Add Maven Checkstyle

.github/workflows/build.yml

@@ -45,5 +45,7 @@ jobs:
         else
            mvn --batch-mode --update-snapshots verify
         fi
+    - name: Checkstyle analysis
+      run: mvn --batch-mode checkstyle:check
     - name: SpotBugs analysis
       run: mvn --batch-mode spotbugs:check

checkstyle.xml

@@ -0,0 +1,31 @@
+<?xml version="1.0"?>
+<!DOCTYPE module PUBLIC
+    "-//Checkstyle//DTD Checkstyle Configuration 1.3//EN"
+    "https://checkstyle.org/dtds/configuration_1_3.dtd">
+
+<!--
+  Lean ruleset: correctness/maintainability checks only, not code style.
+-->
+<module name="Checker">
+    <module name="TreeWalker">
+        <module name="EqualsHashCode"/>
+        <module name="EqualsAvoidNull"/>
+        <module name="EmptyCatchBlock"/>
+        <module name="EmptyStatement"/>
+        <module name="EmptyBlock">
+            <property name="option" value="text"/>
+            <property name="tokens" value="LITERAL_TRY, LITERAL_FINALLY, LITERAL_IF, LITERAL_ELSE"/>
+        </module>
+        <module name="UnusedImports"/>
+        <module name="RedundantImport"/>
+        <module name="SimplifyBooleanExpression"/>
+        <module name="SimplifyBooleanReturn"/>
+        <module name="StringLiteralEquality"/>
+        <module name="ModifiedControlVariable"/>
+        <module name="MissingSwitchDefault"/>
+        <module name="FallThrough"/>
+        <module name="OneStatementPerLine"/>
+        <module name="CovariantEquals"/>
+        <module name="NoFinalizer"/>
+    </module>
+</module>

pom.xml

@@ -403,6 +403,16 @@
 				<artifactId>sonar-maven-plugin</artifactId>
 				<version>5.8.0.7211</version>
 			  </plugin>
+			  <plugin>
+				<groupId>org.apache.maven.plugins</groupId>
+				<artifactId>maven-checkstyle-plugin</artifactId>
+				<version>3.6.0</version>
+				<configuration>
+					<configLocation>checkstyle.xml</configLocation>
+					<failOnViolation>true</failOnViolation>
+					<violationSeverity>warning</violationSeverity>
+				</configuration>
+			  </plugin>
 			  <plugin>
 				<groupId>com.github.spotbugs</groupId>
 				<artifactId>spotbugs-maven-plugin</artifactId>

src/main/java/org/spdx/tools/Verify.java

@@ -228,10 +228,9 @@ public static List<String> verify(String filePath, SerFileType fileType) throws
 			for (String verifyMsg:verify) {
 				if (!retval.contains(verifyMsg)) {
 					// Check for deprecated licenses - should be warnings, not errors
-					if (verifyMsg.contains(" is deprecated.")) {
-						verifyMsg = verifyMsg.replaceAll("error:", "warning:");
-					}
-					retval.add(verifyMsg);
+					String msg = verifyMsg.contains(" is deprecated.")
+							? verifyMsg.replaceAll("error:", "warning:") : verifyMsg;
+					retval.add(msg);
 				}
 			}
 		}

src/main/java/org/spdx/tools/schema/AbstractOwlRdfConverter.java

@@ -221,7 +221,7 @@ private void interpretRestrictions(List<OntClass.Restriction> restrictions) {
 				// get the type from the range of the property
 				property.ranges().forEach(range -> {
 					if (range.isURIResource()) {
-						if (Objects.isNull(typeUri) || typeUri.equals("http://www.w3.org/2000/01/rdf-schema#Literal")) {
+						if (Objects.isNull(typeUri) || "http://www.w3.org/2000/01/rdf-schema#Literal".equals(typeUri)) {
 							typeUri = range.asResource().getURI();
 						}
 					}