checkstyle.xml

<?xml version="1.0"?>
<!DOCTYPE module PUBLIC
	"-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
	"https://checkstyle.org/dtds/configuration_1_3.dtd">

<module name="Checker">

	<!--Basic Settings-->
	<!--Warning severity so the builds do not fail because of checkstyle, this is mainly for the GitHub workflow-->
	<property name="severity" value="warning"/>
	<property name="fileExtensions" value="java"/>
	<module name="BeforeExecutionExclusionFileFilter">
		<property name="fileNamePattern" value="module\-info\.java$"/>
	</module>

	<!--At most 120 characters per line-->
	<module name="LineLength">
		<property name="max" value="120"/>
	</module>

	<!--New line at the end of the file-->
	<module name="NewlineAtEndOfFile"/>

	<module name="TreeWalker">

		<!--Tabs, no spaces-->
		<module name="RegexpSinglelineJava">
			<property name="format" value="^\t* "/>
			<property name="message" value="Indent must use tab characters"/>
			<property name="ignoreComments" value="true"/>
		</module>

		<!--No trailing whitespace-->
		<module name="NoWhitespaceAfter" />

		<!--When statements consume multiple lines, all lines but the first have two tabs of additional indentation-->
		<module name="Indentation">
			<property name="arrayInitIndent" value="8" />
			<property name="basicOffset" value="8" />
			<property name="caseIndent" value="8" />
			<property name="lineWrappingIndentation" value="8" />
			<property name="throwsIndent" value="8" />
		</module>

		<!--Each class begins with an empty line-->
		<module name="EmptyLineSeparator">
			<property name="allowNoEmptyLineBetweenFields" value="true" />
			<property name="tokens"
					  value="IMPORT, STATIC_IMPORT, CLASS_DEF, INTERFACE_DEF,
					  		ENUM_DEF, STATIC_INIT, INSTANCE_INIT, METHOD_DEF,
					  		CTOR_DEF, VARIABLE_DEF, RECORD_DEF, COMPACT_CTOR_DEF" />
		</module>

		<module name="OneStatementPerLine"/>

		<!--Annotations for a structure go on the line before that structure-->
		<module name="AnnotationLocation"/>

		<!--When splitting Strings into multiple lines the last part of the string must be (space character included) " " +-->
		<module name="OperatorWrap">
			<property name="option" value="eol" />
			<property name="tokens" value="PLUS" />
		</module>

		<!--Class names are written in UpperCamelCase-->
		<module name="TypeName"/>

		<!--Methods named in camelCase-->
		<module name="MethodName"/>

		<!--Static constant fields should be named in UPPER_SNAKE_CASE-->
		<module name="ConstantName"/>

		<!--We use JetBrains Annotations for specifying null-ness-->
		<module name="IllegalImport">
			<property name="illegalClasses"
					  value="javax.annotation.Nonnull,
							javax.annotation.Nullable,
							org.eclipse.jdt.annotation.NonNull,
							org.eclipse.jdt.annotation.Nullable,
							org.eclipse.sisu.Nullable,
							org.checkerframework.checker.nullness.qual.NonNull,
							org.checkerframework.checker.nullness.qual.Nullable" />
			<property name="illegalPkgs" value="" />
		</module>

		<!--Modules for code improvements-->
		<module name="MissingOverride"/>
		<module name="EmptyBlock"/>
		<module name="HideUtilityClassConstructor"/>
		<module name="EmptyStatement"/>
		<module name="EqualsHashCode"/>
		<module name="SimplifyBooleanExpression"/>
		<module name="SimplifyBooleanReturn"/>
		<module name="StringLiteralEquality"/>
		<module name="UnusedLocalVariable"/>

	</module>

</module>