Can I only implement equals() but not hashCode() if I only need to compare objects and not yet plan to put the objects into any hash based containers?
Seems all Java bibles say these two MUST be implemented together. :(
-If I always implement hashCode() together with equals() there will be lots of code not really used, and without unit test covering. (I'm not going to unit test hashCode() if not used)
-It's only until when I put the object into a hash based container I know how the objects are intended to be looked up. And only until then I can be sure which hashing strategy to use.
You can, but you'll be breaking the general contract of
equals, and this will lead to weird bugs. Even if you don't think you're using the hash codes, any external code you pass the objects to might rely on them, even if it doesn't seem to be hash-based. If you're not going to give your objects a decent hash method, at least make it throw a runtime exception. It's almost always better to give your objects a decent hashCode, though.