Limitations of @NotNull Annotations and Code Inspection

About

In Java, the most widespread problem causing applications to crash is the so called NullPointerException. Developers can deal with this problem by introducing Annotations (e.g. @Nullable and @NotNull) and using code inspectors. A correctly annotated code that passes code analysis without any warning should never crash - that's the theory.

Code Quest

The following code is correctly annotated and passed code inspection without any warnings. Nevertheless, it will crash immediately when executed.

Main.java

import org.jetbrains.annotations.NotNull;

class Main {

    public static void main(String [] args) {
        @NotNull SubClass instance = new SubClass();
        System.out.println("Execution completed. SubClass has been instantiated: " + instance);
    }
}

SubClass.java

import org.jetbrains.annotations.NotNull;

class SubClass extends SuperClass {

    @NotNull
    private String aString = "initialised";

    SubClass() {
        super();
        aString += " and extended by constructor";
    }

    @Override
    void abstractMethod() {
        int length = aString.length();
        System.out.println(aString + " has " + length + " characters");
    }
}

SuperClass.java

abstract class SuperClass {

    SuperClass() {
        abstractMethod();
    }

    abstract void abstractMethod();
}

Exception

When executing this project, you will get the following exception stacktrace:

Exception in thread "main" java.lang.NullPointerException
    at SubClass.abstractMethod(SubClass.java:15)
    at SuperClass.<init>(SuperClass.java:4)
    at SubClass.<init>(SubClass.java:9)
    at Main.main(Main.java:6)

Code Inspection

Code Inspectors can be separate tools or part of your IDE. In my case, I used IntelliJ IDEA from Jetbrains.

Just checkout the project, open it in IntelliJ and run 'Analyze' -> 'Inspect Code'

The inspection will pass without any warnings: "No suspicious code found. 4 files processed in 'Project 'NullableQuest'."

Hints

• This has nothing to do with the Annotations itself. You can remove them if you want, but the problem will persist.

• This has nothing to do with Threading.

• This has something to do with Java itself.

Solution of this quest

Spoiler alert!

You'll find it here: Solution