing this to prevent collection storage, which is unrelated to the contract. Option e discusses arrays of primitives, where equals and hashCode on the array type have different semantics and are not modified by overriding in a user class.

Common Pitfalls:
A common mistake is to override equals based on one set of fields and then compute hashCode using different fields or failing to override hashCode at all. This inconsistency can lead to subtle bugs that appear only when collections grow or when applications run under different load patterns. Developers should also beware of mutable fields in equals and hashCode, as changing such fields after placing objects into hash based collections can break lookup behavior.

Final Answer:
You should override both methods whenever you define a custom logical equality for your class and intend to use its objects in hash based collections such as HashMap or HashSet, as described in option a.