Best implementation for hashCode method for a collection

Question

How do we decide on the best implementation of hashCode() method for a collection (assuming that equals method has been overridden correctly) ?

Answer 1

First make sure that equals is implemented correctly. From an IBM DeveloperWorks article:

• Symmetry: For two references, a and b, a.equals(b) if and only if b.equals(a)
• Reflexivity: For all non-null references, a.equals(a)
• Transitivity: If a.equals(b) and b.equals(c), then a.equals(c)

Then make sure that their relation with hashCode respects the contact (from the same article):

• Consistency with hashCode(): Two equal objects must have the same hashCode() value

Finally a good hash function should strive to approach the ideal hash function.