There is a story that one of the designers of Java was pushing very hard in favour of Java references being non-nullable by default. But Gosling rejected the idea claiming that it would be too confusing for programmers and it would kill any chances of Java achieving popularity.

Here is a relevant paper, "Non-null References by Default in Java: Alleviating the Nullity Annotation Burden":

http://www.springerlink.com/content/f197143t80671627/

"we conducted an empirical study of 5 open source projects (including the Eclipse 3.3 JDT Core) totaling over 700 KLOC. The results allow us to confirm that in Java programs, at least 2/3 of declarations of reference types are meant to be non-null, by design."