For a long time, there has been no standard shell shipped with the Java programming language to experiment with new language features or libraries, or for rapid prototyping. If you wanted to do this, you could write a test application with a main() method, compile it with javac, and run it. This could be done either in the command line or using a Java IDE; however, in both cases, this is not as convenient as having an interactive shell for the purpose.

Starting an interactive shell in JDK 9 is as simple as running the following command (assuming the bin directory of your JDK 9 installation is in the current path):

jshell

You may find it somewhat puzzling that an interactive shell has not been introduced earlier in the Java platform, as many programming languages, such as Python, Ruby, and a number of others, already come with an interactive shell in their earliest versions. However, this didn't make it on the priority features list until Java 9. The Java Shell makes use of a JShell API that provides capabilities to enable autocompletion or evaluation of expressions and code snippets, among other features. Chapter 6, Experimenting with the Java Shell, is dedicated to discussing the details of the Java Shell so that developers can make the best use of it.