CodingBat-WarmUp1 riddles' solutions and tests CodingBat is a free site of live coding problems to build coding skill in Java and Python (example problem). CodingBat is a project by Nick Parlante, a computer science lecturer at Stanford.
Going through many practice problem is a great way to solidify your understanding of how the code should work. CodingBat problems work great as homework, or for self-study practice, or in a lab, or as live lecture examples. The CodingBat problems are designed to have low overhead: short problem statements (like an exam), nothing to install, immediate feedback in the browser, and there's lots of them to build up those skills. The idea for CodingBat came from my experience teaching CS at Stanford combined with seeing how student's used unit-tests in more advanced courses, and crystalized when I saw an Owen Astrachan demo of a unit-testing thing he uses with his Duke students.
These challenges are found at: https://codingbat.com/java