JShell

// ── Before Java 9 — testing one expression required a full class+build cycle
// Scratch.java
public class Scratch {
    public static void main(String[] args) {
        System.out.println(String.format("%.2f", 3.14159));
    }
}
// $ javac Scratch.java
// $ java Scratch
// 3.14
// Three steps (write file, compile, run) just to check one format string

// ── Java 9+ — JShell evaluates the same expression immediately ─────────
// $ jshell
// jshell> String.format("%.2f", 3.14159)
// $1 ==> "3.14"
//
// No class wrapper, no main method, no separate compile step — the result
// (and JShell's auto-assigned scratch variable $1) appears instantly.

// jshell> 2 + 2
// $1 ==> 4

// jshell> $1 * 10                      // referring back to a previous scratch result
// $2 ==> 40

// jshell> int x = 10
// x ==> 10

// jshell> int x = 20                   // redeclaring "x" simply replaces the earlier one
// x ==> 20

// ── Forward references — calling a not-yet-defined method is accepted ──
// jshell> int square(int n) { return doubleIt(n) * n / 2; }   // doubleIt() doesn't exist yet
// |  created method square(int), however, it cannot be invoked until method
// |  doubleIt(int) is declared

// jshell> int doubleIt(int n) { return n * 2; }   // now define it
// |  created method doubleIt(int)

// jshell> square(5)                     // NOW it resolves and runs correctly
// $3 ==> 25

// ── Bare statements run immediately, no main() wrapper needed ──────────
// jshell> for (int i = 0; i < 3; i++) { System.out.println("Hi " + i); }
// Hi 0
// Hi 1
// Hi 2

// jshell> List<String> names = List.of("Ann", "Bo", "Cy")
// names ==> [Ann, Bo, Cy]

// jshell> names.forEach(System.out::println)
// Ann
// Bo
// Cy

// ── Listing what's currently defined in the session ─────────────────────
// jshell> /vars
// |    int x = 20
// |    List<String> names = [Ann, Bo, Cy]

// jshell> /methods
// |    int square(int)
// |    int doubleIt(int)

// jshell> /list
// 1 : int x = 10;
// 2 : int x = 20;
// 3 : int square(int n) { return doubleIt(n) * n / 2; }
// 4 : int doubleIt(int n) { return n * 2; }

// ── Saving and reloading a session ──────────────────────────────────────
// jshell> /save my-session.jsh
// jshell> /exit
// $ jshell
// jshell> /open my-session.jsh        // replays all saved snippets in order

// ── Loading a plain file of statements into the running session ────────
// setup.jsh:
//   List<String> users = new ArrayList<>();
//   users.add("alice");
//   users.add("bob");
// jshell> /open setup.jsh
// jshell> users
// users ==> [alice, bob]

// ── Using external/project classes from the classpath ───────────────────
// $ jshell -cp target/classes:libs/some-library.jar
// jshell> import com.myapp.UserService
// jshell> UserService svc = new UserService()
// jshell> svc.findUser("alice")