MapTool uses Swing for its user interface (UI) components. This document provides instructions for setting up the tools needed to work on the UI components of MapTool. All of the Swing components are created using the IntelliJ IDEA GUI Designer. IntelliJ IDEA is the recommended IDE for working on the MapTool project. JetBrains provides a Community Edition of IntelliJ IDEA that is free to use. At time of writing you need to scroll down the page a little to find the Community Edition download link.
There is a plugin for IntelliJ IDEA called "UI Designer" that is required to work on the UI components of MapTool. The following tutorial describes how to use the UI Designer.
Note that the UI Designer is only required if you plan to modify the UI, the project can be built using gradle without needing either IntelliJ IDEA or the UI Designer.