Using an IDE

To open the launcher in an IDE, you need to install a plugin for Project Lombok.

Project Lombok is used to generate code automatically. For example:

public class Example {
    @Getter
    private int age;
}

Generates:

public class Example {
    private int age;

    public int getAge() {
        return age;
    }
}

Since it's automatic, you need the plugin.

IntelliJ IDEA

The plugin can be installed by browsing the plugin repository from IntelliJ IDEA's settings.

In addition, you need to enable annotation processors in the settings: