When and why should the hashCode method be overridden in a class?

The hashCode method should be overridden in a class if you override the equals method. This is necessary to maintain the contract between these methods: if two objects are equal according to equals, then their hash codes must be the same. Otherwise, objects may behave incorrectly in hash-based collections, such as HashMap or HashSet.

Example:

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || getClass() != obj.getClass()) return false;
    MyClass other = (MyClass) obj;
    return Objects.equals(field1, other.field1) && field2 == other.field2;
}

@Override
public int hashCode() {
    return Objects.hash(field1, field2);
}

Without overriding hashCode, objects that are equal according to equals may have different hash codes, which can lead to errors when used in hash data structures.