Should null == null be true when comparing objects?

Question

I'm curious what everyone thinks. In SQL (at least in oracle) NULL translates conceptually to "I don't know the value" so NULL = NULL is false. (Maybe it actually results in a NULL which then gets cast to false or something like that...)

This makes sense to me, but in most OO languages null means "no reference" so null==null should probably be true. This is the usual way of doing things in C# for example when overriding Equals.

On the other hand, null is still frequently used to mean "I don't know" in object-oriented languages and implementing null==null to false might result in code that is slightly more meaningful to certain domains.

Tell me what you think.

Answer 1

For general purpose programming, null == null should probably return true.

I can't count the number of times I've run into the

if( obj != null )
{
   //call methods on obj
}

pattern, and it often seems unavoidable. If null == null evaluated to false, this pattern would fall apart, and there wouldn't be a good way to handle this case without exceptions.

Answer 2

I think that null in Java, just like NULL in C++ or None in Python, means specifically "there's nothing here" -- not "I don't know", which is a concept peculiar to SQL, not common in OOP languages.