diff --git a/src/main/java/org/itsallcode/matcher/MismatchReporter.java b/src/main/java/org/itsallcode/matcher/MismatchReporter.java
index 488a523..6723402 100644
--- a/src/main/java/org/itsallcode/matcher/MismatchReporter.java
+++ b/src/main/java/org/itsallcode/matcher/MismatchReporter.java
@@ -9,7 +9,7 @@
  * does not match, the overall state is 'not matching'. Additionally it builds a
  * description of the mismatch.
  */
-public class MismatchReporter {
+public final class MismatchReporter {
     private final Description mismatchDescription;
     private boolean firstMismatch = true;
     private boolean matches = true;
diff --git a/src/main/java/org/itsallcode/matcher/auto/AutoConfigBuilder.java b/src/main/java/org/itsallcode/matcher/auto/AutoConfigBuilder.java
index 7d2a35e..86c6ec2 100644
--- a/src/main/java/org/itsallcode/matcher/auto/AutoConfigBuilder.java
+++ b/src/main/java/org/itsallcode/matcher/auto/AutoConfigBuilder.java
@@ -55,11 +55,11 @@ class AutoConfigBuilder<T> {
 
     MatcherConfig<T> build() {
         Arrays.stream(expected.getClass().getMethods()) //
-                .filter(this::isNotIgnored) //
+                .filter(AutoConfigBuilder::isNotIgnored) //
                 .filter(this::isGetterMethodName) //
-                .filter(this::isGetterMethodSignature) //
+                .filter(AutoConfigBuilder::isGetterMethodSignature) //
                 .sorted(Comparator.comparing(this::hasSimpleReturnType).reversed() //
-                        .thenComparing(this::hasArrayReturnType) //
+                        .thenComparing(AutoConfigBuilder::hasArrayReturnType) //
                         .thenComparing(Method::getName)) //
                 .forEach(this::addConfigForGetter);
         return configBuilder.build();
@@ -178,11 +178,11 @@ private static <T> Matcher<T> createOptionalMatcher(final T expected) {
         return (Matcher<T>) OptionalMatchers.isPresentAnd(AutoMatcher.equalTo(expectedOptional.get()));
     }
 
-    private boolean isNotIgnored(final Method method) {
+    private static boolean isNotIgnored(final Method method) {
         return !IGNORED_METHOD_NAMES.contains(method.getName());
     }
 
-    private boolean isGetterMethodSignature(final Method method) {
+    private static boolean isGetterMethodSignature(final Method method) {
         return method.getParameterCount() == 0 //
                 && !method.getReturnType().equals(Void.TYPE);
     }
@@ -202,7 +202,7 @@ private void addConfigForGetter(final Method method) {
         configBuilder.addProperty(propertyName, createGetter(method), AutoMatcher::equalTo);
     }
 
-    private boolean hasArrayReturnType(final Method method) {
+    private static boolean hasArrayReturnType(final Method method) {
         return method.getReturnType().isArray();
     }
 
diff --git a/src/main/java/org/itsallcode/matcher/auto/OptionalMatchers.java b/src/main/java/org/itsallcode/matcher/auto/OptionalMatchers.java
index 741c93d..653353d 100644
--- a/src/main/java/org/itsallcode/matcher/auto/OptionalMatchers.java
+++ b/src/main/java/org/itsallcode/matcher/auto/OptionalMatchers.java
@@ -18,7 +18,10 @@
  * 
  * @author npathai, sweiler
  */
-class OptionalMatchers {
+final class OptionalMatchers {
+    // This is an utility class that must not be instantiated.
+    private OptionalMatchers() {
+    }
 
     /**
      * Creates a matcher that matches when the examined {@code Optional} contains no
@@ -101,8 +104,4 @@ protected void describeMismatchSafely(final Optional<T> item, final Description
             }
         }
     }
-
-    // This is an utility class that must not be instantiated.
-    private OptionalMatchers() {
-    }
 }
diff --git a/src/main/java/org/itsallcode/matcher/config/MatcherConfig.java b/src/main/java/org/itsallcode/matcher/config/MatcherConfig.java
index dd4259a..18fa7c2 100644
--- a/src/main/java/org/itsallcode/matcher/config/MatcherConfig.java
+++ b/src/main/java/org/itsallcode/matcher/config/MatcherConfig.java
@@ -120,7 +120,8 @@ public <P> Builder<B> addIterableProperty(final String propertyName,
             return addPropertyInternal(propertyName, listMatcher, propertyAccessor);
         }
 
-        private <P> Matcher<Iterable<? extends P>> createListMatcher(final Function<P, Matcher<P>> matcherBuilder,
+        private static <P> Matcher<Iterable<? extends P>> createListMatcher(
+                final Function<P, Matcher<P>> matcherBuilder,
                 final Iterable<? extends P> expectedPropertyValue) {
             if (expectedPropertyValue == null) {
                 return createNullIterableMatcher();
@@ -135,7 +136,7 @@ private <P> Matcher<Iterable<? extends P>> createListMatcher(final Function<P, M
         }
 
         @SuppressWarnings({ "unchecked", "rawtypes" })
-        private <P> Matcher<Iterable<? extends P>> createNullIterableMatcher() {
+        private static <P> Matcher<Iterable<? extends P>> createNullIterableMatcher() {
             return new NullIterableMatcher();
         }
 
diff --git a/src/test/java/org/itsallcode/matcher/model/DemoModelMatcherTest.java b/src/test/java/org/itsallcode/matcher/model/DemoModelMatcherTest.java
index 6d4c98f..2d1e2fb 100644
--- a/src/test/java/org/itsallcode/matcher/model/DemoModelMatcherTest.java
+++ b/src/test/java/org/itsallcode/matcher/model/DemoModelMatcherTest.java
@@ -26,11 +26,12 @@ abstract class DemoModelMatcherTest extends MatcherTestBase<DemoModel> {
     static final String ATTR2 = "attrValue2";
     static final String ATTR3 = "attrValue3";
 
-    static String SIMPLE_MODEL_DESCRIPTION = "{id=<" + ID1 + ">, longVal=null, name=\"" + NAME1
+    static final String SIMPLE_MODEL_DESCRIPTION = "{id=<" + ID1 + ">, longVal=null, name=\"" + NAME1
             + "\", attr=null, children=null, stringArray=null}";
     static final String CHILD1 = "{id=<" + ID2 + ">, longVal=null, name=\"" + NAME2 + "\", attr={value=\"" + ATTR2
             + "\"}, children=an empty iterable, stringArray=null}";
-    static String COMPLEX_MODEL_DESCRIPTION = "{id=<" + ID1 + ">, longVal=null, name=\"" + NAME1 + "\", attr={value=\""
+    static final String COMPLEX_MODEL_DESCRIPTION = "{id=<" + ID1 + ">, longVal=null, name=\"" + NAME1
+            + "\", attr={value=\""
             + ATTR1 + "\"}, children=iterable containing [" + CHILD1 + "], stringArray=null}";
 
     @Test