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Tooling Beginner

JShell for prototyping

Try Java expressions interactively without creating files.

Tooling

Code Comparison

✕ Java 8

// 1. Create Test.java
// 2. javac Test.java
// 3. java Test
// Just to test one expression!

✓ Java 9+

$ jshell
jshell> "hello".chars().count()
$1 ==> 5
jshell> List.of(1,2,3).reversed()
$2 ==> [3, 2, 1]

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Why the modern way wins

⚡

Instant feedback

Type an expression, see the result immediately.

📝

No files needed

No .java files, no compilation step.

🔍

API exploration

Tab completion helps discover methods and parameters.

Old Approach

Create File + Compile + Run

Modern Approach

jshell REPL

Since JDK

Difficulty

Beginner

JDK Support

JShell for prototyping

Available

Widely available since JDK 9 (Sept 2017)

How it works

JShell is a Read-Eval-Print Loop for Java. Test expressions, experiment with APIs, and prototype code without creating files, compiling, or writing a main method. Tab completion and inline docs included.

AOT class preloading

Java 8

// Every startup:
// - Load 10,000+ classes
// - Verify bytecode
// - JIT compile hot paths
// Startup: 2-5 seconds

Java 25

// Training run:
$ java -XX:AOTCacheOutput=app.aot \
    -cp app.jar com.App
// Production:
$ java -XX:AOTCache=app.aot \
    -cp app.jar com.App

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JDK 25+ →

Tooling Advanced

Compact object headers

Java 8

// Default: 128-bit object header
// = 16 bytes overhead per object
// A boolean field object = 32 bytes!
// Mark word (64) + Klass pointer (64)

Java 25

// -XX:+UseCompactObjectHeaders
// 64-bit object header
// = 8 bytes overhead per object
// 50% less header memory
// More objects fit in cache

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JDK 25+ →

Tooling Intermediate

Multi-file source launcher

Java 8

$ javac *.java
$ java Main
// Must compile all files first
// Need a build tool for dependencies

Java 22+

$ java Main.java
// Automatically finds and compiles
// other source files referenced
// by Main.java

Hover to see modern ➜

JDK 22+ →