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.

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

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);
    }
}

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");
    }
}

abstract class SuperClass {

    SuperClass() {
        abstractMethod();
    }

    abstract void abstractMethod();
}

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 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'."

• 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.

Spoiler alert!

You'll find it here: Solution