Java 9 introduced an interactive REPL command-line environment named JShell. This tool allows us to execute Java code snippets and get immediate results. We can easily write code and see the results of its execution without having to create a solution or project. We don't have to wait for the project to finish the build process to check the results of executing many lines of code. JShell, as any other REPL, facilitates exploratory programming, that is, we can easily and interactively try and debug different algorithms and structures.

For example, imagine that we have to interact with an IoT (Internet of Things) library that provides Java bindings. We have to write Java code to use the library to control a drone, also known as a UAV (Unmanned Aerial Vehicle). The drone is an IoT device that interacts with many sensors and actuators, including digital electronic speed controllers linked to engines, propellers, and servomotors.

We want to be able to write a few lines of code to retrieve data from sensors and control the actuators. We just need to make sure things work as explained in the documentation. We want to make sure that the values read from the altimeter change when we move the drone. JShell provides us with the appropriate tool to start interacting with the library in a few seconds. We just need to launch JShell, load the library, and start writing Java 9 code in the REPL. With previous Java versions, we would have needed to create a new project from scratch and write some boilerplate code before we could start writing the first lines of code that interacted with the library. JShell allows us to start working faster and reduces the need to create an entire skeleton to start running Java 9 code. JShell allows interactive exploration of APIs (Application Programming Interfaces) from a REPL.

We can enter any Java 9 definition in JShell. For example, we can declare methods, classes, and variables. We can also enter Java expressions, statements, or imports. Once we have entered the code to declare a method, we can enter a statement that uses the previously defined method and see the results of the execution.

JShell allows us to load source code from a file, and therefore, you will be able to load the source code samples included in this book and evaluate them in JShell. Whenever we have to work with source code, you will know the folder and the file from which you can load it. In addition, JShell allows us to execute JShell commands. We will learn about the most useful commands later, in this chapter.

JShell allows us to call the System.out.printf method to easily format output we want to print. We will take advantage of this method in our sample code.

The semicolon (;) is optional at the end of a statement in JShell. However, we will always use a semicolon at the end of each statement because we don't want to forget that we must use semicolons when we write real-life Java 9 code in projects and solutions. We will only omit the semicolon at the end of a statement when we enter expressions to be evaluated by JShell.

For example, the following two lines are equivalent and both of them will print "Object-Oriented Programming rocks with Java 9!" as a result of their execution in JShell. The first line doesn't include a semicolon (;) at the end of the statement and the second line includes the semicolon (;). We will always use the semicolon (;) as in the second line, to keep consistency.

System.out.printf("Object-Oriented Programming rocks with Java 9!\n")
System.out.printf("Object-Oriented Programming rocks with Java 9!\n");

The following screenshot shows the results of executing the two lines in JShell running on Windows 10:

In some examples, we will take advantage of the fact that JShell provides us networking access. This feature is extremely useful to interact with Web Services. However, you have to make sure that you don't have JShell blocked in your firewall configuration.

By default, JShell provides a set of common imports and we can use the import statement to import the necessary types from any additional package we might need to run our code snippets. We can enter the following command in JShell to list all the imports:

/imports

The following lines show the results of the previous command:

|    import java.io.*
|    import java.math.*
|    import java.net.*
|    import java.nio.file.*
|    import java.util.*
|    import java.util.concurrent.*
|    import java.util.function.*
|    import java.util.prefs.*
|    import java.util.regex.*
|    import java.util.stream.*

As happens when we write Java code outside of JShell, we don't need to import the types from the java.lang package because they are imported by default and they aren't listed when we run the /imports command in JShell. Thus, by default, JShell provides us access to all the types in the following packages:

JShell provides auto-completion features. We just need to press the Tab key whenever we want to have assistance from the auto-complete feature, as done when we work with the Windows Command Prompt or the Terminal in macOS or Linux.

Sometimes, there are too many options that start with the first characters we entered. In these cases, JShell provides us with a list of all the available options to provide us help. For example, we can enter S and press the Tab key. JShell will list all the types imported from the previously listed packages that start with an S. The following screenshot shows the results in JShell:

We want to enter System. Considering the previous list, we will just enter Sys to make sure that System is the only option that starts with Sys. Basically, we are cheating to understand how auto-completion works in JShell. Enter Sys and press the Tab key. JShell will display System.

Now, enter a dot (.) followed by an o (you will have System.o) and press the Tab key. JShell will display System.out.

Next, enter a dot (.) and press the Tab key. JShell will display all the public methods declared in System.out. After the list, JShell will include System.out. again to allow us to continue entering our code. The following screenshot shows the results in JShell:

Enter printl and press the Tab key. JShell will complete to System.out.println(, that is, it will add an n and open parenthesis ((). This way, we just have to enter the arguments for the method because there was just one method that started with printl. Enter "Auto-complete is helpful in JShell"); and press Enter. The following line shows the complete statement:

System.out.println("Auto-complete is helpful in JShell");

The following screenshot shows the results in JShell after running the previous line: