Also test against an empty project where no plugin is applied.

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginCheckstyleTest.groovy

@@ -8,6 +8,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addCheckstyle
 
 class CodeQualityToolsPluginCheckstyleTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addCheckstyle(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert addCheckstyle(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
@@ -82,29 +88,25 @@ class CodeQualityToolsPluginCheckstyleTest extends CommonCodeQualityToolsTest {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.checkstyle.include = ['*.java']
 
-    for (def project : projects) {
-      assert addCheckstyle(project, rootProject, extension)
-      def task = project.tasks.findByName('checkstyle')
+    assert addCheckstyle(javaProject, rootProject, extension)
+    def task = javaProject.tasks.findByName('checkstyle')
 
-      task.with {
-        assert includes.size() == 1
-        assert includes.contains('*.java')
-      }
+    task.with {
+      assert includes.size() == 1
+      assert includes.contains('*.java')
     }
   }
 
   @Test void exclude() {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.checkstyle.exclude = ['**/gen']
 
-    for (def project : projects) {
-      assert addCheckstyle(project, rootProject, extension)
-      def task = project.tasks.findByName('checkstyle')
+    assert addCheckstyle(javaProject, rootProject, extension)
+    def task = javaProject.tasks.findByName('checkstyle')
 
-      task.with {
-        assert excludes.size() == 1
-        assert excludes.contains('**/gen')
-      }
+    task.with {
+      assert excludes.size() == 1
+      assert excludes.contains('**/gen')
     }
   }
 

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginDetektTest.groovy

@@ -6,6 +6,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addDetekt
 
 class CodeQualityToolsPluginDetektTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addDetekt(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert !addDetekt(project, rootProject, new CodeQualityToolsPluginExtensionForTests())

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginFindbugsTest.groovy

@@ -8,6 +8,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addFindbugs
 
 class CodeQualityToolsPluginFindbugsTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addFindbugs(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert addFindbugs(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
@@ -94,20 +100,16 @@ class CodeQualityToolsPluginFindbugsTest extends CommonCodeQualityToolsTest {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.findbugs.effort = 'medium'
 
-    for (def project : projects) {
-      assert addFindbugs(project, rootProject, extension)
-      assert project.findbugs.effort == 'medium'
-    }
+    assert addFindbugs(javaProject, rootProject, extension)
+    assert javaProject.findbugs.effort == 'medium'
   }
 
   @Test void reportLevel() {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.findbugs.reportLevel = 'medium'
 
-    for (def project : projects) {
-      assert addFindbugs(project, rootProject, extension)
-      assert project.findbugs.reportLevel == 'medium'
-    }
+    assert addFindbugs(javaProject, rootProject, extension)
+    assert javaProject.findbugs.reportLevel == 'medium'
   }
 
   @Test void failEarlyFalse() {

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginKtlintTest.groovy

@@ -6,6 +6,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addKtlint
 
 class CodeQualityToolsPluginKtlintTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addKtlint(project, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert !addKtlint(project, new CodeQualityToolsPluginExtensionForTests())

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginLintTest.groovy

@@ -6,6 +6,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addLint
 
 class CodeQualityToolsPluginLintTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addLint(project, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert !addLint(project, new CodeQualityToolsPluginExtensionForTests())

src/test/groovy/com/vanniktech/code/quality/tools/CodeQualityToolsPluginPmdTest.groovy

@@ -8,6 +8,12 @@ import org.junit.Test
 import static com.vanniktech.code.quality.tools.CodeQualityToolsPlugin.addPmd
 
 class CodeQualityToolsPluginPmdTest extends CommonCodeQualityToolsTest {
+  @Test void empty() {
+    emptyProjects.each { project ->
+      assert !addPmd(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
+    }
+  }
+
   @Test void java() {
     javaProjects.each { project ->
       assert addPmd(project, rootProject, new CodeQualityToolsPluginExtensionForTests())
@@ -96,29 +102,25 @@ class CodeQualityToolsPluginPmdTest extends CommonCodeQualityToolsTest {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.pmd.include = ['*.java']
 
-    for (def project : projects) {
-      assert addPmd(project, rootProject, extension)
-      def task = project.tasks.findByName('pmd')
+    assert addPmd(javaProject, rootProject, extension)
+    def task = javaProject.tasks.findByName('pmd')
 
-      task.with {
-        assert includes.size() == 1
-        assert includes.contains('*.java')
-      }
+    task.with {
+      assert includes.size() == 1
+      assert includes.contains('*.java')
     }
   }
 
   @Test void exclude() {
     def extension = new CodeQualityToolsPluginExtensionForTests()
     extension.pmd.exclude = ['**/gen']
 
-    for (def project : projects) {
-      assert addPmd(project, rootProject, extension)
-      def task = project.tasks.findByName('pmd')
+    assert addPmd(javaProject, rootProject, extension)
+    def task = javaProject.tasks.findByName('pmd')
 
-      task.with {
-        assert excludes.size() == 1
-        assert excludes.contains('**/gen')
-      }
+    task.with {
+      assert excludes.size() == 1
+      assert excludes.contains('**/gen')
     }
   }
 

src/test/groovy/com/vanniktech/code/quality/tools/CommonCodeQualityToolsTest.groovy

@@ -17,6 +17,9 @@ import org.junit.Before
 abstract class CommonCodeQualityToolsTest {
   Project rootProject
 
+  Project[] emptyProjects
+  Project emptyProject
+
   Project[] javaProjects
   Project javaProject
   Project javaLibraryProject
@@ -41,6 +44,8 @@ abstract class CommonCodeQualityToolsTest {
   @Before void setUp() {
     rootProject = ProjectBuilder.builder().withName('root').build()
 
+    emptyProject = ProjectBuilder.builder().withName('empty').withParent(rootProject).build()
+
     javaProject = ProjectBuilder.builder().withName('java').withParent(rootProject).build()
     javaProject.plugins.apply(JavaPlugin)
 
@@ -68,6 +73,10 @@ abstract class CommonCodeQualityToolsTest {
     androidFeatureProject = ProjectBuilder.builder().withName('android feature').build()
     androidFeatureProject.plugins.apply(FeaturePlugin)
 
+    emptyProjects = [
+        emptyProject
+    ]
+
     javaProjects = [
         javaProject,
         javaLibraryProject
@@ -86,7 +95,7 @@ abstract class CommonCodeQualityToolsTest {
         androidFeatureProject
     ]
 
-    projects = javaProjects + kotlinProjects + androidProjects
+    projects = emptyProjects + javaProjects + kotlinProjects + androidProjects
   }
 
   static boolean taskDependsOn(final Task task, final String taskName) {