Checker Framework Tutorial

1. Spot the null pointer exception

Begin by viewing NullnessExample.java. (If you have not already, download the source files for the tutorial.) 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.

To run the Nullness Checker, run javac with command-line arguments -processor org.checkerframework.checker.nullness.NullnessChecker, as follows.

(Note: In this tutorial, the commands that you cut-and-paste to run on the command line appear in bold after a $ prompt.)
(Note: You should have already made javacheck an alias to the Checker Framework compiler.)

$ javacheck -processor org.checkerframework.checker.nullness.NullnessChecker NullnessExample.java

The following error will be produced.

NullnessExample.java:9: error: [dereference.of.nullable] dereference of possibly-null reference myObject
        System.out.println(myObject.toString());
                           ^
1 error

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

$ javacheck -processor org.checkerframework.checker.nullness.NullnessChecker NullnessExample.java

No errors should be produced.