What source data is used to determine the hashCode value in an object?

The value of hashCode in an object is usually computed based on those fields of the object that define its uniqueness and equality. In Java, for the correct functioning of collections (such as HashMap, HashSet), it is important that the fields involved in the equals() method are also used when calculating hashCode().

For example, if an object represents a person with fields id, name, and age, and equality is determined only by id, then hashCode should be computed based on id.

Example:

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

If all fields are considered, you can use:

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

The main rule is — fields involved in equals() should be considered in hashCode() to ensure correct behavior of hash data structures.