Checker Framework Tutorial

1. Spot the null pointer exception

Begin by importing the checker-framework-tutorial project into your Eclipse workspace. Select from the menu, File->Import->Existing Project, then browse to the eclipse-projects/checker-framework-tutorial directory. (If you have not already, download the Eclipse projects for the tutorial.)

Open NullnessExample.java. It is a simple Java program with an obvious null pointer exception.

public class NullnessExample {
    public static void main(String[] args) {
        Object myObject = null;
        System.out.println(myObject.toString());
    }
}

2. Run the Nullness Checker

Run the Nullness Checker to see how it can warn you about this error at compile time. The Checker Framework Eclipse plugin documentation tells you how to run a checker and where the warnings appear.

The following warning will be produced.

dereference of possibly-null reference myObject
        System.out.println(myObject.toString());

3. Correct the error

Edit the code to initialize the myObject variable to some non-null value.

public class NullnessExample {
    public static void main(String[] args) {
        Object myObject = new Object();
        System.out.println(myObject.toString());
    }
}

4. Re-run the Nullness Checker

No errors should be produced.