EqualsAndHashCode

EqualsAndHashCode refers to the mechanisms by which objects determine equality and produce hash codes in object-oriented programming, most commonly in Java. In Java, every class inherits two methods from Object: equals(Object) and hashCode(). The equals method defines logical equality between two objects, while hashCode provides a numeric value used by hash-based collections such as HashMap and HashSet. A consistent pairing of these methods is essential for correct behavior in collections and for predictable comparisons.

The equals contract requires several properties: reflexivity (an object equals itself), symmetry (if a.equals(b) then b.equals(a)), transitivity (if a.equals(b) and b.equals c, then a.equals(c)), consistency, and proper handling of null (an object never equals null). A key guideline is that if two objects are considered equal, their hashCode values must be equal. Conversely, unequal objects may share a hashCode, but good practice aims to minimize collisions. Hash codes should be stable for an object as long as its significant fields remain unchanged.

Implementation guidance typically involves selecting the fields that define the object's identity. Common practice is to base both equals and hashCode on the same set of significant fields, and to avoid including mutable or nonessential fields that can violate the contract if they change while the object is in a hash-based collection. When designing a class hierarchy, decisions about using getClass versus instanceof affect equality behavior.

Lombok provides an annotation, @EqualsAndHashCode, which generates these methods automatically. It offers configuration options such as callSuper to include superclass fields, and excludes or Include annotations to tailor which fields participate in the computations.

Practical considerations include avoiding reliance on mutable fields for identity, understanding inheritance implications, and thoroughly testing behavior when objects are used as map keys or set elements.