String to Byte Conversion and Back Again Not Returning Same Result (ASCII)

I'm having a few issues converting a string back to the appropriate value after it has been converted to bytes.

The initial string:

"0000000000Y        Yã"

Where the 'ã' is just a character value.

The conversion code:

byte[] b = s.getBytes(StandardCharsets.US_ASCII);

However when using to convert it back:

String str = new String(b, StandardCharsets.US_ASCII);

I recieve:

"0000000000Y        Y?"

Anyone know why this is?

Thanks.

ã is not an ASCII character, so how it is handled is given by the implementation

https://docs.oracle.com/javase/8/docs/api/java/lang/String.html#getBytes-java.nio.charset.Charset-

>This method always replaces malformed-input and unmappable-character sequences with this charset's default replacement byte array.

For this charset it comes out as '?'

ã is not part of the US_ASCII character set.

The getBytes() method is documented with:

> This method always replaces malformed-input and unmappable-character
> sequences with this charset's default replacement byte array.

(see the documentation)

For US_ASCII, the default replacement byte array seems to be one byte representing the '?' character (ASCII code 0x3F). So this is what gets inserted into the byte array in place of your ã character.

When converting back to String, you get the character corresponding to the replacement byte, being the '?' character.

So, if you convert to bytes, and you want to get back the identical characters, be sure to use a character set that contains every character you intend to use. A safe decision will be UTF-8.

If you need to obey some character encoding (e.g. because some external interface needs that), then Java's replacement strategy makes sense, but of course some characters will get lost.

This is because ã is not an ASCII character. Check an
ASCII table for valid ASCII characters.