> ## Documentation Index
> Fetch the complete documentation index at: https://docs.codecrafters.io/llms.txt
> Use this file to discover all available pages before exploring further.

# Java

> Learn how to solve CodeCrafters challenges in Java

## Local development

To run the code for your challenge locally, you'll need `java` & `mvn` installed. Our test runners use Java 21 (as of November 2023).

The script for your challenge (like `./your_bittorrent.sh`) will automatically compile your code using `mvn` before executing it. It'll
look something like this:

```sh theme={null}
#!/bin/sh
set -e
mvn -B --quiet package -Ddir=/tmp/codecrafters-redis-target
exec java -jar /tmp/codecrafters-redis-target/java_redis.jar "$@"
```

When running tests on our servers, the `mvn` command will only be run once and then commented out. This ensures that each test only runs the `java -jar ...` line.

## File structure

* The files for your solution are placed in the `src/main/java` directory.
* `src/main/java/Main.java` contains the `main` function, which is what the test runner executes.

## Adding more files

You can add more files and directories to `src/main/java`. The test runner will include them when compiling your code.

For example, if you added a file at `src/main/java/Foo.java`, you could use it like so:

<CodeGroup>
  ```java src/main/java/Main.java theme={null}
  public class Main {
      public static void main(String[] args) {
          Foo.foo();  // calls the method foo() from Foo class in Foo.java
      }
  }
  ```

  ```java src/main/java/Foo.java theme={null}
  public class Foo {
      public static void foo() {
          System.out.println("Hello world!");
      }
  }
  ```
</CodeGroup>

By default, all files in `src/main/java` are in the `default` package. If you want to extract files into a different package,
you'll need to add a `package` declaration to the top of the file and change its location. Here's what that'd look like:

Let's say you wanted to extract `Foo.java` into the `com.example` package. Store `Foo.java` at `src/main/java/com/example/Foo.java` and add the following lines:

<CodeGroup>
  ```java src/main/java/com/example/Foo.java theme={null}
  package com.example

  public class Foo {
      public static void foo() {
          System.out.println("Hello world!");
      }
  }
  ```

  ```java src/main/java/Main.java theme={null}
  import com.example.Foo;

  public class Main {
      public static void main(String[] args) {
        Foo.foo();  // Uses the foo function from Foo class in the com.example package
      }
  }
  ```
</CodeGroup>

<Tip>
  Use an IDE like IntelliJ IDEA which will automatically offer to move your file
  to a directory matching the package name.
</Tip>

## Adding dependencies

You can add dependencies to your project by adding them to your `pom.xml` file.

For example, to add the gson library, add the following to the `<dependencies>` section in your `pom.xml`:

```xml theme={null}
<dependency>
  <groupId>com.google.code.gson</groupId>
  <artifactId>gson</artifactId>
  <version>2.8.6</version>
</dependency>
```

When you push your code next, the runner will automatically download the dependency.