diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompleteDescWindow.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompleteDescWindow.java
index ad29a6a..620a213 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompleteDescWindow.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompleteDescWindow.java
@@ -553,7 +553,7 @@ public String toString() {
 	/**
 	 * Action that actually updates the summary text displayed.
 	 */
-	private class TimerAction extends AbstractAction {
+	private final class TimerAction extends AbstractAction {
 
 		private Completion completion;
 		private String anchor;
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompletion.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompletion.java
index 68c4b52..5653c82 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompletion.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/AutoCompletion.java
@@ -1437,7 +1437,7 @@ else if (oldTriggerAction != null) {
 	 * Listens for LookAndFeel changes and updates the various popup windows
 	 * involved in auto-completion accordingly.
 	 */
-	private class LookAndFeelChangeListener implements PropertyChangeListener {
+	private final class LookAndFeelChangeListener implements PropertyChangeListener {
 
 		@Override
 		public void propertyChange(PropertyChangeEvent e) {
@@ -1486,7 +1486,7 @@ public void actionPerformed(ActionEvent e) {
 	 * Listens for events in the parent window of the text component with
 	 * auto-completion enabled.
 	 */
-	private class ParentWindowListener extends ComponentAdapter implements
+	private final class ParentWindowListener extends ComponentAdapter implements
 			WindowFocusListener {
 
 		public void addTo(Window w) {
@@ -1530,7 +1530,7 @@ public void windowLostFocus(WindowEvent e) {
 	/**
 	 * Listens for events from the popup window.
 	 */
-	private class PopupWindowListener extends ComponentAdapter {
+	private final class PopupWindowListener extends ComponentAdapter {
 
 		@Override
 		public void componentHidden(ComponentEvent e) {
@@ -1558,7 +1558,7 @@ public void uninstall(AutoCompletePopupWindow popupWindow) {
 	/**
 	 * Listens for events from the text component we're installed on.
 	 */
-	private class TextComponentListener extends FocusAdapter implements
+	private final class TextComponentListener extends FocusAdapter implements
 			HierarchyListener {
 
 		void addTo(JTextComponent tc) {
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/ParameterizedCompletionContext.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/ParameterizedCompletionContext.java
index ad8c0ca..0671a52 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/ParameterizedCompletionContext.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/ParameterizedCompletionContext.java
@@ -867,7 +867,7 @@ public void updateUI() {
 	/**
 	 * Called when the user presses Enter while entering parameters.
 	 */
-	private class GotoEndAction extends AbstractAction {
+	private final class GotoEndAction extends AbstractAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
@@ -912,7 +912,7 @@ private Action getDefaultEnterAction(JTextComponent tc) {
 	 * Called when the user types the character marking the closing of the
 	 * parameter list, such as '<code>)</code>'.
 	 */
-	private class ClosingAction extends AbstractAction {
+	private final class ClosingAction extends AbstractAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
@@ -969,7 +969,7 @@ public int getCount(String text, char ch) {
 	/**
 	 * Action performed when the user hits the escape key.
 	 */
-	private class HideAction extends AbstractAction {
+	private final class HideAction extends AbstractAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
@@ -993,7 +993,7 @@ public void actionPerformed(ActionEvent e) {
 	 * Listens for various events in the text component while this tool tip
 	 * is visible.
 	 */
-	private class Listener implements FocusListener, CaretListener,
+	private final class Listener implements FocusListener, CaretListener,
 							DocumentListener {
 
 		private boolean markOccurrencesEnabled;
@@ -1209,7 +1209,7 @@ else if (oldAction!=null) {
 	/**
 	 * Action performed when the user hits the tab key.
 	 */
-	private class NextParamAction extends AbstractAction {
+	private final class NextParamAction extends AbstractAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
@@ -1238,7 +1238,7 @@ private static class ParamCopyInfo {
 	/**
 	 * Action performed when the user hits shift+tab.
 	 */
-	private class PrevParamAction extends AbstractAction {
+	private final class PrevParamAction extends AbstractAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/RoundRobinAutoCompletion.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/RoundRobinAutoCompletion.java
index dd81b0d..3d673ba 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/RoundRobinAutoCompletion.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/RoundRobinAutoCompletion.java
@@ -116,7 +116,7 @@ public void resetProvider() {
 	 * <code>CompletionProvider</code> is displayed based on the context in
 	 * which the user presses the trigger key.
 	 */
-	private class CycleAutoCompleteAction extends AutoCompleteAction {
+	private final class CycleAutoCompleteAction extends AutoCompleteAction {
 
 		@Override
 		public void actionPerformed(ActionEvent e) {
@@ -141,7 +141,7 @@ public void actionPerformed(ActionEvent e) {
 						//nothing to do, just let the current provider display
 						break;
 					}
-					else{
+					else {
 						//search for non-empty completions
 						advanceProvider();
 					}
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/SizeGrip.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/SizeGrip.java
index 7a4a0b9..ec043fe 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/SizeGrip.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/SizeGrip.java
@@ -205,7 +205,7 @@ public void updateUI() {
 	 * Listens for mouse events on this panel and resizes the parent window
 	 * appropriately.
 	 */
-	private class MouseHandler extends MouseInputAdapter {
+	private final class MouseHandler extends MouseInputAdapter {
 
 		private Point origPos;
 
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/SuppressFBWarnings.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/SuppressFBWarnings.java
index 02354a7..17fe0a8 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/SuppressFBWarnings.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/SuppressFBWarnings.java
@@ -1,3 +1,7 @@
+/*
+ * This library is distributed under a modified BSD license.  See the included
+ * LICENSE.md file for details.
+ */
 package org.fife.ui.autocomplete;
 
 import java.lang.annotation.Retention;
diff --git a/AutoComplete/src/main/java/org/fife/ui/autocomplete/package-info.java b/AutoComplete/src/main/java/org/fife/ui/autocomplete/package-info.java
index 579f353..01eb011 100644
--- a/AutoComplete/src/main/java/org/fife/ui/autocomplete/package-info.java
+++ b/AutoComplete/src/main/java/org/fife/ui/autocomplete/package-info.java
@@ -1,3 +1,7 @@
+/*
+ * This library is distributed under a modified BSD license.  See the included
+ * LICENSE.md file for details.
+ */
 /**
  * The auto-completion library.
  */
diff --git a/AutoCompleteDemo/src/main/java/org/fife/ui/autocomplete/demo/package-info.java b/AutoCompleteDemo/src/main/java/org/fife/ui/autocomplete/demo/package-info.java
index 8604eed..d0311f8 100644
--- a/AutoCompleteDemo/src/main/java/org/fife/ui/autocomplete/demo/package-info.java
+++ b/AutoCompleteDemo/src/main/java/org/fife/ui/autocomplete/demo/package-info.java
@@ -1,3 +1,7 @@
+/*
+ * This library is distributed under a modified BSD license.  See the included
+ * LICENSE.md file for details.
+ */
 /**
  * A package that demonstrates features of the AutoComplete library.
  */
diff --git a/build.gradle b/build.gradle
index 6c1ebbf..9b9db39 100644
--- a/build.gradle
+++ b/build.gradle
@@ -47,7 +47,7 @@ subprojects {
     }
 
     checkstyle {
-        toolVersion = '9.3'
+        toolVersion = '10.20.1'
         configDirectory = file("$rootProject.projectDir/config/checkstyle")
     }
 
diff --git a/config/checkstyle/checkstyle.xml b/config/checkstyle/checkstyle.xml
index 1848ea3..c641178 100644
--- a/config/checkstyle/checkstyle.xml
+++ b/config/checkstyle/checkstyle.xml
@@ -75,8 +75,8 @@
     <!-- See https://checkstyle.sourceforge.io/config_whitespace.html -->
     <!--<module name="FileTabCharacter"/>-->
 
-    <!-- Miscellaneous other checks.                            -->
-    <!-- See https://checkstyle.sourceforge.io/config_misc.html -->
+    <!-- Miscellaneous other checks.                                               -->
+    <!-- See https://checkstyle.sourceforge.io/checks/regexp/regexpsingleline.html -->
     <module name="RegexpSingleline">
         <property name="format" value="\s+$"/>
         <property name="minimum" value="0"/>
@@ -84,12 +84,12 @@
         <property name="message" value="Line has trailing spaces."/>
     </module>
 
-    <!-- Checks for Headers                                         -->
-    <!-- See https://checkstyle.sourceforge.io/config_header.html   -->
-    <!-- <module name="Header"> -->
-    <!--   <property name="headerFile" value="${checkstyle.header.file}"/> -->
-    <!--   <property name="fileExtensions" value="java"/> -->
-    <!-- </module> -->
+    <!-- Checks for Headers                                               -->
+    <!-- See https://checkstyle.sourceforge.io/checks/header/index.html   -->
+     <module name="Header">
+       <property name="headerFile" value="${config_loc}/javaHeader.txt"/>
+       <property name="fileExtensions" value="java"/>
+     </module>
 
     <module name="TreeWalker">
 
@@ -127,8 +127,8 @@
 
         <module name="JavadocContentLocationCheck"/>
 
-        <!-- Checks for Naming Conventions.                           -->
-        <!-- See https://checkstyle.sourceforge.io/config_naming.html -->
+        <!-- Checks for Naming Conventions.                                 -->
+        <!-- See https://checkstyle.sourceforge.io/checks/naming/index.html -->
         <module name="ConstantName"/>
         <module name="LocalFinalVariableName"/>
         <module name="LocalVariableName"/>
@@ -139,8 +139,8 @@
         <module name="StaticVariableName"/>
         <module name="TypeName"/>
 
-        <!-- Checks for annotations.                                      -->
-        <!-- See https://checkstyle.sourceforge.io/config_annotation.html -->
+        <!-- Checks for annotations.                                            -->
+        <!-- See https://checkstyle.sourceforge.io/checks/annotation/index.html -->
         <module name="AnnotationLocation"/>
         <module name="AnnotationUseStyle"/>
         <module name="MissingDeprecated"/>
@@ -154,8 +154,8 @@
         <module name="UnusedImports"/>
 
 
-        <!-- Checks for Size Violations.                               -->
-        <!-- See https://checkstyle.sourceforge.io/config_sizes.html -->
+        <!-- Checks for Size Violations.                                   -->
+        <!-- See https://checkstyle.sourceforge.io/checks/sizes/index.html -->
         <module name="MethodLength">
             <property name="max" value="300"/>
         </module>
@@ -165,8 +165,8 @@
         </module>
 
 
-        <!-- Checks for whitespace                                        -->
-        <!-- See https://checkstyle.sourceforge.io/config_whitespace.html -->
+        <!-- Checks for whitespace                                              -->
+        <!-- See https://checkstyle.sourceforge.io/checks/whitespace/index.html -->
         <module name="EmptyForIteratorPad"/>
         <module name="GenericWhitespace"/>
         <module name="MethodParamPad"/>
@@ -177,6 +177,12 @@
         </module>
         <module name="ParenPad"/>
         <module name="TypecastParenPad"/>
+        <module name="WhitespaceAfter">
+            <!-- TODO: Add COMMA and possibly SEMI, but it'll be a big diff -->
+            <property name="tokens" value="LITERAL_IF, LITERAL_ELSE, LITERAL_WHILE, LITERAL_DO, LITERAL_FOR,
+                LITERAL_FINALLY, LITERAL_CATCH, DO_WHILE, LITERAL_SWITCH, LITERAL_SYNCHRONIZED, LITERAL_TRY,
+                LITERAL_CASE, LAMBDA, LITERAL_WHEN"/>
+        </module>
         <!--<module name="WhitespaceAround"/>-->
 
 
@@ -186,8 +192,8 @@
         <module name="RedundantModifier"/>
 
 
-        <!-- Checks for blocks. You know, those {}'s                  -->
-        <!-- See https://checkstyle.sourceforge.io/config_blocks.html -->
+        <!-- Checks for blocks. You know, those {}'s                        -->
+        <!-- See https://checkstyle.sourceforge.io/checks/blocks/index.html -->
         <module name="AvoidNestedBlocks"/>
         <module name="EmptyBlock"/>
         <module name="LeftCurly"/>
@@ -196,13 +202,14 @@
         </module>
         <module name="RightCurly"> <!-- Right curlies should be alone in if/else -->
             <property name="option" value="alone"/>
-            <property name="tokens" value="LITERAL_ELSE, METHOD_DEF"/>
+            <property name="tokens" value="CLASS_DEF, ENUM_DEF, LITERAL_ELSE, LITERAL_SWITCH, METHOD_DEF"/>
         </module>
 
-        <!-- Checks for common coding problems                        -->
-        <!-- See https://checkstyle.sourceforge.io/config_coding.html -->
+        <!-- Checks for common coding problems                              -->
+        <!-- See https://checkstyle.sourceforge.io/checks/coding/index.html -->
         <!--<module name="AvoidInlineConditionals"/>-->
         <!--<module name="EmptyStatement"/> Removed to allow "while (doSomething());" -->
+        <module name="ConstructorsDeclarationGrouping"/>
         <module name="EqualsAvoidNull"/>
         <module name="EqualsHashCode"/>
         <module name="ExplicitInitialization"/>
@@ -229,8 +236,8 @@
         <module name="UnnecessarySemicolonInTryWithResources"/>
         <module name="UnusedLocalVariable"/>
 
-        <!-- Checks for class design                                  -->
-        <!-- See https://checkstyle.sourceforge.io/config_design.html -->
+        <!-- Checks for class design                                        -->
+        <!-- See https://checkstyle.sourceforge.io/checks/design/index.html -->
         <!--<module name="DesignForExtension"/>-->
         <module name="FinalClass"/>
         <module name="HideUtilityClassConstructor"/>
@@ -241,8 +248,8 @@
         </module>
 
 
-        <!-- Miscellaneous other checks.                            -->
-        <!-- See https://checkstyle.sourceforge.io/config_misc.html -->
+        <!-- Miscellaneous other checks.                                  -->
+        <!-- See https://checkstyle.sourceforge.io/checks/misc/index.html -->
         <module name="ArrayTypeStyle"/>
         <module name="AvoidEscapedUnicodeCharacters"/>
         <module name="CommentsIndentation"/>
@@ -251,7 +258,7 @@
         <module name="UpperEll"/>
 
         <!-- Checks for metrics.                                       -->
-        <!-- See https://checkstyle.sourceforge.io/config_metrics.html -->
+        <!-- See https://checkstyle.sourceforge.io/checks/metrics/index.html -->
         <!--        <module name="CyclomaticComplexity"/>-->
 
     </module>
diff --git a/config/checkstyle/javaHeader.txt b/config/checkstyle/javaHeader.txt
new file mode 100644
index 0000000..33662f5
--- /dev/null
+++ b/config/checkstyle/javaHeader.txt
@@ -0,0 +1 @@
+/*