JShell: Java's Built-In Scratchpad for Trying Code Fast

JShell, a read-eval-print loop (REPL) included with the JDK since Java 9, allows developers to quickly test Java code snippets without the overhead of creating a class or main method. It supports expressions, variables, methods, and classes, with features like automatic semicolons, scratch variables, and in-place method redefinition. Key commands include /list, /vars, /methods, /save, and /open, enabling efficient experimentation and prototyping.

Here's a familiar feeling. You want to test one small thing in Java — maybe how String.substring behaves, or whether a regex pattern matches. So you create a file, write a class, add a main method, type System.out.println , compile, run. All that ceremony for a one-line answer. JShell does away with the ceremony. It's a read-eval-print loop REPL that ships with the JDK since Java 9. You type Java code, press Enter, and see the result immediately. No class, no main method, no compile step. Think of it as a calculator for Java. Open a terminal and type: jshell If your JDK is installed and on your PATH, you'll see a welcome message and the jshell prompt. That's it. You're in. If you get a "command not found" error, double-check that the JDK not just the JRE is installed and the bin folder is on your PATH. In JShell, the bits of code you type are called snippets . A snippet can be an expression, a variable, a method, or even a full class. Let's try a few: php jshell int age = 25 age == 25 jshell age 2 $2 == 50 jshell "hello".toUpperCase $3 == "HELLO" Notice two things. First, no semicolons needed. JShell adds them for you if you forget. Second, when you type an expression without storing it in a variable, JShell creates a scratch variable for you automatically $2 , $3 , and so on . You can reuse these later: php jshell $2 + 10 $5 == 60 This is handy when you're poking around and don't want to name every intermediate result. You can define full methods in JShell, not just expressions. Here's a simple one: jshell int square int n { ... return n n; ... } | created method square int jshell square 7 $7 == 49 The ... prompt means JShell knows you're in the middle of a multi-line snippet and is waiting for you to finish. Close the braces and press Enter. Classes work the same way. You can define a class, instantiate it, and call methods on it, all without leaving the prompt. This is where JShell gets genuinely useful. If you redefine a method or variable, the old version is replaced. No need to start over. jshell String grade int score { ... if score = 90 return "Pass"; ... return "Fail"; ... } | created method grade int jshell grade 85 $3 == "Fail" That pass threshold feels too strict. Just retype the method with a change: jshell String grade int score { ... if score = 80 return "Pass"; ... return "Fail"; ... } | modified method grade int jshell grade 85 $5 == "Pass" The method updated in place. Any other methods that called grade will use the new version automatically. For longer snippets, use the /edit command to pop open a text editor instead of retyping: jshell /edit grade Make your changes, save, and close the editor. JShell picks up the new version. JShell has about 20 commands, all starting with a forward slash. You won't need all of them. Here are the ones that matter day to day: /list show everything you've typed, with IDs /vars show all variables and their values /methods show all methods you've defined /types show classes, interfaces, and enums /imports show active imports To manage your session: /save myfile.jsh save all snippets to a file /open myfile.jsh load snippets back in later /reset wipe everything and start fresh /drop 3 remove a specific snippet by ID /history see everything typed, in order /exit leave JShell Pro tip: command abbreviations work as long as they're unique. /l runs /list . /v runs /vars . /sa runs /save since /s alone is ambiguous between /save and /set . Press Tab while typing and JShell fills in the rest. This works for commands, class names, and method names. If there's more than one option, Tab shows you all of them. A few shortcuts worth knowing: / /-3 The Shift+Tab then V shortcut is a neat trick: type an expression, hit that combo, and JShell converts it into a variable declaration with the correct type filled in. By default, JShell shows a reasonable amount of feedback. But when you're learning, more detail helps. Switch to verbose mode: jshell /set feedback verbose Verbose mode explains what happened after each snippet — what was created, modified, or dropped, and what type it is. Once you're comfortable, switch back to normal or concise: jshell /set feedback normal jshell /set feedback concise There's also silent mode if you want zero output useful when piping JShell into scripts . One complaint people have about REPLs: you lose everything when you close them. JShell solves this with /save and /open . Before you exit, save your work: /save practice.jsh Next time you start JShell, load it back: /open practice.jsh You can also pass a file directly when launching: jshell practice.jsh This makes JShell practical for more than throwaway experiments. Keep a .jsh file of helper methods you use often, and load it whenever you start a session. JShell isn't a replacement for your IDE. It's a tool for specific moments: LocalDate.of 2026, 2, 31 does? Find out in two seconds instead of writing a test class.The thing I like most about JShell is that it removes the fear of experimentation. In a normal Java project, trying something feels expensive. In JShell, it costs nothing. You type, you see, you move on. jshell to start it. main method required. $1 , $2 , ... hold results you didn't name. /list , /vars , /methods , /save , /open , /reset , and /exit cover most of what you need. / reruns your last snippet. /set feedback verbose while learning, then dial it back. /save and reload them with /open .Based on dev.java/learn — JShell: The Java Shell Tool https://dev.java/learn/jshell-tool/