Java 7 - Null-safe types

Yesterday I published part of a proposal for enhancing null handling in Java 7. Reading the comments, this got a few people upset. Unfortunately, what I didn't make clear was that I was also documenting the other part - null-safe types for Java.

The null-safe types part of the proposal is the tougher one to write-up, and probably the tougher one to implement. I was going to try and finish the details before publishing, but lets get the discussion going anyway...

Null-safe types are similar to Option Types in Nice. The difficulty is taking the idea and blending it with Java in a way that works and doesn't break the style of Java.

So, what is the null-safe types proposal? Well, lets examine this method:

  public Person lookupPerson(String personId) { ... }

This is a fairly typical method that loks up a person from a person identifier. But what happens if we pass a null identifier into the method? And can we get a null response? The current solution to this is adding explicit documentation:

  /**
   * Find person by identifier.
   * @param personId  the person id, not null
   * @return the person, never null
   * @throws IllegalArgumentException if the person is not found
   */
  public Person lookupPerson(String personId) { ... }

Now we know how the method is to be used. But this is just documentation. There is no compile-time checking. If we don't read the documentation then we may get a NPE at runtime.

The null-safe types proposal adds a new modifier to types - hash # - that prevents that variable/parameter/return from being null.

  public #Person lookupPerson(#String personId) { ... }

Now that we are using these declarations, the compiler can check that the calling code does not pass a null String to the method.

  String badId = null;
  #Person person = lookupPerson(badId);   // compile error

  #String id = "1234";
  #Person person = lookupPerson(id);      // ok

The draft of the formal null-safe types proposal is available with more details. And there are a lot more details, and probably a lot I've missed. If you've got any suggested enhancements or fixes to the proposal, or want to help out, let me know. All feedback welcomed.

And remember that this proposal is intended to complement the null-ignore part of the proposal.