\ufffd, aka REPLACE_CHAR, should be output by decoders
decodeXXXLoop only if the input bytes have the semantics of a REPLACE_CHAR,
not simply because the input is malformed or unmappable.  After all, there
is MALFORMED[n] and UNMAPPABLE[n].

--------------------------------------------------------
public class Decode {
    private static boolean isAscii(char c) {
	return c < '\u0080';
    }

    private static boolean isPrintable(char c) {
	return ('\u0020' < c) && (c < '\u007f');
    }

    public static void main(String[] args) throws Throwable {
	if (args.length < 2)
	    throw new Exception("Usage: java Decode CHARSET BYTE [BYTE ...]");
	String cs = args[0];
	byte[] bytes = new byte[args.length-1];
	for (int i = 1; i < args.length; i++) {
	    String arg = args[i];
	    bytes[i-1] =
		(arg.length() == 1 && isAscii(arg.charAt(0))) ?
		(byte) arg.charAt(0) :
		arg.equals("ESC") ? 0x1b :
		arg.equals("SO")  ? 0x0e :
		arg.equals("SI")  ? 0x0f :
		arg.equals("SS2") ? (byte) 0x8e :
		arg.equals("SS3") ? (byte) 0x8f :
		arg.matches("0x.*") ? Integer.decode(arg).byteValue() :
		Integer.decode("0x"+arg).byteValue();
	}
	String s = new String(bytes, cs);

	for (int j = 0; j < s.length(); j++) {
	    if (j > 0)
		System.out.print(' ');
	    char c = s.charAt(j);
	    if (isPrintable(c))
		System.out.print(c);
	    else if (c == '\u001b') System.out.print("ESC");
	    else
		System.out.printf("\\u%04x", (int) c);
	}
	System.out.print("\n");
    }
}
--------------------------------------------------------
 $ jver 6 javac Decode.java && for cs in ISO-2022-JP ISO-2022-JP-2 x-windows-50220 x-windows-50221 x-windows-iso2022jp ; do echo $cs;  jver 6 java Decode $cs ESC 24 40 00 00; done; echo EUC-TW ; jver 6 java Decode EUC-TW 8e 98 ad e5
ISO-2022-JP
\ufffd
ISO-2022-JP-2
\ufffd
x-windows-50220
\ufffd
x-windows-50221
\ufffd
x-windows-iso2022jp
\ufffd
EUC-TW
\ufffd