I’m trying to convert a char array to a long integer and a long integer to a char array, but the conversion fails at value 128.

public long asciiToLong(char[] in) {
    CharBuffer charBuffer = CharBuffer.wrap(in);
    ByteBuffer byteBuffer = StandardCharsets.US_ASCII.encode(charBuffer);
    
    byte[] byteArray = new byte[byteBuffer.remaining()];
    byteBuffer.get(byteArray);

    long result = ByteBuffer.wrap(new String(byteArray).getBytes()).getLong();
    return result;
}

public char[] longToAscii(long in) {
    ByteBuffer buffer = ByteBuffer.allocate(Long.BYTES);
    buffer.putLong(in);
    byte[] b =  buffer.array();
    CharBuffer charBuffer = StandardCharsets.US_ASCII.decode(ByteBuffer.wrap(b));

    char[] result = new char[charBuffer.remaining()];
    charBuffer.get(result);
    
    return result;
}

When I test it with the following code it fails at 128.

for (long l = 0 ; l < 100000 ; l++) {
    char[] ascii = longToAscii(l);
    System.out.printf("%d -> ", l);
    long l2 = asciiToLong(ascii);
    System.err.printf("%dn", l2);
    if (l != l2) {
        throw new RuntimeException();
    }
}

Output :

    0 -> 0
    1 -> 1
    2 -> 2
    3 -> 3
    // console output truncated..
    126 -> 126
    127 -> 127
    128 -> 63 // fails

Changing the StandardCharset used to decode the long integer fails at the same position with different value.