What are the main principles of implementing the hashCode method in Java that conform to the contract and must be followed?

The main principles of implementing the hashCode method in Java, consistent with the contract, are as follows:

1. Consistency: Multiple calls to hashCode() on the same object during a single execution should return the same value, provided the object has not been modified.

2. Relation to equals: If two objects are equal according to the equals() method, then calling hashCode() on both objects should return the same value.

3. Optional requirement: If two objects have the same hashCode, they are not necessarily equal (but this can affect the performance of hash-based structures).

Example of a simple hashCode:

@Override
public int hashCode() {
    int result = 17;
    result = 31 * result + (field1 != null ? field1.hashCode() : 0);
    result = 31 * result + field2;
    return result;
}

Adhering to these rules is important for the correct operation of hash-based collections such as HashMap and HashSet."}]}]}