Exploring Java 9’s JShell – The Java Shell

With the release of Java 9, developers were introduced to a new tool called JShell, commonly referred to as the Java Shell. JShell provides an interactive and lightweight way to write, test, and experiment with Java code. In this article, we will delve into the concept of JShell, its significance, and how to use it to improve your Java programming experience.

What Is JShell?

JShell is a Read-Eval-Print Loop (REPL) tool that allows you to interactively enter Java code and immediately see the results. It’s similar to a command-line interface where you can type Java code snippets, and JShell will execute them in real-time.

Why Is JShell Useful?

JShell offers several benefits to developers:

1. Rapid Prototyping

With JShell, you can quickly prototype and experiment with Java code without the need to create a full Java class or application. This makes it an ideal tool for learning and exploring the language’s features.

2. Code Testing and Debugging

JShell allows you to test and debug small code snippets before incorporating them into your larger Java applications. It’s particularly useful for verifying code behavior and logic.

3. Learning and Teaching

For educators and students, JShell is a valuable tool for teaching and learning Java. It simplifies the process of demonstrating and explaining Java concepts in an interactive manner.

Using JShell

To use JShell, open a terminal or command prompt and type “jshell.” You will enter the JShell environment, where you can start entering Java code. Here’s a basic example:

$ jshell
|  Welcome to JShell -- Version 10
|  For an introduction type: /help intro

jshell> int x = 5;
x ==> 5
jshell> int y = 7;
y ==> 7
jshell> int sum = x + y;
sum ==> 12

In this example, we declared two variables, “x” and “y,” assigned values to them, and then calculated their sum. JShell provides immediate feedback by showing the results of each statement.

Using JShell as a Calculator

JShell can be used as a powerful calculator for performing various mathematical calculations. For instance, you can calculate square roots, trigonometric functions, and more:

jshell> Math.sqrt(25);
$1 ==> 5.0
jshell> Math.sin(Math.toRadians(30));
$2 ==> 0.49999999999999994

Here, JShell is used to calculate the square root of 25 and the sine of 30 degrees.

Defining Methods and Classes

JShell also allows you to define methods and classes interactively. This can be handy for experimenting with new code structures:

jshell> void sayHello(String name) {
   ...> System.out.println("Hello, " + name + "!");
   ...> }
|  created method sayHello(String)

jshell> sayHello("John");
Hello, John!

In this example, we defined a simple “sayHello” method and called it with an argument.

Working with Variables

JShell retains variables and methods throughout your session, allowing you to build on previous work:

jshell> String greeting = "Hi there!";
greeting ==> "Hi there!"

jshell> System.out.println(greeting);
Hi there!

You can see that the “greeting” variable and the “sayHello” method defined earlier are still available for use.

Exiting JShell

To exit JShell, simply type “/exit” or press Ctrl+D. This will end your JShell session, and you’ll return to the command prompt.

Conclusion

JShell, the Java Shell, is a valuable tool for Java developers and learners. It provides an interactive and efficient way to experiment with Java code, test small code snippets, and learn the language’s features. Whether you’re a beginner or an experienced developer, JShell can significantly improve your Java programming experience and productivity.