A billion-dollar apology from the creator of the famous NullPointerException.

This is one of the most famous keywords in every programmer's dictionary. Even after decades, programmers couldn't escape this error every now and then. It's by far the most prevalent kind of error in Java and many other programming languages as well.

Tony Hoare, the inventor of the null reference apologized in 2009 and denotes this kind of errors as his billion-dollar mistake.

I call it my billion-dollar mistake. It was the invention of the null reference in 1965. At that time, I was designing the first comprehensive type system for references in an object oriented language (ALGOL W). My goal was to ensure that all use of references should be absolutely safe, with checking performed automatically by the compiler. But I couldn't resist the temptation to put in a null reference, simply because it was so easy to implement. This has led to innumerable errors, vulnerabilities, and system crashes, which have probably caused a billion dollars of pain and damage in the last forty years.