diff --git a/java/com/google/gerrit/plugins/codeowners/backend/config/BackendConfig.java b/java/com/google/gerrit/plugins/codeowners/backend/config/BackendConfig.java
index dbe3744..2077d64 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/config/BackendConfig.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/config/BackendConfig.java
@@ -33,8 +33,6 @@
 import com.google.gerrit.server.git.validators.ValidationMessage;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
-import java.util.ArrayList;
-import java.util.List;
 import java.util.Optional;
 import org.eclipse.jgit.lib.Config;
 
@@ -110,7 +108,7 @@
     requireNonNull(fileName, "fileName");
     requireNonNull(projectLevelConfig, "projectLevelConfig");
 
-    List<CommitValidationMessage> validationMessages = new ArrayList<>();
+    ImmutableList.Builder<CommitValidationMessage> validationMessages = ImmutableList.builder();
 
     String backendName = projectLevelConfig.getString(SECTION_CODE_OWNERS, null, KEY_BACKEND);
     if (backendName != null) {
@@ -166,7 +164,7 @@
       }
     }
 
-    return ImmutableList.copyOf(validationMessages);
+    return validationMessages.build();
   }
 
   /**
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/config/CodeOwnersPluginConfigValidator.java b/java/com/google/gerrit/plugins/codeowners/backend/config/CodeOwnersPluginConfigValidator.java
index 2353a31..651ce61 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/config/CodeOwnersPluginConfigValidator.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/config/CodeOwnersPluginConfigValidator.java
@@ -33,8 +33,6 @@
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
 import java.io.IOException;
-import java.util.ArrayList;
-import java.util.List;
 import org.eclipse.jgit.errors.ConfigInvalidException;
 import org.eclipse.jgit.lib.Config;
 import org.eclipse.jgit.lib.ObjectId;
@@ -166,7 +164,7 @@
    */
   public ImmutableList<CommitValidationMessage> validateConfig(
       ProjectState projectState, String fileName, Config cfg) {
-    List<CommitValidationMessage> validationMessages = new ArrayList<>();
+    ImmutableList.Builder<CommitValidationMessage> validationMessages = ImmutableList.builder();
     validationMessages.addAll(backendConfig.validateProjectLevelConfig(fileName, cfg));
     validationMessages.addAll(generalConfig.validateProjectLevelConfig(fileName, cfg));
     validationMessages.addAll(statusConfig.validateProjectLevelConfig(fileName, cfg));
@@ -174,7 +172,7 @@
         requiredApprovalConfig.validateProjectLevelConfig(projectState, fileName, cfg));
     validationMessages.addAll(
         overrideApprovalConfig.validateProjectLevelConfig(projectState, fileName, cfg));
-    return ImmutableList.copyOf(validationMessages);
+    return validationMessages.build();
   }
 
   /**
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/config/GeneralConfig.java b/java/com/google/gerrit/plugins/codeowners/backend/config/GeneralConfig.java
index 7e71f09..5540f0a 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/config/GeneralConfig.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/config/GeneralConfig.java
@@ -38,9 +38,7 @@
 import com.google.gerrit.server.project.RefPatternMatcher;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
-import java.util.ArrayList;
 import java.util.Arrays;
-import java.util.List;
 import java.util.Objects;
 import java.util.Optional;
 import java.util.regex.PatternSyntaxException;
@@ -113,7 +111,7 @@
     requireNonNull(fileName, "fileName");
     requireNonNull(projectLevelConfig, "projectLevelConfig");
 
-    List<CommitValidationMessage> validationMessages = new ArrayList<>();
+    ImmutableList.Builder<CommitValidationMessage> validationMessages = ImmutableList.builder();
 
     try {
       projectLevelConfig.getEnum(
@@ -175,7 +173,7 @@
               ValidationMessage.Type.ERROR));
     }
 
-    return ImmutableList.copyOf(validationMessages);
+    return validationMessages.build();
   }
 
   /**
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/config/StatusConfig.java b/java/com/google/gerrit/plugins/codeowners/backend/config/StatusConfig.java
index c3d4dee..d75a3fa 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/config/StatusConfig.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/config/StatusConfig.java
@@ -31,8 +31,6 @@
 import com.google.gerrit.server.project.RefPatternMatcher;
 import com.google.inject.Inject;
 import com.google.inject.Singleton;
-import java.util.ArrayList;
-import java.util.List;
 import java.util.regex.PatternSyntaxException;
 import org.eclipse.jgit.lib.Config;
 
@@ -81,7 +79,7 @@
     requireNonNull(fileName, "fileName");
     requireNonNull(projectLevelConfig, "projectLevelConfig");
 
-    List<CommitValidationMessage> validationMessages = new ArrayList<>();
+    ImmutableList.Builder<CommitValidationMessage> validationMessages = ImmutableList.builder();
 
     try {
       projectLevelConfig.getBoolean(SECTION_CODE_OWNERS, null, KEY_DISABLED, false);
@@ -117,7 +115,7 @@
       }
     }
 
-    return ImmutableList.copyOf(validationMessages);
+    return validationMessages.build();
   }
 
   /**
diff --git a/java/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParser.java b/java/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParser.java
index 01ab9b9..5f35099 100644
--- a/java/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParser.java
+++ b/java/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParser.java
@@ -191,7 +191,7 @@
     private static final Pattern PAT_PER_FILE =
         Pattern.compile(BOL + "per-file[\\s]+([^=#]+)=[\\s]*([^#]+)" + EOL);
 
-    private List<ValidationError> validationErrors;
+    private ImmutableList.Builder<ValidationError> validationErrors;
 
     CodeOwnerConfig parse(
         ObjectId revision, CodeOwnerConfig.Key codeOwnerConfigKey, String codeOwnerConfigAsString) {
@@ -264,12 +264,12 @@
           new String[] {
             removeExtraSpaces(perFileMatcher.group(1)), removeExtraSpaces(perFileMatcher.group(2))
           };
-      String[] dirGlobs = splitGlobs(globsAndOwners[0]);
+      ImmutableSet<String> dirGlobs = splitGlobs(globsAndOwners[0]);
       String directive = globsAndOwners[1];
       if (directive.equals(TOK_SET_NOPARENT)) {
         return CodeOwnerSet.builder()
             .setIgnoreGlobalAndParentCodeOwners()
-            .setPathExpressions(ImmutableSet.copyOf(dirGlobs))
+            .setPathExpressions(dirGlobs)
             .build();
       }
 
@@ -277,7 +277,7 @@
       if ((codeOwnerConfigReference = parseInclude(directive)) != null) {
         return CodeOwnerSet.builder()
             .addImport(codeOwnerConfigReference)
-            .setPathExpressions(ImmutableSet.copyOf(dirGlobs))
+            .setPathExpressions(dirGlobs)
             .build();
       }
 
@@ -296,7 +296,7 @@
 
       CodeOwnerSet.Builder codeOwnerSet =
           CodeOwnerSet.builder()
-              .setPathExpressions(ImmutableSet.copyOf(dirGlobs))
+              .setPathExpressions(dirGlobs)
               .setCodeOwners(
                   ownerEmails.stream().map(CodeOwnerReference::create).collect(toImmutableSet()));
       ownerEmails.stream()
@@ -320,8 +320,8 @@
      * @return the globs as array
      */
     @VisibleForTesting
-    static String[] splitGlobs(String commaSeparatedGlobs) {
-      ArrayList<String> globList = new ArrayList<>();
+    static ImmutableSet<String> splitGlobs(String commaSeparatedGlobs) {
+      ImmutableSet.Builder<String> globList = ImmutableSet.builder();
       StringBuilder nextGlob = new StringBuilder();
       int curlyBracesIndentionLevel = 0;
       int squareBracesIndentionLevel = 0;
@@ -354,7 +354,7 @@
       if (nextGlob.length() > 0) {
         globList.add(nextGlob.toString());
       }
-      return globList.toArray(new String[globList.size()]);
+      return globList.build();
     }
 
     private static boolean isComment(String line) {
@@ -433,7 +433,7 @@
      */
     public ImmutableList<ValidationError> getValidationErrors() {
       if (validationErrors != null) {
-        return ImmutableList.copyOf(validationErrors);
+        return validationErrors.build();
       }
       return ImmutableList.of();
     }
@@ -441,7 +441,7 @@
     @Override
     public void error(ValidationError error) {
       if (validationErrors == null) {
-        validationErrors = new ArrayList<>(4);
+        validationErrors = ImmutableList.builder();
       }
       validationErrors.add(error);
     }
diff --git a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnerConfigsInBranchCollection.java b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnerConfigsInBranchCollection.java
index 2ed2706..9b13e81 100644
--- a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnerConfigsInBranchCollection.java
+++ b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnerConfigsInBranchCollection.java
@@ -69,8 +69,7 @@
      * The resource kind of the members in the {@link CodeOwnerConfigsInBranchCollection} REST
      * collection.
      */
-    static final TypeLiteral<RestView<PathResource>> PATH_KIND =
-        new TypeLiteral<RestView<PathResource>>() {};
+    static final TypeLiteral<RestView<PathResource>> PATH_KIND = new TypeLiteral<>() {};
 
     static PathResource parse(BranchResource branchResource, IdString pathId)
         throws BadRequestException {
diff --git a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInBranchCollection.java b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInBranchCollection.java
index 7b0f6c3..0f4011c 100644
--- a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInBranchCollection.java
+++ b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInBranchCollection.java
@@ -67,8 +67,7 @@
     /**
      * The resource kind of the members in the {@link CodeOwnersInBranchCollection} REST collection.
      */
-    static final TypeLiteral<RestView<PathResource>> PATH_KIND =
-        new TypeLiteral<RestView<PathResource>>() {};
+    static final TypeLiteral<RestView<PathResource>> PATH_KIND = new TypeLiteral<>() {};
 
     static PathResource parse(BranchResource branchResource, IdString pathId)
         throws BadRequestException {
diff --git a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInChangeCollection.java b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInChangeCollection.java
index 20e8dfa..642d605 100644
--- a/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInChangeCollection.java
+++ b/java/com/google/gerrit/plugins/codeowners/restapi/CodeOwnersInChangeCollection.java
@@ -135,8 +135,7 @@
     /**
      * The resource kind of the members in the {@link CodeOwnersInChangeCollection} REST collection.
      */
-    static final TypeLiteral<RestView<PathResource>> PATH_KIND =
-        new TypeLiteral<RestView<PathResource>>() {};
+    static final TypeLiteral<RestView<PathResource>> PATH_KIND = new TypeLiteral<>() {};
 
     static PathResource parse(
         RevisionResource revisionResource, ObjectId branchRevision, IdString pathId)
diff --git a/javatests/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParserTest.java b/javatests/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParserTest.java
index ae42bcd..260e635 100644
--- a/javatests/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParserTest.java
+++ b/javatests/com/google/gerrit/plugins/codeowners/backend/findowners/FindOwnersCodeOwnerConfigParserTest.java
@@ -924,7 +924,6 @@
 
   private static void assertSplitGlobs(String commaSeparatedGlobs, String... expectedGlobs) {
     assertThat(FindOwnersCodeOwnerConfigParser.Parser.splitGlobs(commaSeparatedGlobs))
-        .asList()
         .containsExactlyElementsIn(expectedGlobs);
   }
 }