What are the main principles of the relationship between the equals and hashCode methods in Java?

Main principles of the relationship between equals and hashCode in Java:

• If two objects are equal according to equals, their hashCode must be the same.
• If the hashCode of two objects is different, equals must return false.
• If equals returns false, hashCode can be the same, but this is undesirable as it degrades the performance of hash-based structures.

This ensures the correct operation of collections that use hashing (HashMap, HashSet), where hashCode determines the bucket, and equals provides the exact comparison of objects.

Example:

@Override
public boolean equals(Object obj) {
    if (this == obj) return true;
    if (obj == null || getClass() != obj.getClass()) return false;
    Person person = (Person) obj;
    return Objects.equals(id, person.id);
}

@Override
public int hashCode() {
    return Objects.hash(id);
}

Here, equals and hashCode are based on the same field id, which ensures their consistency.