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Lesson 11: JShell – Java’s Interactive REPL (Java 9+)

javasleep

March 13, 2025

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JShell is Java’s Read-Eval-Print Loop (REPL), introduced in Java 9, allowing developers to run Java code interactively without compiling a full program.

1. What is JShell?

✅ Instantly execute Java code – No need to create a full main() method.
✅ Great for quick testing and learning – Experiment with APIs before using them in projects.
✅ Improves developer productivity – Quickly check logic without setting up a project.

2. How to Start JShell?

🔹 Step 1: Open your terminal or command prompt.
🔹 Step 2: Run JShell:

jshell

🔹 Step 3: Start typing Java code interactively!

✅ Example: Running simple Java commands in JShell

jshell> int x = 5;
x ==> 5

jshell> x + 10
$2 ==> 15

✔ JShell evaluates expressions immediately.

3. Key Features of JShell

4. Running Code in JShell

📌 Example 1: Defining and Using a Variable

jshell> int age = 25;
age ==> 25

jshell> age + 5
$2 ==> 30

✔ JShell automatically evaluates expressions.

📌 Example 2: Writing Methods in JShell

jshell> int square(int n) { return n * n; }
|  Created method square(int)

jshell> square(4)
$3 ==> 16

✔ No need to define a class or main() method!

📌 Example 3: Creating a Simple Class

jshell> class Person {
   ...>     String name;
   ...>     int age;
   ...>     Person(String n, int a) { name = n; age = a; }
   ...>     void display() { System.out.println(name + " is " + age + " years old."); }
   ...> }
|  Created class Person

jshell> Person p = new Person("Alice", 30);
p ==> Person@6a6824be

jshell> p.display()
Alice is 30 years old.

✔ JShell allows defining and using classes dynamically.

5. Useful JShell Commands

✅ Example: Checking all variables

jshell> /vars
age : int = 25
p : Person = Person@6a6824be

6. Using JShell in Real-World Scenarios

✅ Quickly test Java code snippets before adding them to a project
✅ Experiment with Java Streams, Collections, and Lambdas
✅ Debug logic without writing an entire program

📌 Example: Testing Streams API in JShell

jshell> List.of("Alice", "Bob", "Charlie").stream()
   ...>     .filter(name -> name.startsWith("A"))
   ...>     .forEach(System.out::println);
Alice

✔ JShell helps quickly verify Java 8+ Stream API behavior.

Lesson Reflection

1. How does JShell improve developer productivity compared to writing full Java programs?
2. When would you use JShell instead of a traditional Java file?
3. Can you think of a scenario where JShell would be useful in debugging?

Answers to Reflection Questions on JShell

1️⃣ How does JShell improve developer productivity compared to writing full Java programs?

✅ Eliminates the Need for Boilerplate Code

• Traditional Java programs require a class, a main() method, and imports, even for simple tasks.
• JShell allows running Java code instantly without writing a full program.

📌 Example: Testing a Simple Calculation

🔴 Traditional Java Approach (Full Class & Main Method)

public class Main {
    public static void main(String[] args) {
        int result = 10 * 5;
        System.out.println(result);
    }
}

✅ JShell Approach (No Setup Needed!)

jshell> 10 * 5
$1 ==> 50

🚀 JShell executes it instantly—no need for a full program!

2️⃣ When would you use JShell instead of a traditional Java file?

✅ 1. When testing small Java snippets quickly

• Instead of creating a full project, you can test methods, loops, and expressions interactively.

✅ 2. When learning Java APIs or new features

• If you want to explore Java Streams, Optional, or new Java features, JShell is the fastest way to test them.

📌 Example: Testing Streams in JShell

jshell> List.of("Alice", "Bob", "Charlie").stream()
   ...>     .map(String::toUpperCase)
   ...>     .forEach(System.out::println);
ALICE
BOB
CHARLIE

✔ JShell lets you experiment with Java Streams in real time!

✅ 3. When debugging logic without recompiling code

• JShell remembers previous variables and methods, so you can test fixes instantly.

3️⃣ Can you think of a scenario where JShell would be useful in debugging?

🚀 Scenario: Debugging a Method Without Running the Full Application

📌 Problem:
A developer is working on a method to reverse a string but isn’t sure if it works correctly.
🔴 Traditional Debugging Approach:

• Edit code → Compile → Run → Print debug output → Repeat if needed.

✅ JShell Debugging Approach:

jshell> String reverse(String str) { return new StringBuilder(str).reverse().toString(); }
|  Created method reverse(String)

jshell> reverse("hello")
$2 ==> "olleh"

✔ JShell helps test the method immediately—no compilation needed!

🔍 Key Takeaways

✔ JShell removes boilerplate code and allows quick Java execution.
✔ JShell is useful for testing, learning Java APIs, and debugging code interactively.
✔ It is best used for quick experiments, but full Java files are still needed for real-world applications.

The next Java 10+ feature: Local Variable Type Inference (var)? 😊🚀