added Error Prone (https://errorprone.info/) and fixed reported errors and warnings

- CI runs Error Prone with Java 11
- use Gradle task `errorprone` to run it on development machine
- fixes are mostly cosmetic except:
  - use Locale.ENGLISH for String.toLowerCase()
  - use explicit character encoding when reading/writing files
  - TabbedPane: wrong logic in mouse-wheel scrolling
  - SplitPane: simplified property change listener (fixes hiding field `propertyChangeListener` of superclass)

.github/workflows/ci.yml

@@ -43,6 +43,10 @@ jobs:
           distribution: adopt  # Java 8 and 11 are pre-installed on ubuntu-latest
           cache: gradle
 
+      - name: Check with Error Prone
+        if: matrix.java == '11'
+        run: ./gradlew errorprone clean -Dtoolchain=${{ matrix.toolchain }}
+
       - name: Build with Gradle
         run: ./gradlew build -Dtoolchain=${{ matrix.toolchain }}
 

build.gradle.kts

@@ -14,6 +14,8 @@
  * limitations under the License.
  */
 
+import net.ltgt.gradle.errorprone.errorprone
+
 val releaseVersion = "3.1.1"
 val developmentVersion = "3.2-SNAPSHOT"
 
@@ -43,6 +45,10 @@ if( !toolchainJavaVersion.isNullOrEmpty() )
 println()
 
 
+plugins {
+	alias( libs.plugins.errorprone ) apply false
+}
+
 allprojects {
 	tasks {
 		withType<JavaCompile>().configureEach {
@@ -81,4 +87,55 @@ allprojects {
 			isFailOnError = false
 		}
 	}
+
+
+	//---- Error Prone ----
+
+	tasks.register( "errorprone" ) {
+		group = "verification"
+		tasks.withType<JavaCompile>().forEach {
+			dependsOn( it )
+		}
+	}
+
+	val useErrorProne = gradle.startParameter.taskNames.contains( "errorprone" )
+	if( useErrorProne ) {
+		plugins.withType<JavaPlugin> {
+			apply( plugin = libs.plugins.errorprone.get().pluginId )
+
+			dependencies {
+				"errorprone"( libs.errorprone )
+			}
+
+			tasks.withType<JavaCompile>().configureEach {
+				options.compilerArgs.add( "-Werror" )
+				options.errorprone {
+					disable(
+						"ReferenceEquality",	// reports usage of '==' for objects
+						"StringSplitter",		// reports String.split()
+						"JavaTimeDefaultTimeZone",	// reports Year.now()
+						"MissingSummary",		// reports `/** @since 2 */`
+						"InvalidBlockTag",		// reports @uiDefault in javadoc
+						"AlreadyChecked",		// reports false positives
+						"InlineMeSuggester",	// suggests using Error Prone annotations for deprecated methods
+						"TypeParameterUnusedInFormals",
+						"UnsynchronizedOverridesSynchronized",
+					)
+					when( project.name ) {
+						"flatlaf-intellij-themes" -> disable(
+							"MutablePublicArray",	// reports FlatAllIJThemes.INFOS
+						)
+						"flatlaf-theme-editor" -> disable(
+							"CatchAndPrintStackTrace",
+						)
+						"flatlaf-testing" -> disable(
+							"CatchAndPrintStackTrace",
+							"JdkObsolete",			// reports Hashtable used for JSlider.setLabelTable()
+							"JavaUtilDate",			// reports usage of class Date
+						)
+					}
+				}
+			}
+		}
+	}
 }

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatLaf.java

@@ -391,7 +391,7 @@ public abstract class FlatLaf
 				Method m = UIManager.class.getMethod( "createLookAndFeel", String.class );
 				aquaLaf = (BasicLookAndFeel) m.invoke( null, "Mac OS X" );
 			} else
-				aquaLaf = (BasicLookAndFeel) Class.forName( aquaLafClassName ).getDeclaredConstructor().newInstance();
+				aquaLaf = Class.forName( aquaLafClassName ).asSubclass( BasicLookAndFeel.class ).getDeclaredConstructor().newInstance();
 		} catch( Exception ex ) {
 			LoggingFacade.INSTANCE.logSevere( "FlatLaf: Failed to initialize Aqua look and feel '" + aquaLafClassName + "'.", ex );
 			throw new IllegalStateException();
@@ -581,10 +581,13 @@ public abstract class FlatLaf
 		Object activeFont = new ActiveFont( null, null, -1, 0, 0, 0, 0 );
 
 		// override fonts
+		List<String> fontKeys = new ArrayList<>( 50 );
 		for( Object key : defaults.keySet() ) {
 			if( key instanceof String && (((String)key).endsWith( ".font" ) || ((String)key).endsWith( "Font" )) )
-				defaults.put( key, activeFont );
+				fontKeys.add( (String) key );
 		}
+		for( String key : fontKeys )
+			defaults.put( key, activeFont );
 
 		// add fonts that are not set in BasicLookAndFeel
 		defaults.put( "RootPane.font", activeFont );
@@ -1541,7 +1544,7 @@ public abstract class FlatLaf
 			int newStyle = (style != -1)
 				? style
 				: (styleChange != 0)
-					? baseStyle & ~((styleChange >> 16) & 0xffff) | (styleChange & 0xffff)
+					? (baseStyle & ~((styleChange >> 16) & 0xffff)) | (styleChange & 0xffff)
 					: baseStyle;
 
 			// new size

flatlaf-core/src/main/java/com/formdev/flatlaf/FlatPropertiesLaf.java

@@ -21,6 +21,7 @@ import java.io.FileInputStream;
 import java.io.IOException;
 import java.io.InputStream;
 import java.util.ArrayList;
+import java.util.Locale;
 import java.util.Properties;
 
 /**
@@ -96,7 +97,7 @@ public class FlatPropertiesLaf
 	protected ArrayList<Class<?>> getLafClassesForDefaultsLoading() {
 		ArrayList<Class<?>> lafClasses = new ArrayList<>();
 		lafClasses.add( FlatLaf.class );
-		switch( baseTheme.toLowerCase() ) {
+		switch( baseTheme.toLowerCase( Locale.ENGLISH ) ) {
 			default:
 			case "light":
 				lafClasses.add( FlatLightLaf.class );

flatlaf-core/src/main/java/com/formdev/flatlaf/LinuxFontPolicy.java

@@ -23,11 +23,14 @@ import java.awt.GraphicsEnvironment;
 import java.awt.Toolkit;
 import java.io.BufferedReader;
 import java.io.File;
-import java.io.FileReader;
+import java.io.FileInputStream;
 import java.io.IOException;
+import java.io.InputStreamReader;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Collections;
 import java.util.List;
+import java.util.Locale;
 import java.util.StringTokenizer;
 import javax.swing.text.StyleContext;
 import com.formdev.flatlaf.util.LoggingFacade;
@@ -68,7 +71,7 @@ class LinuxFontPolicy
 			if( word.endsWith( "," ) )
 				word = word.substring( 0, word.length() - 1 ).trim();
 
-			String lword = word.toLowerCase();
+			String lword = word.toLowerCase( Locale.ENGLISH );
 			if( lword.equals( "italic" ) || lword.equals( "oblique" ) )
 				style |= Font.ITALIC;
 			else if( lword.equals( "bold" ) )
@@ -104,7 +107,7 @@ class LinuxFontPolicy
 			size = 1;
 
 		// handle logical font names
-		String logicalFamily = mapFcName( family.toLowerCase() );
+		String logicalFamily = mapFcName( family.toLowerCase( Locale.ENGLISH ) );
 		if( logicalFamily != null )
 			family = logicalFamily;
 
@@ -143,7 +146,7 @@ class LinuxFontPolicy
 				return createFont( Font.DIALOG, style, size, dsize );
 
 			// check whether last work contains some font weight (e.g. Ultra-Bold or Heavy)
-			String lastWord = family.substring( index + 1 ).toLowerCase();
+			String lastWord = family.substring( index + 1 ).toLowerCase( Locale.ENGLISH );
 			if( lastWord.contains( "bold" ) || lastWord.contains( "heavy" ) || lastWord.contains( "black" ) )
 				style |= Font.BOLD;
 
@@ -257,6 +260,7 @@ class LinuxFontPolicy
 		return createFont( family, style, size, dsize );
 	}
 
+	@SuppressWarnings( "MixedMutabilityReturnType" ) // Error Prone
 	private static List<String> readConfig( String filename ) {
 		File userHome = new File( System.getProperty( "user.home" ) );
 
@@ -277,7 +281,9 @@ class LinuxFontPolicy
 
 		// read config file
 		ArrayList<String> lines = new ArrayList<>( 200 );
-		try( BufferedReader reader = new BufferedReader( new FileReader( file ) ) ) {
+		try( BufferedReader reader = new BufferedReader( new InputStreamReader(
+			new FileInputStream( file ), StandardCharsets.US_ASCII ) ) )
+		{
 			String line;
 			while( (line = reader.readLine()) != null )
 				lines.add( line );

flatlaf-core/src/main/java/com/formdev/flatlaf/UIDefaultsLoader.java

@@ -348,8 +348,11 @@ class UIDefaultsLoader
 		// convert binary color to string
 		if( newValue instanceof Color ) {
 			Color color = (Color) newValue;
+			int rgb = color.getRGB() & 0xffffff;
 			int alpha = color.getAlpha();
-			return String.format( (alpha != 255) ? "#%06x%02x" : "#%06x", color.getRGB() & 0xffffff, alpha );
+			return (alpha != 255)
+				? String.format( "#%06x%02x", rgb, alpha )
+				: String.format( "#%06x", rgb );
 		}
 
 		throw new IllegalArgumentException( "property value type '" + newValue.getClass().getName() + "' not supported in references" );
@@ -702,8 +705,6 @@ class UIDefaultsLoader
 	/**
 	 * Parses a hex color in  {@code #RGB}, {@code #RGBA}, {@code #RRGGBB} or {@code #RRGGBBAA}
 	 * format and returns it as color object.
-	 *
-	 * @throws IllegalArgumentException
 	 */
 	static ColorUIResource parseColor( String value ) {
 		int rgba = parseColorRGBA( value );
@@ -716,8 +717,6 @@ class UIDefaultsLoader
 	 * Parses a hex color in  {@code #RGB}, {@code #RGBA}, {@code #RRGGBB} or {@code #RRGGBBAA}
 	 * format and returns it as {@code rgba} integer suitable for {@link java.awt.Color},
 	 * which includes alpha component in bits 24-31.
-	 *
-	 * @throws IllegalArgumentException
 	 */
 	static int parseColorRGBA( String value ) {
 		int len = value.length();

flatlaf-core/src/main/java/com/formdev/flatlaf/icons/FlatInternalFrameCloseIcon.java

@@ -53,8 +53,8 @@ public class FlatInternalFrameCloseIcon
 
 		g.setColor( FlatButtonUI.buttonStateColor( c, c.getForeground(), null, null, hoverForeground, pressedForeground ) );
 
-		float mx = width / 2;
+		float mx = width / 2f;
-		float my = height / 2;
+		float my = height / 2f;
 		float r = 3.25f;
 
 		Path2D path = new Path2D.Float( Path2D.WIND_EVEN_ODD, 4 );

flatlaf-core/src/main/java/com/formdev/flatlaf/icons/FlatTabbedPaneCloseIcon.java

@@ -94,8 +94,8 @@ public class FlatTabbedPaneCloseIcon
 		Color fg = FlatButtonUI.buttonStateColor( c, closeForeground, null, null, closeHoverForeground, closePressedForeground );
 		g.setColor( FlatUIUtils.deriveColor( fg, c.getForeground() ) );
 
-		float mx = width / 2;
+		float mx = width / 2f;
-		float my = height / 2;
+		float my = height / 2f;
 		float r = ((bg != null) ? closeCrossFilledSize : closeCrossPlainSize) / 2;
 
 		// paint cross

flatlaf-core/src/main/java/com/formdev/flatlaf/json/JsonParser.java

@@ -502,9 +502,9 @@ class JsonParser {
   }
 
   private boolean isHexDigit() {
-    return current >= '0' && current <= '9'
+    return (current >= '0' && current <= '9')
-        || current >= 'a' && current <= 'f'
+        || (current >= 'a' && current <= 'f')
-        || current >= 'A' && current <= 'F';
+        || (current >= 'A' && current <= 'F');
   }
 
   private boolean isEndOfText() {

flatlaf-core/src/main/java/com/formdev/flatlaf/json/Location.java

@@ -69,7 +69,7 @@ public class Location {
     if (obj == null) {
       return false;
     }
-    if (getClass() != obj.getClass()) {
+    if (!(obj instanceof Location)) {
       return false;
     }
     Location other = (Location)obj;

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatLabelUI.java

@@ -24,6 +24,7 @@ import java.awt.Rectangle;
 import java.beans.PropertyChangeEvent;
 import java.util.Arrays;
 import java.util.HashSet;
+import java.util.Locale;
 import java.util.Map;
 import java.util.Set;
 import javax.swing.Icon;
@@ -179,7 +180,7 @@ public class FlatLabelUI
 			// BASE_SIZE rule is parsed in javax.swing.text.html.StyleSheet.addRule()
 			String style = "<style>BASE_SIZE " + c.getFont().getSize() + "</style>";
 
-			String lowerText = text.toLowerCase();
+			String lowerText = text.toLowerCase( Locale.ENGLISH );
 			int headIndex;
 			int styleIndex;
 
@@ -228,7 +229,7 @@ public class FlatLabelUI
 						int tagBegin = i + 1;
 						for( i += 2; i < textLength; i++ ) {
 							if( !Character.isLetterOrDigit( text.charAt( i ) ) ) {
-								String tag = text.substring( tagBegin, i ).toLowerCase();
+								String tag = text.substring( tagBegin, i ).toLowerCase( Locale.ENGLISH );
 								if( tagsUseFontSizeSet.contains( tag ) )
 									return true;
 

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatPopupFactory.java

@@ -391,7 +391,7 @@ public class FlatPopupFactory
 
 	//---- class NonFlashingPopup ---------------------------------------------
 
-	private class NonFlashingPopup
+	private static class NonFlashingPopup
 		extends Popup
 	{
 		private Popup delegate;

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatScrollPaneUI.java

@@ -360,8 +360,8 @@ public class FlatScrollPaneUI
 	protected void updateViewport( PropertyChangeEvent e ) {
 		super.updateViewport( e );
 
-		JViewport oldViewport = (JViewport) (e.getOldValue());
+		JViewport oldViewport = (JViewport) e.getOldValue();
-		JViewport newViewport = (JViewport) (e.getNewValue());
+		JViewport newViewport = (JViewport) e.getNewValue();
 
 		removeViewportListeners( oldViewport );
 		addViewportListeners( newViewport );

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatSplitPaneUI.java

@@ -87,7 +87,6 @@ public class FlatSplitPaneUI
 	@Styleable protected Color oneTouchHoverArrowColor;
 	@Styleable protected Color oneTouchPressedArrowColor;
 
-	private PropertyChangeListener propertyChangeListener;
 	private Map<String, Object> oldStyleValues;
 
 	public static ComponentUI createUI( JComponent c ) {
@@ -126,19 +125,9 @@ public class FlatSplitPaneUI
 	}
 
 	@Override
-	protected void installListeners() {
+	protected PropertyChangeListener createPropertyChangeListener() {
-		super.installListeners();
+		return FlatStylingSupport.createPropertyChangeListener( splitPane, this::installStyle,
-
+			super.createPropertyChangeListener() );
-		propertyChangeListener = FlatStylingSupport.createPropertyChangeListener( splitPane, this::installStyle, null );
-		splitPane.addPropertyChangeListener( propertyChangeListener );
-	}
-
-	@Override
-	protected void uninstallListeners() {
-		super.uninstallListeners();
-
-		splitPane.removePropertyChangeListener( propertyChangeListener );
-		propertyChangeListener = null;
 	}
 
 	@Override

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatStylingSupport.java

@@ -278,6 +278,7 @@ public class FlatStylingSupport
 	 * @throws IllegalArgumentException on syntax errors
 	 * @throws ClassCastException if value type does not fit to expected type
 	 */
+	@SuppressWarnings( "ReturnValueIgnored" ) // Error Prone
 	public static Map<String, Object> parseAndApply( Map<String, Object> oldStyleValues,
 		Object style, BiFunction<String, Object, Object> applyProperty )
 			throws UnknownStyleException, IllegalArgumentException

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTabbedPaneUI.java

@@ -1967,7 +1967,7 @@ public class FlatTabbedPaneUI
 
 	//---- class TabCloseButton -----------------------------------------------
 
-	private class TabCloseButton
+	private static class TabCloseButton
 		extends JButton
 		implements UIResource
 	{
@@ -1977,7 +1977,7 @@ public class FlatTabbedPaneUI
 
 	//---- class ContainerUIResource ------------------------------------------
 
-	private class ContainerUIResource
+	private static class ContainerUIResource
 		extends JPanel
 		implements UIResource
 	{
@@ -2382,7 +2382,7 @@ public class FlatTabbedPaneUI
 			if( isPreciseWheel &&
 				getScrollButtonsPlacement() == BOTH &&
 				getScrollButtonsPolicy() == AS_NEEDED_SINGLE &&
-				(isLeftToRight() || !horizontal) || // scroll buttons are hidden in right-to-left
+				(isLeftToRight() || !horizontal) && // scroll buttons are hidden in right-to-left
 				scrollBackwardButtonPrefSize != null )
 			{
 				// special cases for scrolling with touchpad or high-resolution wheel:
@@ -3051,7 +3051,7 @@ public class FlatTabbedPaneUI
 							break;
 
 						case CENTER:
-							shiftTabs( 0, (diff) / 2 );
+							shiftTabs( 0, diff / 2 );
 							topHeight += diff / 2;
 							bottomHeight += diff - (diff / 2);
 							break;

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTitlePane.java

@@ -682,7 +682,7 @@ public class FlatTitlePane
 				// Seems to be a bug in sun.awt.X11.XNETProtocol.requestState(),
 				// which does some strange state XOR-ing...
 				if( (oldState & Frame.MAXIMIZED_BOTH) == Frame.MAXIMIZED_VERT )
-					newState = oldState & ~Frame.MAXIMIZED_BOTH | Frame.MAXIMIZED_HORIZ;
+					newState = (oldState & ~Frame.MAXIMIZED_BOTH) | Frame.MAXIMIZED_HORIZ;
 			}
 
 			frame.setExtendedState( newState );

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatTreeUI.java

@@ -575,7 +575,7 @@ public class FlatTreeUI
 			if( isSelected && isWideSelection() ) {
 				Color oldColor = g.getColor();
 				g.setColor( selectionInactiveBackground );
-				paintWideSelection( g, clipBounds, insets, bounds, path, row, isExpanded, hasBeenExpanded, isLeaf );
+				paintWideSelection( g, bounds, row );
 				g.setColor( oldColor );
 			}
 			return;
@@ -633,7 +633,7 @@ public class FlatTreeUI
 
 			if( isWideSelection() ) {
 				// wide selection
-				paintWideSelection( g, clipBounds, insets, bounds, path, row, isExpanded, hasBeenExpanded, isLeaf );
+				paintWideSelection( g, bounds, row );
 			} else {
 				// non-wide selection
 				paintCellBackground( g, rendererComponent, bounds, row, true );
@@ -702,9 +702,7 @@ public class FlatTreeUI
 		return oldColor;
 	}
 
-	private void paintWideSelection( Graphics g, Rectangle clipBounds, Insets insets, Rectangle bounds,
+	private void paintWideSelection( Graphics g, Rectangle bounds, int row ) {
-		TreePath path, int row, boolean isExpanded, boolean hasBeenExpanded, boolean isLeaf )
-	{
 		float arcTop, arcBottom;
 		arcTop = arcBottom = UIScale.scale( selectionArc / 2f );
 

flatlaf-core/src/main/java/com/formdev/flatlaf/ui/FlatUIUtils.java

@@ -725,7 +725,7 @@ public class FlatUIUtils
 	{
 		dotSize = UIScale.scale( dotSize );
 		gap = UIScale.scale( gap );
-		int gripSize = (dotSize * dotCount) + ((gap * (dotCount - 1)));
+		int gripSize = (dotSize * dotCount) + (gap * (dotCount - 1));
 
 		// calculate grip position
 		float gx;

flatlaf-core/src/main/java/com/formdev/flatlaf/util/Graphics2DProxy.java

@@ -28,7 +28,6 @@ import java.awt.Paint;
 import java.awt.Polygon;
 import java.awt.Rectangle;
 import java.awt.RenderingHints;
-import java.awt.RenderingHints.Key;
 import java.awt.Shape;
 import java.awt.Stroke;
 import java.awt.font.FontRenderContext;
@@ -368,12 +367,12 @@ public class Graphics2DProxy
 	}
 
 	@Override
-	public void setRenderingHint( Key hintKey, Object hintValue ) {
+	public void setRenderingHint( RenderingHints.Key hintKey, Object hintValue ) {
 		delegate.setRenderingHint( hintKey, hintValue );
 	}
 
 	@Override
-	public Object getRenderingHint( Key hintKey ) {
+	public Object getRenderingHint( RenderingHints.Key hintKey ) {
 		return delegate.getRenderingHint( hintKey );
 	}
 

flatlaf-core/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf {

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestFlatStyleableValue.java

@@ -1214,6 +1214,7 @@ public class TestFlatStyleableValue
 
 	//---- class TestIcon -----------------------------------------------------
 
+	@SuppressWarnings( "EqualsHashCode" ) // Error Prone
 	public static class TestIcon
 		implements Icon
 	{

flatlaf-core/src/test/java/com/formdev/flatlaf/ui/TestUtils.java

@@ -30,6 +30,7 @@ import com.formdev.flatlaf.FlatSystemProperties;
  */
 public class TestUtils
 {
+	@SuppressWarnings( "MutablePublicArray" ) // Error Prone
 	public static final float[] FACTORS = { 1f, 1.25f, 1.5f, 1.75f, 2f, 2.25f, 2.5f, 2.75f, 3f, 3.25f, 3.5f, 3.75f, 4f, 5f, 6f };
 
 	public static void setup( boolean withFocus ) {

flatlaf-demo/src/main/java/com/formdev/flatlaf/demo/DemoFrame.java

@@ -440,9 +440,9 @@ class DemoFrame
 
 		Class<? extends LookAndFeel> lafClass = UIManager.getLookAndFeel().getClass();
 		try {
-			FlatLaf.setup( lafClass.newInstance() );
+			FlatLaf.setup( lafClass.getDeclaredConstructor().newInstance() );
 			FlatLaf.updateUI();
-		} catch( InstantiationException | IllegalAccessException ex ) {
+		} catch( Exception ex ) {
 			LoggingFacade.INSTANCE.logSevere( null, ex );
 		}
 	}

flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/FlatSVGIcon.java

@@ -279,7 +279,7 @@ public class FlatSVGIcon
 
 	private static synchronized URI loadFromStream( InputStream in ) throws IOException {
 		try( InputStream in2 = in ) {
-			return svgUniverse.loadSVG( in2, "/flatlaf-stream-" + (streamNumber++) );
+			return svgUniverse.loadSVG( in2, "/flatlaf-stream-" + streamNumber++ );
 		}
 	}
 
@@ -474,7 +474,7 @@ public class FlatSVGIcon
 		URI uri = this.uri;
 		if( uri == null ) {
 			URL url = getIconURL( name, dark );
-			if( url == null & dark )
+			if( url == null && dark )
 				url = getIconURL( name, false );
 
 			if( url == null ) {

flatlaf-extras/src/main/java/com/formdev/flatlaf/extras/FlatUIDefaultsInspector.java

@@ -770,6 +770,7 @@ public class FlatUIDefaultsInspector
 				return String.valueOf( value );
 		}
 
+		@SuppressWarnings( "FormatString" ) // Error Prone
 		private static String color2hex( Color color ) {
 			int rgb = color.getRGB();
 			boolean hasAlpha = color.getAlpha() != 255;

flatlaf-extras/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.extras {

flatlaf-fonts/flatlaf-fonts-inter/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.fonts.inter {

flatlaf-fonts/flatlaf-fonts-jetbrains-mono/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.fonts.jetbrains_mono {

flatlaf-fonts/flatlaf-fonts-roboto-mono/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.fonts.roboto_mono {

flatlaf-fonts/flatlaf-fonts-roboto/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.fonts.roboto {

flatlaf-intellij-themes/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.intellijthemes {

flatlaf-natives/flatlaf-natives-jna/src/main/java/com/formdev/flatlaf/natives/jna/windows/FlatWindowsNativeWindowBorder.java

@@ -869,7 +869,6 @@ public class FlatWindowsNativeWindowBorder
 		int GetSystemMetricsForDpi( int nIndex, int dpi );
 
 		boolean IsZoomed( HWND hWnd );
-		HANDLE GetProp( HWND hWnd, String lpString );
 
 		HMENU GetSystemMenu( HWND hWnd, boolean bRevert );
 		boolean SetMenuItemInfo( HMENU hmenu, int item, boolean fByPositon, MENUITEMINFO lpmii );

flatlaf-swingx/src/main/java/com/formdev/flatlaf/swingx/ui/FlatTaskPaneUI.java

@@ -229,7 +229,7 @@ public class FlatTaskPaneUI
 
 			// compute chevron position
 			int cx = (int) (x + width / 2 - cw / 2);
-			int cy = (int) (y + (height / 2 - ch));
+			int cy = (int) (y + height / 2 - ch);
 			float offset = ch + UIScale.scale( 1f );
 
 			// set stroke with scaled width

flatlaf-swingx/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.swingx {

flatlaf-testing/flatlaf-testing-modular-app/src/main/module-info/module-info.java

@@ -14,7 +14,7 @@
  * limitations under the License.
  */
 
-/**
+/*
  * @author Karl Tauber
  */
 module com.formdev.flatlaf.testing.modular.app {

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatComponents2Test.java

@@ -1456,7 +1456,7 @@ public class FlatComponents2Test
 
 	//---- TestTableRowHeaderModel --------------------------------------------
 
-	private class TestTableRowHeaderModel
+	private static class TestTableRowHeaderModel
 		extends AbstractTableModel
 		implements TableModelListener
 	{

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatDisabledIconsTest.java

@@ -932,7 +932,7 @@ public class FlatDisabledIconsTest
 		@Override
 		public int filterRGB(int x, int y, int rgb) {
 			// Reduce the color bandwidth in quarter (>> 2) and Shift 0x88.
-			return (rgb & 0xff000000) + 0x888888 + ((((rgb >> 16) & 0xff) >> 2) << 16) + ((((rgb >> 8) & 0xff) >> 2) << 8) + (((rgb) & 0xff) >> 2);
+			return (rgb & 0xff000000) + 0x888888 + ((((rgb >> 16) & 0xff) >> 2) << 16) + ((((rgb >> 8) & 0xff) >> 2) << 8) + ((rgb & 0xff) >> 2);
 		}
 	}
 }

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatMenusTest.java

@@ -937,7 +937,7 @@ public class FlatMenusTest
 
 	//---- class PopupMenu ----------------------------------------------------
 
-	private class PopupMenu extends JPopupMenu {
+	private static class PopupMenu extends JPopupMenu {
 		private PopupMenu() {
 			initComponents();
 		}

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/FlatSubMenusTest.java

@@ -398,7 +398,7 @@ public class FlatSubMenusTest
 
 	//---- class PopupMenu ----------------------------------------------------
 
-	private class PopupMenu extends JPopupMenu {
+	private static class PopupMenu extends JPopupMenu {
 		private PopupMenu() {
 			initComponents();
 		}

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/uidefaults/UIDefaultsDump.java

@@ -22,16 +22,20 @@ import java.awt.Font;
 import java.awt.Insets;
 import java.io.BufferedReader;
 import java.io.File;
-import java.io.FileReader;
+import java.io.FileInputStream;
-import java.io.FileWriter;
+import java.io.FileOutputStream;
 import java.io.IOException;
 import java.io.InputStream;
+import java.io.InputStreamReader;
+import java.io.OutputStreamWriter;
 import java.io.PrintWriter;
 import java.io.Reader;
 import java.io.StringReader;
 import java.io.StringWriter;
+import java.io.Writer;
 import java.lang.reflect.Array;
 import java.lang.reflect.Field;
+import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.Arrays;
 import java.util.LinkedHashMap;
@@ -229,7 +233,8 @@ public class UIDefaultsDump
 		}
 		if( origFile != null ) {
 			try {
-				Map<String, String> defaults1 = parse( new FileReader( origFile ) );
+				Map<String, String> defaults1 = parse( new InputStreamReader(
+					new FileInputStream( origFile ), StandardCharsets.UTF_8 ) );
 				Map<String, String> defaults2 = parse( new StringReader( stringWriter.toString() ) );
 
 				content = diff( defaults1, defaults2 );
@@ -242,7 +247,9 @@ public class UIDefaultsDump
 
 		// write to file
 		file.getParentFile().mkdirs();
-		try( FileWriter fileWriter = new FileWriter( file ) ) {
+		try( Writer fileWriter = new OutputStreamWriter(
+			new FileOutputStream( file ), StandardCharsets.UTF_8 ) )
+		{
 			fileWriter.write( content );
 		} catch( IOException ex ) {
 			ex.printStackTrace();
@@ -419,7 +426,7 @@ public class UIDefaultsDump
 		} else if( value instanceof Character ) {
 			char ch = ((Character)value).charValue();
 			if( ch >= ' ' && ch <= '~' )
-				out.printf( "'%c'", value );
+				out.printf( "'%c'", ch );
 			else
 				out.printf( "'\\u%h'", (int) ch );
 		} else if( value.getClass().isArray() )

flatlaf-testing/src/main/java/com/formdev/flatlaf/testing/uidefaults/UIDefaultsKeysDump.java

@@ -19,10 +19,13 @@ package com.formdev.flatlaf.testing.uidefaults;
 import java.io.BufferedReader;
 import java.io.BufferedWriter;
 import java.io.File;
-import java.io.FileReader;
+import java.io.FileInputStream;
-import java.io.FileWriter;
+import java.io.FileOutputStream;
 import java.io.IOException;
+import java.io.InputStreamReader;
+import java.io.OutputStreamWriter;
 import java.io.Writer;
+import java.nio.charset.StandardCharsets;
 import java.util.Arrays;
 import java.util.HashSet;
 import java.util.Locale;
@@ -47,7 +50,9 @@ public class UIDefaultsKeysDump
 
 		// load existing keys file
 		HashSet<String> keys = new HashSet<>();
-		try( BufferedReader reader = new BufferedReader( new FileReader( keysFile ) ) ) {
+		try( BufferedReader reader = new BufferedReader( new InputStreamReader(
+			new FileInputStream( keysFile ), StandardCharsets.UTF_8 ) ) )
+		{
 			String key;
 			while( (key = reader.readLine()) != null ) {
 				keys.add( key );
@@ -64,7 +69,9 @@ public class UIDefaultsKeysDump
 		collectKeys( FlatTestLaf.class.getName(), keys );
 
 		// write key file
-		try( Writer fileWriter = new BufferedWriter( new FileWriter( keysFile ) ) ) {
+		try( Writer fileWriter = new BufferedWriter( new OutputStreamWriter(
+			new FileOutputStream( keysFile ), StandardCharsets.UTF_8 ) ) )
+		{
 			String[] sortedKeys = keys.toArray( new String[keys.size()] );
 			Arrays.sort( sortedKeys );
 			for( String key : sortedKeys ) {

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatLafThemeEditor.java

@@ -46,6 +46,6 @@ public class FlatLafThemeEditor
 			System.setProperty( "apple.awt.application.appearance", "system" );
 		}
 
-		FlatThemeFileEditor.main( args );
+		FlatThemeFileEditor.launch( args );
 	}
 }

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeEditorOverlay.java

@@ -52,6 +52,7 @@ class FlatThemeEditorOverlay
 	private Font font;
 	private Font baseFont;
 
+	@SuppressWarnings( "FormatString" ) // Error Prone
 	@Override
 	public void paint( Graphics g, JComponent c ) {
 		// paint the syntax text area

flatlaf-theme-editor/src/main/java/com/formdev/flatlaf/themeeditor/FlatThemeFileEditor.java

@@ -102,7 +102,7 @@ class FlatThemeFileEditor
 	private final FlatThemePropertiesBaseManager propertiesBaseManager = new FlatThemePropertiesBaseManager();
 	private final JButton newButton;
 
-	static void main( String[] args ) {
+	static void launch( String[] args ) {
 		File dir = (args.length > 0)
 			? new File( args[0] )
 			: null;
@@ -1306,7 +1306,7 @@ class FlatThemeFileEditor
 				super.addElement( obj );
 			} else {
 				int index = binarySearch( this, obj, comparator );
-				insertElementAt( obj, (index < 0) ? ((-index)-1) : index );
+				insertElementAt( obj, (index < 0) ? (-index - 1) : index );
 			}
 		}
 

gradle/libs.versions.toml

@@ -54,6 +54,12 @@ junit-api = { module = "org.junit.jupiter:junit-jupiter-api", version.ref = "jun
 junit-params = { module = "org.junit.jupiter:junit-jupiter-params", version.ref = "junit" }
 junit-engine = { module = "org.junit.jupiter:junit-jupiter-engine", version.ref = "junit" }
 
+# errorprone
+errorprone = "com.google.errorprone:error_prone_core:2.19.1"
+
 
 [bundles]
 junit = [ "junit-api", "junit-params" ]
+
+[plugins]
+errorprone = { id = "net.ltgt.errorprone", version = "3.1.0" }