Question from Kathy and Bert's book - p.413

[Mansi Vyas]

On p. 413:
"if two objects are considered equal using the equals() method, then they must have identical hashcode values. So to be truly safe, your rule of thumb should be if you override equals(), override hashCode() as well."
But what about the indexOf() method in the Vector class. This method tests for equality using the equals() method. I'm assuming that you need to override this method for it to work correctly. In this case, would you need to override the hashCode() method as well?
Mansi

[Damien Howard]

To properly follow the hashcode/equals contract, you should properly override both so that equal objects have the same hashcode values

[bennido kool kat]

If I override both methods and I don't follow the contract, will the compiler or JVM give an error ?

[Andres Gonzalez]

Originally posted by bennido kool kat:
If I override both methods and I don't follow the contract, will the compiler or JVM give an error ?

No. As long as you do a correct override (same return type, no new check exceptions, public access modifier, etc) It will not complain at all. you will only get inconsistent results in your application.
[ July 10, 2003: Message edited by: Andres Gonzalez ]

[Damien Howard]

If I override both methods and I don't follow the contract, will the compiler or JVM give an error ?

No it won't give an error. The contract is an unofficial thing. A convention in a way. You can override them to do anything you like, but if you don't follow the contract then you may find design problems with your code, especially concerning collections.
K&B discuss some examples in their book. I'm too lazy to find any at the moment.