When I started to prepare a course of lectures on the Java language for the Moscow Institute of Physics and Technology, I became to search for material that can be used as illustrations. ‘One picture is worth a thousand words’ as they say, and sometimes it’s worth even more because it seems impossible to explain e. g. how a hash table works without drawing something. My task was to explain to students many Java concepts, from string pool to advanced data structures. I was looking for a solution that is able to make the visualization of Java data structures in the easiest and precise way, ideally compatible with ‘presentation as code’ technology.
There exists Aleksey Shipilev’s JOL (Java Object Layout) tool known among Java performance engineers. JOL analyzes the memory layout for any given object, including all the auxiliary data and fields, and the graph of objects reachable from the given instance. This tool gives accurate estimations of the object size and also shows the addresses in memory where objects are located. However, it’s not yet able to visualize the object graph and gives too many low-level details that are irrelevant for the students who just started to learn Java.