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:

#!/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:

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

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

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!");
    }
}

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
    }
}

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

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.

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Local development

File structure

Adding more files

Adding dependencies

​Local development

​File structure

​Adding more files

​Adding dependencies

Local development

File structure

Adding more files

Adding dependencies