JShell: Java without the ceremony

For most of Java's life, trying one line meant creating a file, declaring a class, writing a main, compiling and running. JShell removed all of that, and it is the fastest way to answer "what does this actually do".

#What it looks like

$ jshell
jshell> int x = 2 + 3
x ==> 5
 
jshell> x * 10
$2 ==> 50
 
jshell> "hello".toUpperCase()
$3 ==> "HELLO"
 
jshell> /exit

No class. No main. No semicolon required at the end of a line. Each expression is evaluated and its result printed — and given a name ($2, $3) so you can use it in the next line.

That is the whole idea: Java as a conversation, the way the terminal is a conversation with the machine.

#What it is for

Checking what a method does. Rather than reading the documentation for String.split and guessing:

jshell> "a,b,,,".split(",").length
$1 ==> 2
 
jshell> "a,b,,,".split(",", -1).length
$2 ==> 5

Two lines, and you now know something that catches people out in production.

Trying an API before committing to it. Import and use it; if it is awkward in JShell it will be awkward in your code.

Settling an argument. "Does Integer.valueOf(128) == Integer.valueOf(128) return true?" is a question with an answer, and it takes eight seconds to get.

Teaching and learning. Every example in this course that prints something is a thing you can paste and watch.

#The commands worth knowing

Anything starting with / is JShell itself rather than Java:

/list is the one you will use. A session drifts, and seeing what you actually typed is often the answer.

#What it will not do

It is a scratchpad, not a development environment. It has no project, no dependency management (you can add a classpath with --class-path, and it is fiddly), and no persistence beyond /save.

It is also not how you should learn program structure. A language where everything is a top-level expression is not Java, and a beginner who lives in JShell arrives at their first class confused about why anything needs a main. Use it to check a fact, then go and write the file.

#Misconceptions

• "JShell is a different Java." Same language, same library, same JVM. It wraps what you type in the machinery Java needs and hides it.
• "Semicolons are optional in Java now." In JShell, at the end of an entry. In a file, no.
• "It is for beginners." It is quickest for people who already know what they are checking. Beginners get the most from it alongside real files, not instead of them.