Pull method getAddMethodName up to JavacSingularizer

src/core/lombok/javac/handlers/JavacSingularsRecipes.java

@@ -404,5 +404,7 @@ public class JavacSingularsRecipes {
 				}
 			}
 		}
+
+		protected abstract String getAddMethodName();
 	}
 }

src/core/lombok/javac/handlers/singulars/JavacGuavaSingularizer.java

@@ -193,7 +193,7 @@ abstract class JavacGuavaSingularizer extends JavacSingularizer {
 	}
 	
 	protected abstract LombokImmutableList<String> getArgumentSuffixes();
-	protected abstract String getAddMethodName();
+
 	protected abstract String getAddAllTypeName();
 	
 	protected int getTypeArgumentsCount() {

src/core/lombok/javac/handlers/singulars/JavacJavaUtilListSetSingularizer.java

@@ -134,7 +134,8 @@ abstract class JavacJavaUtilListSetSingularizer extends JavacJavaUtilSingularize
 		finishAndInjectMethod(maker, returnType, builderType, source, mods, body, name, List.of(param));
 	}
 
-	private String getAddMethodName() {
+	@Override
+	protected String getAddMethodName() {
 		return "add";
 	}
 }

src/core/lombok/javac/handlers/singulars/JavacJavaUtilMapSingularizer.java

@@ -197,7 +197,8 @@ public class JavacJavaUtilMapSingularizer extends JavacJavaUtilSingularizer {
 		}
 	}
 
-	private String getAddMethodName() {
+	@Override
+	protected String getAddMethodName() {
 		return "put";
 	}
 }