8236705: Use single character variant of String.replace when applicable

Reviewed-by: igerasim, alanb

src/java.base/share/classes/jdk/internal/module/ModulePath.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2014, 2019, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2014, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -566,7 +566,7 @@ private ModuleDescriptor deriveModuleDescriptor(JarFile jf)
         if (attrs != null) {
             String mainClass = attrs.getValue(Attributes.Name.MAIN_CLASS);
             if (mainClass != null) {
-                mainClass = mainClass.replace("/", ".");
+                mainClass = mainClass.replace('/', '.');
                 if (Checks.isClassName(mainClass)) {
                     String pn = packageName(mainClass);
                     if (packages.contains(pn)) {

src/java.base/share/classes/jdk/internal/module/Resources.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2016, 2017, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2016, 2020, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -64,7 +64,7 @@ public static String toPackageName(String name) {
         if (index == -1 || index == name.length()-1) {
             return "";
         } else {
-            return name.substring(0, index).replace("/", ".");
+            return name.substring(0, index).replace('/', '.');
         }
     }