He's interested in object-oriented and functional programming. His research thesis is that the two paradigms are two sides of the same coin, to be identified as much as possible. To prove this, he has worked with a number of language designs in the past, in particular the Pizza and GJ extensions to Java.

He has also influenced the development of Java as a co-designer of Java generics and as the original author of the current javac reference compiler.

His recent work is centered on Scala, a new programming language which unifies FP and OOP while staying completely inter-operable with Java.