Why you should not use null as a method parameter

Object Oriented concept was first introduced with Lisp in the late 1950s. Lisp has atoms and attributes, where the atoms represented an real world object. OOP has evolved quite a bit after that. SOLID design principle were introduced that helped us craft a better software. I will not go in details of how these principles help us, there is already lot of material out there. Instead I’ll focus on some mistakes we do while implementing these patterns. One of them is passing null as a method parameter especially when the called method doesn’t use that parameter or has some default behavior.

What are we talking about?

Let’s take a simple example to elaborate this:

public interface IdentityProvider {
    public User login(String username, String password, String type);

    // other methods

The anti-pattern which I’m talking about uses null as a flag to toggle behavior. For example, take the following usages:

// 1. A valid type is passsed
idp.login('username', 'password', AccountType.ADMIN);

// 2. Using null to take default value
idp.login('username', 'password', null);

The first call here is absolutely fine. But in the second call we are passing null so that the login is attempted against the standard/default account type.

What’s wrong with this?

This type of call violate a fundamental design principle - abstraction. Let’s take an example to understand how.

The Carpenter and the table

You: Hey! I need a table, can you make me one?
Carpenter: Sure, I'll need some wood, screws and glue.
You: Oh Okay! I have wood and screws but I didn't get glue.
Carpenter: No problem, we can make one without glue. I'll just put more screws for the strength.
You: Awesome! Here you go!
(and you hand over the *ingredients* to the carpenter and get your table)

In programming, one might call the makeTable method like this:

carpenter.makeTable(wood, screw, glue);

Note that glue is optional, so someone can call the method like this as well:

carpenter.makeTable(wood, screw, null);

So what’s wrong?

Well, you know the implementation details, that’s wrong. The makeTable method is not abstract any more because you know that you can pass null and it will work fine. You know that internally it can work without glue.

When methods are declared with parameters, it marks a contract that, that method needs those argument to work correctly. If you were just a user of a third party library, you would not know that glue is required or not. You would have 2 options - read to docs if it says whether it can take null or figure it out by trial and error; in both the cases you are trying to know whether it works without glue and in both the cases you will have some idea about the internal implementation.

How to fix it?

One simple way to fix it using method overloading:

Table makeTable(Wood wood, Screw screws, Glue glue);
Table makeTable(Wood wood, Screw screws);

The other one is having a separate method for without glue:

Table makeTable(Wood wood, Screw screws, Glue glue);
Table makeTableWithoutGlue(Wood wood, Screw screws);

There could be many more way to fix the design. But these are probably the simplest one. It makes the code clearer. It leaves less space for errors and assumptions.

Don’t get me wrong!

Don’t get me wrong. Internally the second method might call the first method with a null argument. But that’s internally. That’s the implementation of the carpenter class and the guy implementing is aware of it. The null paradigm should not go outside the class/library. Not to the programmer using it.

You will wonder that I’m not an API designer, neither anyone else apart from my team is using the code we have written. Why should I do this? Well, it has the same benefits as other design principles - SOLID, KISS, etc - helps with writing maintainable and clean code. In one part of the code we might be a API designer while in some other part we might be user for that API. We have to interchangeably wear those 2 hats multiple times a day. In fact multiple times in an hour.