Groovy NullObject should be null or not?

Question

This example can be easily tested in the groovy console.

var a is evaluated to not null while b is evaluated to null.

Both are instances of org.codehaus.groovy.runtim.NullObject

def b = null

println b.getClass()
println b == null

def a = null.getClass().newInstance()

println a.getClass()
println a == null

Does anyone knows why?

This is a tricky thing when dealing with reflection code.

Answer 1

In the equals method of NullObject, it only returns true if you are comparing it to null

As an instance of NullObject is not strictly null, it returns false...

Whether NullObject should return true if you call equals against another NullObject is probably a question best asked on the mailing list... I'll have a look and see if I can find any previous question.

Answer 2

Actually I am wondering if this is not a bug. As an explanation... NullObject is a runtime/intermediate kind of Object. If you do anything on null, then NullObject is used. This, and the the implementation of NullObject#equals speaks for a==null returning true. It returns fails, because there is some internal code before that, that is for example determining if compareTo is called instead of equals and such things. Now this piece of code starts with

if (left == right) return true; 
if (left == null || right == null) return false;

so null==null will return true, but NullObject==null will return false. On the other hand NullObject should not leak out if possible. Maybe we should fix newInstance() to return null.

I filled http://jira.codehaus.org/browse/GROOVY-5769 for this