diff --git a/build-tools/src/main/java/org/elasticsearch/gradle/util/GradleUtils.java b/build-tools/src/main/java/org/elasticsearch/gradle/util/GradleUtils.java
index 880d7a069e9a7..cab4d55210864 100644
--- a/build-tools/src/main/java/org/elasticsearch/gradle/util/GradleUtils.java
+++ b/build-tools/src/main/java/org/elasticsearch/gradle/util/GradleUtils.java
@@ -186,7 +186,7 @@ public static void extendSourceSet(Project project, String parentSourceSetName,
      * task execution time.
      */
     public static String getProjectPathFromTask(String taskPath) {
-        int lastDelimiterIndex = taskPath.lastIndexOf(":");
+        int lastDelimiterIndex = taskPath.lastIndexOf(':');
         return lastDelimiterIndex == 0 ? ":" : taskPath.substring(0, lastDelimiterIndex);
     }
 
diff --git a/client/rest/src/main/java/org/elasticsearch/client/RestClient.java b/client/rest/src/main/java/org/elasticsearch/client/RestClient.java
index ed087bef0ac76..8b769d3d311d4 100644
--- a/client/rest/src/main/java/org/elasticsearch/client/RestClient.java
+++ b/client/rest/src/main/java/org/elasticsearch/client/RestClient.java
@@ -159,10 +159,10 @@ public class RestClient implements Closeable {
     public static RestClientBuilder builder(String cloudId) {
         // there is an optional first portion of the cloudId that is a human readable string, but it is not used.
         if (cloudId.contains(":")) {
-            if (cloudId.indexOf(":") == cloudId.length() - 1) {
+            if (cloudId.indexOf(':') == cloudId.length() - 1) {
                 throw new IllegalStateException("cloudId " + cloudId + " must begin with a human readable identifier followed by a colon");
             }
-            cloudId = cloudId.substring(cloudId.indexOf(":") + 1);
+            cloudId = cloudId.substring(cloudId.indexOf(':') + 1);
         }
 
         String decoded = new String(Base64.getDecoder().decode(cloudId), UTF_8);
diff --git a/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/RegisteredDomainProcessor.java b/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/RegisteredDomainProcessor.java
index 6b949889fc960..acccdd92fc9b1 100644
--- a/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/RegisteredDomainProcessor.java
+++ b/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/RegisteredDomainProcessor.java
@@ -91,7 +91,7 @@ private DomainInfo getRegisteredDomain(IngestDocument d) {
             }
             return null;
         }
-        if (registeredDomain.indexOf(".") == -1) {
+        if (registeredDomain.indexOf('.') == -1) {
             // we have domain with no matching public suffix, but "." in it
             return null;
         }
@@ -117,7 +117,7 @@ private DomainInfo(String eTLD) {
         }
 
         private DomainInfo(String registeredDomain, String domain) {
-            int index = registeredDomain.indexOf(".") + 1;
+            int index = registeredDomain.indexOf('.') + 1;
             if (index > 0 && index < registeredDomain.length()) {
                 this.domain = domain;
                 this.eTLD = registeredDomain.substring(index);
diff --git a/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/UriPartsProcessor.java b/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/UriPartsProcessor.java
index 2a7025a02ac30..66e6df5fde58d 100644
--- a/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/UriPartsProcessor.java
+++ b/modules/ingest-common/src/main/java/org/elasticsearch/ingest/common/UriPartsProcessor.java
@@ -155,7 +155,7 @@ private static Map<String, Object> getUriParts(URI uri, URL fallbackUrl) {
         if (userInfo != null) {
             uriParts.put("user_info", userInfo);
             if (userInfo.contains(":")) {
-                int colonIndex = userInfo.indexOf(":");
+                int colonIndex = userInfo.indexOf(':');
                 uriParts.put("username", userInfo.substring(0, colonIndex));
                 uriParts.put("password", colonIndex < userInfo.length() ? userInfo.substring(colonIndex + 1) : "");
             }
diff --git a/modules/lang-painless/src/doc/java/org/elasticsearch/painless/ContextApiSpecGenerator.java b/modules/lang-painless/src/doc/java/org/elasticsearch/painless/ContextApiSpecGenerator.java
index 29a73ea1742e0..d4b11d159ae44 100644
--- a/modules/lang-painless/src/doc/java/org/elasticsearch/painless/ContextApiSpecGenerator.java
+++ b/modules/lang-painless/src/doc/java/org/elasticsearch/painless/ContextApiSpecGenerator.java
@@ -121,7 +121,7 @@ public JavaClassFilesystemResolver(Path root, Map<String, Path> pkgRoots) {
         public InputStream openClassFile(String className) throws IOException {
             // TODO(stu): handle primitives & not stdlib
             if (className.contains(".")) {
-                int dollarPosition = className.indexOf("$");
+                int dollarPosition = className.indexOf('$');
                 if (dollarPosition >= 0) {
                     className = className.substring(0, dollarPosition);
                 }
@@ -131,10 +131,10 @@ public InputStream openClassFile(String className) throws IOException {
                     Path classPath = root.resolve(path + ".java");
                     return new FileInputStream(classPath.toFile());
                 } else {
-                    String packageName = className.substring(0, className.lastIndexOf("."));
+                    String packageName = className.substring(0, className.lastIndexOf('.'));
                     Path packageRoot = pkgRoots.get(packageName);
                     if (packageRoot != null) {
-                        Path classPath = packageRoot.resolve(className.substring(className.lastIndexOf(".") + 1) + ".java");
+                        Path classPath = packageRoot.resolve(className.substring(className.lastIndexOf('.') + 1) + ".java");
                         return new FileInputStream(classPath.toFile());
                     }
                 }
diff --git a/modules/repository-azure/src/main/java/org/elasticsearch/repositories/azure/AzureStorageSettings.java b/modules/repository-azure/src/main/java/org/elasticsearch/repositories/azure/AzureStorageSettings.java
index f804fe1e46282..93a388dea98ae 100644
--- a/modules/repository-azure/src/main/java/org/elasticsearch/repositories/azure/AzureStorageSettings.java
+++ b/modules/repository-azure/src/main/java/org/elasticsearch/repositories/azure/AzureStorageSettings.java
@@ -338,7 +338,7 @@ private String getProperty(String propertyName) {
         for (int i = 0; i < settings.length; i++) {
             String setting = settings[i].trim();
             if (setting.length() > 0) {
-                final int idx = setting.indexOf("=");
+                final int idx = setting.indexOf('=');
                 if (idx == -1 || idx == 0 || idx == settings[i].length() - 1) {
                     new IllegalArgumentException("Invalid connection string: " + getConnectString());
                 }
diff --git a/plugins/analysis-icu/src/main/java/org/elasticsearch/plugin/analysis/icu/IcuTokenizerFactory.java b/plugins/analysis-icu/src/main/java/org/elasticsearch/plugin/analysis/icu/IcuTokenizerFactory.java
index e869ebf20f59e..0a73eb38bd45c 100644
--- a/plugins/analysis-icu/src/main/java/org/elasticsearch/plugin/analysis/icu/IcuTokenizerFactory.java
+++ b/plugins/analysis-icu/src/main/java/org/elasticsearch/plugin/analysis/icu/IcuTokenizerFactory.java
@@ -58,7 +58,7 @@ private static ICUTokenizerConfig getIcuConfig(Environment env, Settings setting
             List<String> ruleFiles = settings.getAsList(RULE_FILES);
 
             for (String scriptAndResourcePath : ruleFiles) {
-                int colonPos = scriptAndResourcePath.indexOf(":");
+                int colonPos = scriptAndResourcePath.indexOf(':');
                 if (colonPos == -1 || colonPos == scriptAndResourcePath.length() - 1) {
                     throw new IllegalArgumentException(RULE_FILES + " should contain comma-separated \"code:rulefile\" pairs");
                 }
diff --git a/server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java b/server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java
index e107b0d063778..4972a784cc2bd 100644
--- a/server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java
+++ b/server/src/main/java/org/elasticsearch/action/admin/indices/rollover/MetadataRolloverService.java
@@ -380,7 +380,7 @@ static String generateRolloverIndexName(String sourceIndexName) {
         String resolvedName = IndexNameExpressionResolver.resolveDateMathExpression(sourceIndexName);
         final boolean isDateMath = sourceIndexName.equals(resolvedName) == false;
         if (INDEX_NAME_PATTERN.matcher(resolvedName).matches()) {
-            int numberIndex = sourceIndexName.lastIndexOf("-");
+            int numberIndex = sourceIndexName.lastIndexOf('-');
             assert numberIndex != -1 : "no separator '-' found";
             int counter = Integer.parseInt(
                 sourceIndexName.substring(numberIndex + 1, isDateMath ? sourceIndexName.length() - 1 : sourceIndexName.length())
diff --git a/server/src/main/java/org/elasticsearch/action/fieldcaps/ResponseRewriter.java b/server/src/main/java/org/elasticsearch/action/fieldcaps/ResponseRewriter.java
index d39dd28b32611..38b0287522207 100644
--- a/server/src/main/java/org/elasticsearch/action/fieldcaps/ResponseRewriter.java
+++ b/server/src/main/java/org/elasticsearch/action/fieldcaps/ResponseRewriter.java
@@ -109,7 +109,7 @@ private static boolean isNestedField(String field, Set<String> nestedParents) {
     }
 
     private static boolean isNotMultifield(String field, Set<String> objectFields) {
-        int lastDotPos = field.lastIndexOf(".");
+        int lastDotPos = field.lastIndexOf('.');
         if (lastDotPos == -1) {
             return true;
         }
diff --git a/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java b/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java
index 2730cc8909790..22672756bdaf0 100644
--- a/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java
+++ b/server/src/main/java/org/elasticsearch/cluster/metadata/IndexMetadata.java
@@ -2890,7 +2890,7 @@ public static int getRoutingFactor(int sourceNumberOfShards, int targetNumberOfS
      * number
      */
     public static int parseIndexNameCounter(String indexName) {
-        int numberIndex = indexName.lastIndexOf("-");
+        int numberIndex = indexName.lastIndexOf('-');
         if (numberIndex == -1) {
             throw new IllegalArgumentException("no - separator found in index name [" + indexName + "]");
         }
diff --git a/server/src/main/java/org/elasticsearch/common/regex/Regex.java b/server/src/main/java/org/elasticsearch/common/regex/Regex.java
index 1b60a5dce9038..532fc2ae9a019 100644
--- a/server/src/main/java/org/elasticsearch/common/regex/Regex.java
+++ b/server/src/main/java/org/elasticsearch/common/regex/Regex.java
@@ -44,7 +44,7 @@ public static boolean isMatchAllPattern(String str) {
     }
 
     public static boolean isSuffixMatchPattern(String str) {
-        return str.length() > 1 && str.indexOf("*") == str.length() - 1;
+        return str.length() > 1 && str.indexOf('*') == str.length() - 1;
     }
 
     /** Return an {@link Automaton} that matches the given pattern. */
diff --git a/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java b/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
index 7f954a570348a..7a807f767611b 100644
--- a/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
+++ b/server/src/main/java/org/elasticsearch/index/mapper/ObjectMapper.java
@@ -127,7 +127,7 @@ public final void addDynamic(String name, String prefix, Mapper mapper, Document
             // of the mapper name. So for a mapper 'foo.bar.baz', we locate 'foo' and then
             // call addDynamic on it with the name 'bar.baz', and next call addDynamic on 'bar' with the name 'baz'.
             else {
-                int firstDotIndex = name.indexOf(".");
+                int firstDotIndex = name.indexOf('.');
                 String immediateChild = name.substring(0, firstDotIndex);
                 String immediateChildFullName = prefix == null ? immediateChild : prefix + "." + immediateChild;
                 ObjectMapper.Builder parentBuilder = findObjectBuilder(immediateChildFullName, context);
diff --git a/server/src/main/java/org/elasticsearch/plugins/PluginsService.java b/server/src/main/java/org/elasticsearch/plugins/PluginsService.java
index d2f31697e1021..038b03c5dd93f 100644
--- a/server/src/main/java/org/elasticsearch/plugins/PluginsService.java
+++ b/server/src/main/java/org/elasticsearch/plugins/PluginsService.java
@@ -865,7 +865,7 @@ static final URI uncheckedToURI(URL url) {
 
     static final String toPackageName(String className) {
         assert className.endsWith(".") == false;
-        int index = className.lastIndexOf(".");
+        int index = className.lastIndexOf('.');
         if (index == -1) {
             throw new IllegalStateException("invalid class name:" + className);
         }
diff --git a/server/src/main/java/org/elasticsearch/plugins/PluginsUtils.java b/server/src/main/java/org/elasticsearch/plugins/PluginsUtils.java
index 0533f535a19f1..becc5ef05e846 100644
--- a/server/src/main/java/org/elasticsearch/plugins/PluginsUtils.java
+++ b/server/src/main/java/org/elasticsearch/plugins/PluginsUtils.java
@@ -196,7 +196,7 @@ public static void checkForFailedPluginRemovals(final Path pluginsDirectory) thr
             if (iterator.hasNext()) {
                 final Path removing = iterator.next();
                 final String fileName = removing.getFileName().toString();
-                final String name = fileName.substring(1 + fileName.indexOf("-"));
+                final String name = fileName.substring(1 + fileName.indexOf('-'));
                 final String message = String.format(
                     Locale.ROOT,
                     "found file [%s] from a failed attempt to remove the plugin [%s]; execute [elasticsearch-plugin remove %2$s]",
diff --git a/server/src/main/java/org/elasticsearch/rest/RestUtils.java b/server/src/main/java/org/elasticsearch/rest/RestUtils.java
index 5535ea65dfc41..c6d4f9ae38853 100644
--- a/server/src/main/java/org/elasticsearch/rest/RestUtils.java
+++ b/server/src/main/java/org/elasticsearch/rest/RestUtils.java
@@ -38,7 +38,7 @@ public static void decodeQueryString(String s, int fromIndex, Map<String, String
             return;
         }
 
-        int queryStringLength = s.contains("#") ? s.indexOf("#") : s.length();
+        int queryStringLength = s.contains("#") ? s.indexOf('#') : s.length();
 
         String name = null;
         int pos = fromIndex; // Beginning of the unprocessed region
diff --git a/server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java b/server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java
index 067cf2d800957..05c56da2cda48 100644
--- a/server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java
+++ b/server/src/main/java/org/elasticsearch/rest/action/search/RestSearchAction.java
@@ -314,7 +314,7 @@ private static void parseSearchSource(final SearchSourceBuilder searchSourceBuil
         if (sSorts != null) {
             String[] sorts = Strings.splitStringByCommaToArray(sSorts);
             for (String sort : sorts) {
-                int delimiter = sort.lastIndexOf(":");
+                int delimiter = sort.lastIndexOf(':');
                 if (delimiter != -1) {
                     String sortField = sort.substring(0, delimiter);
                     String reverse = sort.substring(delimiter + 1);
diff --git a/server/src/main/java/org/elasticsearch/script/ScriptCache.java b/server/src/main/java/org/elasticsearch/script/ScriptCache.java
index 95aad9b086671..d4746922dcc0a 100644
--- a/server/src/main/java/org/elasticsearch/script/ScriptCache.java
+++ b/server/src/main/java/org/elasticsearch/script/ScriptCache.java
@@ -271,7 +271,7 @@ public CompilationRate(String value) {
                     "parameter must contain a positive integer and a timevalue, i.e. 10/1m, but was [" + value + "]"
                 );
             }
-            int idx = value.indexOf("/");
+            int idx = value.indexOf('/');
             String count = value.substring(0, idx);
             String time = value.substring(idx + 1);
             try {
diff --git a/test/fixtures/s3-fixture/src/main/java/fixture/s3/S3HttpHandler.java b/test/fixtures/s3-fixture/src/main/java/fixture/s3/S3HttpHandler.java
index 2698d96ab7ab9..21fa0c59352da 100644
--- a/test/fixtures/s3-fixture/src/main/java/fixture/s3/S3HttpHandler.java
+++ b/test/fixtures/s3-fixture/src/main/java/fixture/s3/S3HttpHandler.java
@@ -331,7 +331,7 @@ public static RequestComponents parseRequestComponents(final String request) {
             final ArrayList<String> queryParameters = new ArrayList<>();
             Arrays.stream(Strings.tokenizeToStringArray(queryString, "&")).forEach(param -> {
                 if (param.startsWith("x-")) {
-                    final int equalPos = param.indexOf("=");
+                    final int equalPos = param.indexOf('=');
                     customQueryParameters.computeIfAbsent(param.substring(0, equalPos), k -> new ArrayList<>())
                         .add(param.substring(equalPos + 1));
                 } else {
diff --git a/test/fixtures/testcontainer-utils/src/main/java/org/elasticsearch/test/fixtures/ResourceUtils.java b/test/fixtures/testcontainer-utils/src/main/java/org/elasticsearch/test/fixtures/ResourceUtils.java
index 8fe64ea34d8d4..537a899e9d1ec 100644
--- a/test/fixtures/testcontainer-utils/src/main/java/org/elasticsearch/test/fixtures/ResourceUtils.java
+++ b/test/fixtures/testcontainer-utils/src/main/java/org/elasticsearch/test/fixtures/ResourceUtils.java
@@ -26,7 +26,7 @@ public static Path copyResourceToFile(Class<?> clazz, Path targetFolder, String
                 throw new RuntimeException("Failed to load " + resourcePath + " from classpath");
             }
             InputStream inputStream = resourceUrl.openStream();
-            File outputFile = new File(targetFolder.toFile(), resourcePath.substring(resourcePath.lastIndexOf("/")));
+            File outputFile = new File(targetFolder.toFile(), resourcePath.substring(resourcePath.lastIndexOf('/')));
             Files.copy(inputStream, outputFile.toPath(), StandardCopyOption.REPLACE_EXISTING);
             return outputFile.toPath();
         } catch (IOException e) {
diff --git a/test/framework/src/main/java/org/elasticsearch/test/fixture/AbstractHttpFixture.java b/test/framework/src/main/java/org/elasticsearch/test/fixture/AbstractHttpFixture.java
index 8e7fae85e57f5..9a9088036ea6b 100644
--- a/test/framework/src/main/java/org/elasticsearch/test/fixture/AbstractHttpFixture.java
+++ b/test/framework/src/main/java/org/elasticsearch/test/fixture/AbstractHttpFixture.java
@@ -222,7 +222,7 @@ public Request(final long id, final String method, final URI uri, final Map<Stri
             final Map<String, String> params = new HashMap<>();
             if (uri.getQuery() != null && uri.getQuery().length() > 0) {
                 for (String param : uri.getQuery().split("&")) {
-                    int i = param.indexOf("=");
+                    int i = param.indexOf('=');
                     if (i > 0) {
                         params.put(param.substring(0, i), param.substring(i + 1));
                     } else {
diff --git a/test/logger-usage/src/main/java/org/elasticsearch/test/loggerusage/ESLoggerUsageChecker.java b/test/logger-usage/src/main/java/org/elasticsearch/test/loggerusage/ESLoggerUsageChecker.java
index d7cde5676a27f..15a0f938da268 100644
--- a/test/logger-usage/src/main/java/org/elasticsearch/test/loggerusage/ESLoggerUsageChecker.java
+++ b/test/logger-usage/src/main/java/org/elasticsearch/test/loggerusage/ESLoggerUsageChecker.java
@@ -116,8 +116,8 @@ public record WrongLoggerUsage(String className, String methodName, String logMe
          */
         public String getErrorLines() {
             String fullClassName = Type.getObjectType(className).getClassName();
-            String simpleClassName = fullClassName.substring(fullClassName.lastIndexOf(".") + 1, fullClassName.length());
-            int innerClassIndex = simpleClassName.indexOf("$");
+            String simpleClassName = fullClassName.substring(fullClassName.lastIndexOf('.') + 1, fullClassName.length());
+            int innerClassIndex = simpleClassName.indexOf('$');
             if (innerClassIndex > 0) {
                 simpleClassName = simpleClassName.substring(0, innerClassIndex);
             }
diff --git a/test/yaml-rest-runner/src/main/java/org/elasticsearch/test/rest/yaml/section/TransformAndSetSection.java b/test/yaml-rest-runner/src/main/java/org/elasticsearch/test/rest/yaml/section/TransformAndSetSection.java
index ff31cf8a57c34..a6626c1229319 100644
--- a/test/yaml-rest-runner/src/main/java/org/elasticsearch/test/rest/yaml/section/TransformAndSetSection.java
+++ b/test/yaml-rest-runner/src/main/java/org/elasticsearch/test/rest/yaml/section/TransformAndSetSection.java
@@ -78,7 +78,7 @@ public void execute(ClientYamlTestExecutionContext executionContext) throws IOEx
             String key = entry.getKey();
             String value = entry.getValue();
             if (value.startsWith("#base64EncodeCredentials(") && value.endsWith(")")) {
-                value = entry.getValue().substring("#base64EncodeCredentials(".length(), entry.getValue().lastIndexOf(")"));
+                value = entry.getValue().substring("#base64EncodeCredentials(".length(), entry.getValue().lastIndexOf(')'));
                 String[] idAndPassword = value.split(",");
                 if (idAndPassword.length == 2) {
                     String credentials = executionContext.response(idAndPassword[0].trim())
diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleType.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleType.java
index e378cbb171a38..3203f85b2a7eb 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleType.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/ilm/TimeseriesLifecycleType.java
@@ -378,7 +378,7 @@ public static String validateMonotonicallyIncreasingPhaseTimings(Collection<Phas
                         }
                         // if multiple phases are cited replace last occurrence of "," with " and"
                         StringBuilder builder = new StringBuilder();
-                        int last_comma_index = error.lastIndexOf(",");
+                        int last_comma_index = error.lastIndexOf(',');
                         builder.append(error, 0, last_comma_index);
                         builder.append(" and");
                         builder.append(error.substring(last_comma_index + 1));
diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/SecuritySettings.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/SecuritySettings.java
index 010a0ead1051c..790707ee46397 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/SecuritySettings.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/SecuritySettings.java
@@ -44,7 +44,7 @@ public static Settings addUserSettings(final Settings settings) {
         if (settings.get(authHeaderSettingName) == null) {
             Optional<String> userOptional = SecurityField.USER_SETTING.get(settings); // TODO migrate to securesetting!
             userOptional.ifPresent(userSetting -> {
-                final int i = userSetting.indexOf(":");
+                final int i = userSetting.indexOf(':');
                 if (i < 0 || i == userSetting.length() - 1) {
                     throw new IllegalArgumentException(
                         "invalid [" + SecurityField.USER_SETTING.getKey() + "] setting. must be in the form of \"<username>:<password>\""
diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authc/jwt/JwtRealmSettings.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authc/jwt/JwtRealmSettings.java
index e75ff1ac321bc..b05c81653963a 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authc/jwt/JwtRealmSettings.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/security/authc/jwt/JwtRealmSettings.java
@@ -545,7 +545,7 @@ private static void validateFallbackClaimSetting(
     }
 
     private static void verifyFallbackClaimName(String key, String fallbackClaimName) {
-        final String claimName = key.substring(key.lastIndexOf(".") + 1);
+        final String claimName = key.substring(key.lastIndexOf('.') + 1);
         verifyNonNullNotEmpty(key, fallbackClaimName, null);
         if (claimName.equals(fallbackClaimName)) {
             return;
diff --git a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/execution/Wid.java b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/execution/Wid.java
index 39f59aa742690..f92df9be4648e 100644
--- a/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/execution/Wid.java
+++ b/x-pack/plugin/core/src/main/java/org/elasticsearch/xpack/core/watcher/execution/Wid.java
@@ -37,7 +37,7 @@ public Wid(String watchId, ZonedDateTime executionTime) {
 
     public Wid(String value) {
         this.value = value;
-        int index = value.lastIndexOf("_");
+        int index = value.lastIndexOf('_');
         if (index <= 0) {
             throw illegalArgument("invalid watcher execution id [{}]", value);
         }
diff --git a/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestUtils.java b/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestUtils.java
index 4e0f0b8661631..438edecc76fbc 100644
--- a/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestUtils.java
+++ b/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestUtils.java
@@ -160,7 +160,7 @@ public void close() {
                     if (columns == null) {
                         columns = new CsvColumn[entries.length];
                         for (int i = 0; i < entries.length; i++) {
-                            int split = entries[i].indexOf(":");
+                            int split = entries[i].indexOf(':');
                             String name, typeName;
 
                             if (split < 0) {
@@ -245,7 +245,7 @@ static String[] multiValuesAwareCsvToStringArray(String csvLine, int lineNumber)
         int pos = 0;          // current position in the csv String
         int commaPos;         // current "," character position
         int previousCommaPos = 0;
-        while ((commaPos = csvLine.indexOf(",", pos)) != -1 || pos <= csvLine.length()) {
+        while ((commaPos = csvLine.indexOf(',', pos)) != -1 || pos <= csvLine.length()) {
             if (commaPos > 0 && csvLine.charAt(commaPos - 1) == ESCAPE_CHAR) {// skip the escaped comma
                 pos = commaPos + 1;// moving on to the next character after comma
                 continue;
diff --git a/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestsDataLoader.java b/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestsDataLoader.java
index 224f3d34980e6..302fda9b331e3 100644
--- a/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestsDataLoader.java
+++ b/x-pack/plugin/esql/qa/testFixtures/src/main/java/org/elasticsearch/xpack/esql/CsvTestsDataLoader.java
@@ -261,7 +261,7 @@ private static void loadCsvData(
                     if (columns == null) {
                         columns = new String[entries.length];
                         for (int i = 0; i < entries.length; i++) {
-                            int split = entries[i].indexOf(":");
+                            int split = entries[i].indexOf(':');
                             String name, typeName;
 
                             if (split < 0) {
diff --git a/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/execution/search/extractor/AbstractFieldHitExtractor.java b/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/execution/search/extractor/AbstractFieldHitExtractor.java
index 0defe009c6f97..549c7ffab7f29 100644
--- a/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/execution/search/extractor/AbstractFieldHitExtractor.java
+++ b/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/execution/search/extractor/AbstractFieldHitExtractor.java
@@ -133,7 +133,7 @@ private Object extractNestedField(SearchHit hit) {
         List<String> remainingPath = new ArrayList<>();
         // first, search for the "root" DocumentField under which the remaining path of nested document values is
         while ((field = hit.field(tempHitname)) == null) {
-            int indexOfDot = tempHitname.lastIndexOf(".");
+            int indexOfDot = tempHitname.lastIndexOf('.');
             if (indexOfDot < 0) {// there is no such field in the hit
                 return null;
             }
diff --git a/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/util/ReflectionUtils.java b/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/util/ReflectionUtils.java
index 1447cb4dde143..6009e7514b413 100644
--- a/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/util/ReflectionUtils.java
+++ b/x-pack/plugin/ql/src/main/java/org/elasticsearch/xpack/ql/util/ReflectionUtils.java
@@ -48,9 +48,9 @@ public static <E> Class<E> detectSuperTypeForRuleLike(Class<?> c) {
     // remove packaging from the name - strategy used for naming rules by default
     public static String ruleLikeNaming(Class<?> c) {
         String className = c.getName();
-        int parentPackage = className.lastIndexOf(".");
+        int parentPackage = className.lastIndexOf('.');
         if (parentPackage > 0) {
-            int grandParentPackage = className.substring(0, parentPackage).lastIndexOf(".");
+            int grandParentPackage = className.substring(0, parentPackage).lastIndexOf('.');
             return (grandParentPackage > 0 ? className.substring(grandParentPackage + 1) : className.substring(parentPackage));
         } else {
             return className;
diff --git a/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/SpecReader.java b/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/SpecReader.java
index c6e21b64ce6ae..fef23f11b4a65 100644
--- a/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/SpecReader.java
+++ b/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/SpecReader.java
@@ -46,7 +46,7 @@ public static List<Object[]> readScriptSpec(List<URL> urls, Parser parser) throw
 
     public static List<Object[]> readURLSpec(URL source, Parser parser) throws Exception {
         String fileName = pathAndName(source.getFile()).v2();
-        String groupName = fileName.substring(0, fileName.lastIndexOf("."));
+        String groupName = fileName.substring(0, fileName.lastIndexOf('.'));
 
         Map<String, Integer> testNames = new LinkedHashMap<>();
         List<Object[]> testCases = new ArrayList<>();
diff --git a/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/TestUtils.java b/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/TestUtils.java
index 7f5249fe4c2db..8ff7cac54bf5c 100644
--- a/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/TestUtils.java
+++ b/x-pack/plugin/ql/test-fixtures/src/main/java/org/elasticsearch/xpack/ql/TestUtils.java
@@ -286,7 +286,7 @@ public static BufferedReader reader(URL resource) throws IOException {
     public static Tuple<String, String> pathAndName(String string) {
         String folder = StringUtils.EMPTY;
         String file = string;
-        int lastIndexOf = string.lastIndexOf("/");
+        int lastIndexOf = string.lastIndexOf('/');
         if (lastIndexOf > 0) {
             folder = string.substring(0, lastIndexOf - 1);
             if (lastIndexOf + 1 < string.length()) {
diff --git a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/file/FileUserRolesStore.java b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/file/FileUserRolesStore.java
index b644ba174e2ef..ffc14ca96a768 100644
--- a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/file/FileUserRolesStore.java
+++ b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/file/FileUserRolesStore.java
@@ -128,7 +128,7 @@ public static Map<String, String[]> parseFile(Path path, @Nullable Logger logger
             if (line.startsWith("#")) {  // comment
                 continue;
             }
-            int i = line.indexOf(":");
+            int i = line.indexOf(':');
             if (i <= 0 || i == line.length() - 1) {
                 logger.error("invalid entry in users_roles file [{}], line [{}]. skipping...", path.toAbsolutePath(), lineNr);
                 continue;
diff --git a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/ldap/ActiveDirectorySessionFactory.java b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/ldap/ActiveDirectorySessionFactory.java
index 495fdfe4cc0f2..12f51e0ec7912 100644
--- a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/ldap/ActiveDirectorySessionFactory.java
+++ b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/authc/ldap/ActiveDirectorySessionFactory.java
@@ -261,7 +261,7 @@ ServerSet getServerSet() {
     ADAuthenticator getADAuthenticator(String username) {
         if (username.indexOf('\\') > 0) {
             return downLevelADAuthenticator;
-        } else if (username.indexOf("@") > 0) {
+        } else if (username.indexOf('@') > 0) {
             return upnADAuthenticator;
         }
         return defaultADAuthenticator;
diff --git a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java
index c7965872f8185..349df95f16b57 100644
--- a/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java
+++ b/x-pack/plugin/security/src/main/java/org/elasticsearch/xpack/security/profile/ProfileService.java
@@ -789,7 +789,7 @@ void getOrCreateProfileWithBackoff(
     void maybeIncrementDifferentiatorAndCreateNewProfile(Subject subject, ProfileDocument profileDocument, ActionListener<Profile> listener)
         throws IOException {
         final String uid = profileDocument.uid();
-        final int index = uid.lastIndexOf("_");
+        final int index = uid.lastIndexOf('_');
         if (index == -1) {
             listener.onFailure(new ElasticsearchException("profile uid [{}] does not contain any underscore character", uid));
             return;
diff --git a/x-pack/plugin/sql/sql-client/src/main/java/org/elasticsearch/xpack/sql/client/StringUtils.java b/x-pack/plugin/sql/sql-client/src/main/java/org/elasticsearch/xpack/sql/client/StringUtils.java
index a5d45f2e5b525..4f5239e1ca8ec 100644
--- a/x-pack/plugin/sql/sql-client/src/main/java/org/elasticsearch/xpack/sql/client/StringUtils.java
+++ b/x-pack/plugin/sql/sql-client/src/main/java/org/elasticsearch/xpack/sql/client/StringUtils.java
@@ -121,7 +121,7 @@ public static String normalize(String path) {
         }
         String pathToUse = path.replace("\\", SLASH);
 
-        int prefixIndex = pathToUse.indexOf(":");
+        int prefixIndex = pathToUse.indexOf(':');
         String prefix = "";
         if (prefixIndex != -1) {
             prefix = pathToUse.substring(0, prefixIndex + 1);
diff --git a/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/pivot/SchemaUtil.java b/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/pivot/SchemaUtil.java
index 2ddc9d6d4abda..48b156ce39fc2 100644
--- a/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/pivot/SchemaUtil.java
+++ b/x-pack/plugin/transform/src/main/java/org/elasticsearch/xpack/transform/transforms/pivot/SchemaUtil.java
@@ -326,7 +326,7 @@ static void insertNestedObjectMappings(Map<String, String> fieldMappings) {
             int pos;
             String objectKey = key;
             // lastIndexOf returns -1 on mismatch, but to disallow empty strings check for > 0
-            while ((pos = objectKey.lastIndexOf(".")) > 0) {
+            while ((pos = objectKey.lastIndexOf('.')) > 0) {
                 objectKey = objectKey.substring(0, pos);
                 additionalMappings.putIfAbsent(objectKey, "object");
             }
diff --git a/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java b/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java
index 3afe2fa731860..d11cb7521976a 100644
--- a/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java
+++ b/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/notification/email/EmailService.java
@@ -248,7 +248,7 @@ static Set<String> getRecipientDomains(Email email) {
         )
             .map(InternetAddress::getAddress)
             // Pull out only the domain of the email address, so foo@bar.com -> bar.com
-            .map(emailAddress -> emailAddress.substring(emailAddress.lastIndexOf("@") + 1))
+            .map(emailAddress -> emailAddress.substring(emailAddress.lastIndexOf('@') + 1))
             .collect(Collectors.toSet());
     }
 
diff --git a/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/trigger/schedule/support/DayTimes.java b/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/trigger/schedule/support/DayTimes.java
index 7586543d0869e..52e5a50ec7d01 100644
--- a/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/trigger/schedule/support/DayTimes.java
+++ b/x-pack/plugin/watcher/src/main/java/org/elasticsearch/xpack/watcher/trigger/schedule/support/DayTimes.java
@@ -68,11 +68,11 @@ public static DayTimes parse(String time) throws ElasticsearchParseException {
         }
         int[] hour;
         int[] minute;
-        int i = time.indexOf(":");
+        int i = time.indexOf(':');
         if (i < 0) {
             throw new ElasticsearchParseException("could not parse time [{}]. time format must be in the form of hh:mm", time);
         }
-        if (i == time.length() - 1 || time.indexOf(":", i + 1) >= 0) {
+        if (i == time.length() - 1 || time.indexOf(':', i + 1) >= 0) {
             throw new ElasticsearchParseException("could not parse time [{}]. time format must be in the form of hh:mm", time);
         }
         String hrStr = time.substring(0, i);