Passing null to a method

Asked 2008-08-28T13:34:40.313

I am in the middle of reading the excellent Clean Code

One discussion is regarding passing nulls into a method.

public class MetricsCalculator {
    public double xProjection(Point p1, Point p2) {
        return (p2.x - p1.x) * 1.5;
    }
}
...
calculator.xProjection(null, new Point(12,13));

It represents different ways of handling this:

public double xProjection(Point p1, Point p2) {
    if (p1 == null || p2 == null) {
        throw new IllegalArgumentException("Invalid argument for xProjection");
    }
    return (p2.x - p1.x) * 1.5;
}

public double xProjection(Point p1, Point p2) {
    assert p1 != null : "p1 should not be null";
    assert p2 != null : "p2 should not be null";
    return (p2.x - p1.x) * 1.5;
}

I prefer the assertions approach, but I don't like the fact that assertions are turned off by default.

The book finally states:

In most programming languages there is no good way to deal with a null that is passed by a caller accidentally. Because this is the case, the rational approach is to forbid passing null by default.

It doesn't really go into how you would enforce this restriction?

Do any of you have strong opinions either way.

java null assert

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2 Answers

Slightly off-topic, but one feature of findbugs that I think is very useful is to be able to annotate the parameters of methods to describe which parameters should not be passed a null value.

Using static analysis of your code, findbugs can then point out locations where the method is called with a potentially null value.

This has two advantages:

The annotation describes your intention for how the method should be called, aiding documentation
FindBugs can point to potential problem callers of the method, allowing you to track down potential bugs.

Only useful when you have access to the code that calls your methods, but that is usually the case.

answered 2008-08-28T22:34:06.913

The best way to handle this really would be the use of exceptions. Ultimately, the asserts are going to end up giving a similar experience to the end user but provide no way for the developer calling your code to handle the situation before showing an exception to the end user. Ultimatley, you want to ensure that you test for invalid inputs as early as possible (especially in public facing code) and provide the appropriate exceptions that the calling code can catch.

answered 2008-08-28T16:59:05.923

Passing null to a method

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