Import Checkstyle Project

If no project is currently open in IntelliJ IDEA, click Import Project on the Welcome screen. Otherwise, select File > Import Project

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Then Next > Next > ... Until Finish.

Generate sources that a required by checkstyle by right click over pom.xml file and click on menu "Maven / Generate Sources and Update Folders" screenshot

Debug

Open the Check's source file by double click on it in a source tree as is shown:

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Debug the Check by putting the breakpoint at controversial place (double-click) on the left part of line number as it is shown:

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Then right-click the corresponding Unit-test file or class definition > Debug "testName"

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Then manage you debug operations F8 (Step Over), Shift+F8 (Step Out), F7 (Step Into), Alt+F9 (Run to Cursor)

Organize Imports

One of the Checkstyle checks we run on our own code require certain order of import statements. Few changes in IDE settings are required to help your IDE do it automatically.
To change formatter settings please go to File->Settings in menu.
Then in the tree go to: Editor->Code Style->Java, open Import tab (follow numbers on a picture) and apply settings highlighted:

Organize Imports settings in IDEA

Inspections

Checkstyle has its own very strict set of inspections. To import and enable them go to Settings -> Editor -> Inspections -> Manage -> Import... and locate file config/intellij-idea-inspections.xml.

Inspections in IntelliJ IDEA

Not all files in repository should be analyzed. For example test input files contain numerous violations on purpose. Our shared inspection scope can be used to exclude such files.
To add custom scope copy file config/intellij-idea-inspection-scope.xml to .idea/scopes directory. Now it should be ready to use in inspect code window:

Scope for inspections

Disable Auto Indent On Paste

IDEA has it's own rules of indentation when pasting code and it applies these rules to all the lines of the copied code. This may result in wrong formatting of code in many checkstyle files. A simple solution to this problem is by using the key combinations Ctrl + Shift + Alt + V or Edit | Paste Simple. However it is recommended that this settings be changed by default as shown below Disable Auto Indent on Paste