src/java.base/share/classes/java/lang/StringConcatHelper.java

*** 340,345 ****
--- 340,353 ----
      static String newString(byte[] buf, byte coder) {
          // Use the private, non-copying constructor (unsafe!)
          return new String(buf, coder);
      }
  
+     /**
+      * Provides the initial coder for the String.
+      * @return initial coder
+      */
+     static byte initialCoder() {
+         return String.COMPACT_STRINGS ? String.LATIN1 : String.UTF16;
+     }
+ 
  }