97 Things Every Java Programmer Should Know

Chapter 43. Java’s Unspeakable Types

Ben Evans

What is null?

New Java programmers often struggle with this idea. A simple example reveals the truth:

String s = null;
Integer i = null;
Object o = null;

The symbol null must therefore be a value.

As every value in Java has a type, null must therefore have a type. What is it?

It obviously cannot be any type that we ordinarily encounter. A variable of type String cannot hold a value of type Object—the Liskov substitution properties simply do not work that way.

Nor does Java 11 local variable type inference help:

jshell> var v = null;
| Error:
| cannot infer type for local variable v
| (variable initializer is 'null')
| var v = null;
| ^ — — — — — -^

The pragmatic Java programmer may simply scratch their head and decide, as many have done, that it doesn’t really matter all that much. Instead, they can pretend “null is merely a special literal that can be of any reference type.”

However, for those of us who find this approach unsatisfying, the true answer can be found in the Java Language Specification (JLS), in Section 4.1:

There is also a special null type, the type of the expression null (§3.10.7, §15.8.1), which has no name.

Because the null type has no name, it is impossible to declare a variable of the null type or to cast to the null type.

There it is. Java allows us to write down values whose types we cannot declare as ...