Rather than merely test for code correctness, Exercism uses peer review to improve general programming techniques. Users are encouraged to comment on others' solutions, and refine their own based on feedback.
Codeacademy doesn't overwhelm beginners with information. It gives bite-sized information, the bare minimum needed to finish a task and get to the next one. As the course progresses, it slowly fills in background information.
JavaString does a good job of introducing concepts in a straight forward way. However there is no actually projects to demonstrate the code, no interactive components, and the descriptions can be a bit bare bones. JavaString alone won't be enough to teach you concepts, only introduce them so y...
Each exercise chapter focuses on a characteristics such as, arrays, strings, logic, recursion etc. This is useful as it does not overload the beginner nor the expert, as they get the chance to sharpen their skills.