Fix gradle integration

Author: blacelle

plugin-gradle/src/main/java/com/diffplug/gradle/spotless/JavaExtension.java

@@ -19,6 +19,7 @@
 
 import java.io.File;
 import java.util.ArrayList;
+import java.util.Collection;
 import java.util.List;
 import java.util.Objects;
 
@@ -283,9 +284,9 @@ public class CleanthatJavaConfig {
 
 		private String sourceJdk = CleanthatJavaStep.defaultSourceJdk();
 
-		private List<String> mutators = CleanthatJavaStep.defaultMutators();
+		private List<String> mutators = new ArrayList<>(CleanthatJavaStep.defaultMutators());
 
-		private List<String> excludedMutators = CleanthatJavaStep.defaultExcludedMutators();
+		private List<String> excludedMutators = new ArrayList<>(CleanthatJavaStep.defaultExcludedMutators());
 
 		CleanthatJavaConfig() {
 			addStep(createStep());
@@ -319,14 +320,20 @@ public CleanthatJavaConfig clearMutators() {
 			return this;
 		}
 
-		// The fully qualified name of a class implementing eu.solven.cleanthat.engine.java.refactorer.meta.IMutator
-		// or '*' to include all default mutators
+		// An id of a mutator (see IMutator.getIds()) or
+		// tThe fully qualified name of a class implementing eu.solven.cleanthat.engine.java.refactorer.meta.IMutator
 		public CleanthatJavaConfig addMutator(String mutator) {
 			this.mutators.add(mutator);
 			replaceStep(createStep());
 			return this;
 		}
 
+		public CleanthatJavaConfig addMutators(Collection<String> mutators) {
+			this.mutators.addAll(mutators);
+			replaceStep(createStep());
+			return this;
+		}
+
 		// useful to exclude a mutator amongst the default list of mutators
 		public CleanthatJavaConfig excludeMutator(String mutator) {
 			this.excludedMutators.add(mutator);

plugin-gradle/src/test/java/com/diffplug/gradle/spotless/CleanthatJavaIntegrationTest.java

@@ -31,7 +31,9 @@ void integration() throws IOException {
 				"spotless {",
 				"    java {",
 				"        target file('test.java')",
-				"        cleanthat().sourceCompatibility('11')",
+				"        cleanthat()" +
+				"          .sourceCompatibility('11')",
+				"          .addMutators(['LiteralsFirstInComparisons', 'OptionalNotEmpty'])",
 				"    }",
 				"}");