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Checker Framework tutorial

Welcome!

This is a hands-on, play-along tutorial for null-safe programming with the Checker Framework.

Why?

Suppose you have created a neat little utility that turns the String A wonderful autumn afternoon into a-wonderful-autumn-afternoon:

public static String makeSlug(String s) {
    return s.replaceAll("\\s+", "-").toLowerCase();
}

Somewhere else in the app this utility is used to create a page title:

    String title = hasTitle()
            ? getTitle()
            : System.getProperty("defaults.title");

    String titleSlug = makeSlug(title);

What could go wrong?

If you have been programming Java for any amount of time, the call to makeSlug() probably has you shifting around uneasily in your chair: this code may blow up with a NullPointerException.

Though it doesn't make it explicit, makeSlug does not accept a null argument. We cannot be sure that title is not null. We don't know if getTitle() always returns a non-null value, but at least System.getProperty() will return null if the property does not exist. Therefore the code is not correct.

This is the kind of reasoning we do every day: read the code, follow the flow of references, and track which references may be null at a given point to make sure the code is correct.

Wouldn't it be nice if this sort of reasoning about what may and may not be null at a given place in the code could somehow be expressed in code? And wouldn't it be nice to then have a machine check the flow of nullable and non-null references for us, and establish if the code is correct with respect to null?

This is where the Checker Framework comes in.

This tutorial

todo

Mission statement on the next page.