The Java Collections Framework is the most widely used API – probably in your application too. You use it all the time, but do you really know your way around it? You can be surprised even by common classes like ArrayList and HashMap, let alone by the handling of null values or immutability in lesser-known classes. In this interactive session, we’ll look at some obvious code and its less-than-obvious effect. It’s up to you to guess the outcome! But everything will be explained, and you’ll end up knowing how to avoid pitfalls, both well-known and obscure, in using the Collections Framework.

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Maurice Naftalin: Five decades working in IT: developer, designer, architect, manager, teacher, and author. Working with Java since 1.0. Author, Mastering Lambdas; co-author, Java Generics and Collections (2e scheduled for Java 21). Disorganises the unconference JAlba. Java Champion and Rock Star. Speaks at conferences a lot.

Dmitry Vyazelenko: Independent consultant working on Aeron ( and helping organizations solve their Java performance issues. Public speaker and disorganizer of the JCrete and JAlba unconferences.