Most of the interactive materials currently available use multiple choice questions as their main interaction. This helps with jargon and detail, but not with core programming skills. The most effective way to learn programming is to do frequent progressive programming exercises and JOT is an experiment to try to distill this simple truth into a do-it-yourself tutorial.
A tutorial like this can't replace textbooks, reference materials or teachers, only supplement them, and it cannot provide intelligent personal support. However, it can help you to interpret the feedback you get from the Java compiler and runner, check the coding style of your programs, and set precise programming goals so that it can automatically check whether your programs are working properly.
At present JOT is entirely Web based. Simple Web page features are used, and standards are followed, to try to ensure that it works on most recent browsers. Programs are tested by compiling and running them directly on our server. Needless to say, there are security restrictions in place to prevent submitted programs from doing any damage to our server, and to limit the resources they can use.
There are a lot of exercises, but making them available online means that skeleton programs can be provided as a starting point, and so people can do them quickly, focusing on the main point of the exercise, without the overhead of typing in and debugging a lot of surrounding boilerplate code. Online compiling and testing means that people don't even have to have a Java compiler installed to work through the exercises.