diff --git a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
index ce3b36e..225cbd6 100644
--- a/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
+++ b/src/main/java/com/googlesource/gerrit/plugins/findowners/OwnersValidator.java
@@ -140,7 +140,7 @@
         try (Repository repo = repoManager.openRepository(project)) {
           String name = getOwnersFileName(project);
           messages =
-              performValidation(repo, receiveEvent.commit, receiveEvent.revWalk, name, false);
+              performValidation(receiveEvent.commit, receiveEvent.revWalk, name, false);
         }
       }
     } catch (NoSuchProjectException | IOException | ExecutionException e) {
@@ -154,13 +154,13 @@
 
   @VisibleForTesting
   List<CommitValidationMessage> performValidation(
-      Repository repo, RevCommit c, RevWalk revWalk, String ownersFileName, boolean verbose)
+      RevCommit c, RevWalk revWalk, String ownersFileName, boolean verbose)
       throws IOException, ExecutionException {
     // Collect all messages from all files.
     List<CommitValidationMessage> messages = new LinkedList<>();
     // Collect all email addresses from all files and check each address only once.
     Map<String, Set<String>> email2lines = new HashMap<>();
-    Map<String, ObjectId> content = getChangedOwners(repo, c, revWalk, ownersFileName);
+    Map<String, ObjectId> content = getChangedOwners(c, revWalk, ownersFileName);
     for (String path : content.keySet()) {
       ObjectLoader ol = revWalk.getObjectReader().open(content.get(path));
       try (InputStream in = ol.openStream()) {
@@ -228,7 +228,7 @@
       int line = 0;
       for (String l = br.readLine(); l != null; l = br.readLine()) {
         line++;
-        checkLine(messages, email2lines, path, line, l, verbose);
+        checkLine(messages, email2lines, path, line, l);
       }
     }
   }
@@ -247,9 +247,7 @@
   private static void collectEmail(
       Map<String, Set<String>> map, String email, String file, int lineNumber) {
     if (!email.equals("*")) {
-      if (map.get(email) == null) {
-        map.put(email, new HashSet<>());
-      }
+      map.computeIfAbsent(email, (String k) -> new HashSet<>());
       map.get(email).add(file + ":" + lineNumber);
     }
   }
@@ -272,8 +270,7 @@
       Map<String, Set<String>> email2lines,
       String path,
       int lineNumber,
-      String line,
-      boolean verbose) {
+      String line) {
     Matcher m;
     if (patComment.matcher(line).find()
         || patNoParent.matcher(line).find()
@@ -294,10 +291,9 @@
    * from "Path to the changed file" to "ObjectId of the file".
    */
   private static Map<String, ObjectId> getChangedOwners(
-      Repository repo, RevCommit c, RevWalk revWalk, String ownersFileName) throws IOException {
+      RevCommit c, RevWalk revWalk, String ownersFileName) throws IOException {
     final Map<String, ObjectId> content = new HashMap<>();
     visitChangedEntries(
-        repo,
         c,
         revWalk,
         new TreeWalkVisitor() {
@@ -321,7 +317,7 @@
    * is found this method calls the onVisit() method of the class TreeWalkVisitor.
    */
   private static void visitChangedEntries(
-      Repository repo, RevCommit c, RevWalk revWalk, TreeWalkVisitor visitor) throws IOException {
+      RevCommit c, RevWalk revWalk, TreeWalkVisitor visitor) throws IOException {
     try (TreeWalk tw = new TreeWalk(revWalk.getObjectReader())) {
       tw.setRecursive(true);
       tw.setFilter(TreeFilter.ANY_DIFF);
diff --git a/src/test/java/com/googlesource/gerrit/plugins/findowners/OwnersValidatorTest.java b/src/test/java/com/googlesource/gerrit/plugins/findowners/OwnersValidatorTest.java
index dfbc0fa..7f1c39a 100644
--- a/src/test/java/com/googlesource/gerrit/plugins/findowners/OwnersValidatorTest.java
+++ b/src/test/java/com/googlesource/gerrit/plugins/findowners/OwnersValidatorTest.java
@@ -48,11 +48,14 @@
 import org.junit.After;
 import org.junit.Before;
 import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.junit.runners.JUnit4;
 
 /** Test OwnersValidator, which checks syntax of changed OWNERS files. */
+@RunWith(JUnit4.class)
 public class OwnersValidatorTest {
 
-  private class MockedEmails extends Emails {
+  private static class MockedEmails extends Emails {
     Set<String> registered;
 
     MockedEmails() {
@@ -62,6 +65,7 @@
               "u1@g.com", "u2@g.com", "u2.m@g.com", "user1@google.com", "u1+review@g.com");
     }
 
+    @Override
     public ImmutableSetMultimap<String, Account.Id> getAccountsFor(String... emails) {
       // Used by checkEmails; each email should have exactly one Account.Id
       ImmutableSetMultimap.Builder<String, Account.Id> builder = ImmutableSetMultimap.builder();
@@ -112,7 +116,7 @@
     assertThat(OwnersValidator.getOwnersFileName(ANDROID_CONFIG)).isEqualTo(OWNERS_ANDROID);
   }
 
-  private static final Map<String, String> FILES_WITHOUT_OWNERS =
+  private static final ImmutableMap<String, String> FILES_WITHOUT_OWNERS =
       ImmutableMap.of("README", "any\n", "d1/test.c", "int x;\n");
 
   @Test
@@ -124,7 +128,7 @@
     }
   }
 
-  private static final Map<String, String> FILES_WITH_NO_ERROR =
+  private static final ImmutableMap<String, String> FILES_WITH_NO_ERROR =
       ImmutableMap.of(
           OWNERS,
           "\n\n#comments ...\n  ###  more comments\n"
@@ -136,7 +140,7 @@
               + "per-file *.txt = * # everyone can approve\n"
               + "set  noparent  # comment\n");
 
-  private static final Set<String> EXPECTED_VERBOSE_OUTPUT =
+  private static final ImmutableSet<String> EXPECTED_VERBOSE_OUTPUT =
       ImmutableSet.of(
           "MSG: validate: " + OWNERS,
           "MSG: owner: user1@google.com",
@@ -153,7 +157,7 @@
     }
   }
 
-  private static final Map<String, String> FILES_WITH_WRONG_SYNTAX =
+  private static final ImmutableMap<String, String> FILES_WITH_WRONG_SYNTAX =
       ImmutableMap.of(
           "README",
           "# some content\nu2@g.com\n",
@@ -164,13 +168,13 @@
           "d3/" + OWNERS,
           "\nfile: common/Owners\n");
 
-  private static final Set<String> EXPECTED_WRONG_SYNTAX =
+  private static final ImmutableSet<String> EXPECTED_WRONG_SYNTAX =
       ImmutableSet.of(
           "ERROR: syntax: " + OWNERS + ":2: wrong syntax",
           "ERROR: unknown: u3@g.com at d2/" + OWNERS + ":2",
           "ERROR: ignored: d3/" + OWNERS + ":2: file: common/Owners");
 
-  private static final Set<String> EXPECTED_VERBOSE_WRONG_SYNTAX =
+  private static final ImmutableSet<String> EXPECTED_VERBOSE_WRONG_SYNTAX =
       ImmutableSet.of(
           "MSG: validate: d3/" + OWNERS,
           "MSG: validate: d2/" + OWNERS,
@@ -193,13 +197,13 @@
     }
   }
 
-  private static final Map<String, String> FILES_WITH_WRONG_EMAILS =
+  private static final ImmutableMap<String, String> FILES_WITH_WRONG_EMAILS =
       ImmutableMap.of("d1/" + OWNERS, "u1@g.com\n", "d2/" + OWNERS_ANDROID, "u2@g.com\n");
 
-  private static final Set<String> EXPECTED_VERBOSE_DEFAULT =
+  private static final ImmutableSet<String> EXPECTED_VERBOSE_DEFAULT =
       ImmutableSet.of("MSG: validate: d1/" + OWNERS, "MSG: owner: u1@g.com");
 
-  private static final Set<String> EXPECTED_VERBOSE_ANDROID =
+  private static final ImmutableSet<String> EXPECTED_VERBOSE_ANDROID =
       ImmutableSet.of("MSG: validate: d2/" + OWNERS_ANDROID, "MSG: owner: u2@g.com");
 
   @Test
@@ -255,7 +259,7 @@
     OwnersValidator validator = new OwnersValidator(null, null, null, myEmails);
     String ownersFileName = OwnersValidator.getOwnersFileName(cfg);
     List<CommitValidationMessage> m =
-        validator.performValidation(repo, c, rw, ownersFileName, verbose);
+        validator.performValidation(c, rw, ownersFileName, verbose);
     return transformMessages(m);
   }