src/java.base/share/classes/sun/nio/cs/UTF_8.java

*** 547,556 ****
--- 547,560 ----
                      da[dp++] = replacement().charAt(0);
                  }
              }
              return dp;
          }
+ 
+         public boolean isASCIICompatible() {
+             return true;
+         }
      }
  
      private static final class Encoder extends CharsetEncoder
                                   implements ArrayEncoder {
  
*** 740,746 ****
--- 744,754 ----
                      da[dp++] = (byte)(0x80 | (c & 0x3f));
                  }
              }
              return dp;
          }
+ 
+         public boolean isASCIICompatible() {
+             return true;
+         }
      }
  }