Java

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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:

#!/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.

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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.

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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:

public class Main {
    public static void main(String[] args) {
        Foo.foo();  // calls the method foo() from Foo class in Foo.java
    }
}

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:

package com.example

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

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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:

<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.