Java 9 introduces JShell and a Read-Eval-Print Loop (REPL) for the Java Programming Language. REPL allows you to evaluate code snippets such as declarations, statements, expressions. You can test your code as you create it, and way before you are done with your whole project.
The JShell Java Enhancement Proposal
(JEP) for the project indicated that the new feature is the result of academic feedback. Schools are adopting languages that have REPL functionality because it lowers the initial learning curve of programming. The interactive REPL tool gives rapid evaluation of code to young developers.
In the video below, Jim Connors gives a short demonstration on how you can now use the tool as part of the latest Java 9 build.