Getting Started with JShell: A Quick Guide

Getting Started with JShell: A Quick Guide | Generated by AI

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JShell is an interactive REPL (Read-Eval-Print Loop) tool included with the Java Development Kit (JDK) starting from JDK 9. It allows you to execute Java code snippets interactively without needing to write a full program. Below is a concise guide on how to use JShell:

1. Starting JShell

Ensure JDK is installed: Make sure you have JDK 9 or later installed. You can check by running java -version in your terminal or command prompt.
Launch JShell:
- Open a terminal or command prompt.
- Type jshell and press Enter.
- You’ll see a prompt like jshell> indicating JShell is ready.

2. Basic Usage

Running Java Code:
- Type Java expressions, statements, or declarations directly at the prompt.
- Example:
```
jshell> System.out.println("Hello, JShell!")
Hello, JShell!
```
- JShell evaluates the input and displays the result immediately.
Variables and Expressions:
- Declare variables or evaluate expressions:
```
jshell> int x = 10
x ==> 10
jshell> x * 2
$2 ==> 20
```
- JShell automatically assigns temporary names (e.g., $2) to expression results.

Defining Methods and Classes:

You can define methods or classes:

jshell> void sayHello() { System.out.println("Hello!"); }
|  created method sayHello()
jshell> sayHello()
Hello!

jshell> class Test { int x = 5; }
|  created class Test
jshell> Test t = new Test()
t ==> Test@7c417213
jshell> t.x
$5 ==> 5

3. Key Commands

JShell provides built-in commands starting with /. Here are some essential ones:

List all code: /list – Shows all snippets entered in the session.
```
jshell> /list
```
Edit code: /edit <id> – Opens a GUI editor for the snippet with the given ID (from /list).
Save session: /save myfile.java – Saves all snippets to a file.
Load file: /open myfile.java – Loads and executes code from a file.
View variables: /vars – Lists all declared variables.
```
jshell> /vars
|    int x = 10
```
View methods: /methods – Lists all defined methods.
Exit JShell: /exit – Closes the JShell session.
Help: /help – Displays all available commands.

4. Importing Packages

JShell automatically imports common packages (e.g., java.util, java.io). To use others, import them manually:
```
jshell> import java.time.LocalDate
jshell> LocalDate.now()
$3 ==> 2025-06-27
```

5. Editing and Correcting Code

Modify existing code:
- Use /list to find the ID of a snippet.
- Redefine it by typing a new version. JShell overwrites the old definition.
```
jshell> int x = 5
x ==> 5
jshell> int x = 10
x ==> 10
```
Drop a snippet: /drop <id> – Removes a specific snippet by its ID.

6. Tab Completion

Press Tab to auto-complete class names, methods, or commands.
Example:
```
jshell> System.out.pr<tab>
```
This suggests println, print, etc.

7. Running External Scripts

Load and run a Java file:
```
jshell> /open MyScript.java
```
You can also start JShell with a script:
```
jshell MyScript.java
```

8. Customizing JShell

Startup scripts: Create a file (e.g., startup.jsh) with imports or common code, then load it with /open startup.jsh or set it as the default startup script:
```
jshell --start startup.jsh
```
Feedback modes: Adjust output verbosity with /set feedback <mode> (e.g., normal, concise, verbose).

9. Tips

Quick prototyping: Use JShell to test small code snippets, experiment with APIs, or learn Java syntax.
Error handling: JShell provides clear error messages and lets you correct mistakes on the fly.
Multi-line input: For complex code, JShell supports multi-line editing. End a line with a backslash (\) to continue:
```
jshell> int sum(int a, int b) { \
    ...> return a + b; \
    ...> }
```

10. Troubleshooting

JShell not found: Ensure the JDK bin directory is in your system’s PATH.
Slow startup: On some systems, JShell may take a moment to initialize. Ensure your JDK is up to date.
Reset session: Use /reset to clear all snippets and start fresh.

For more details, check the official JDK documentation or run /help in JShell. If you have specific tasks or code you want to try in JShell, let me know, and I can guide you further!

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