How to add an byte value (8-bit) to string and read it back in byte in Java

Question

I'm trying to develop an Android app, where I need to add an byte value (8-bit) inside a string and read it back again in byte[].

However I'm getting some different value when I convert the string to byte[] again using getBytes(). I think its some encoding or charset issue.

BTW I'm new to java programming I mostly code in C.

Code:

void function(void)
{   
    String a = "bla";
    char x = (0xD0 & 0xFF);  //Need to add & read back '0xD0'
    a += x;
    Log.d(TAG,"TEST: "+a);

    String mm = "-- ";
    byte[] buffer = null;
    try {
        buffer = a.getBytes("US-ASCII");
    } catch (UnsupportedEncodingException e) {
        Log.e(TAG, e.getMessage());
    }
    for (int i = 0; i < buffer.length; i++) {
        mm+=" "+Integer.toHexString( buffer[i] );
    }

    Log.e(TAG, "Len:"+buffer.length+mm);
}

Output:

TEST: bla￐
Len:4--  62 6c 61 3f

Expected:

Len:4--  62 6c 61 d0

Found the solution: Now I'm using encoding UTF-16LE, which does not loose data, and transmitting even bytes, skip odd bytes

Solution:

void function(void)
{   
    String a = "bla";
    char x = 0xD0;
    a += x;
    Log.d(TAG,"TEST: "+a);

    String mm = "-- ";
    byte[] buffer = null;
    try {
        buffer = a.getBytes("UTF-16LE");
    } catch (UnsupportedEncodingException e) {
        Log.e(TAG, e.getMessage());
    }
    for (int i = 0; i < buffer.length; ) {
        mm += i +":"+Integer.toHexString( buffer[i] ) + ",";
        /* Skip odd bytes as using "UTF-16LE" encoding */
        i+=2;
    }

    Log.e(TAG, "Len:"+buffer.length+mm);
}

Result:

Len:8-- 0:62,2:6c,4:61,6:ffffffd0,

bytes are signed -128 to 127, chars are positive 0 to 65535. You are probably adding char 0xFFD0 to your String, due to sign extension. — AJNeufeld
– AJNeufeld, Commented Apr 15, 2016 at 13:55
Yes but casting as (byte)x gives Len:6-- 62 6c 61 2d 34 38, and by anding with 0xff i am converting it to unsigned. I'm not getting how 3f is getting there. — Rahul Chawre
– Rahul Chawre, Commented Apr 15, 2016 at 14:03
Thanks, but without casting it gives error. And changing byte to char gives same result — Rahul Chawre
– Rahul Chawre, Commented Apr 15, 2016 at 14:19
What you are doing is inherently wrong. Strings are not arrays of bytes; they are arrays of characters, which are 2 bytes. Your getBytes() at the end is converting 2 byte characters into 1 byte ... well ... bytes. There is a loss of information that occurs at that step, if characters are outside the US-ASCII range. If you want to accumulate bytes, you should use a ByteBuffer or similar structure. JSYK: Your buffer result 62 6c 61 2d 34 38 is "bla-48" ... the byte was converted to a string and appended. — AJNeufeld
– AJNeufeld, Commented Apr 15, 2016 at 14:45

Joop Eggen · Accepted Answer · 2016-04-15 15:13:21Z

As java was created, they made a distinction between binary data (byte[], InputStream, OutputStream) and Unicode text (String, char, Reader, Writer). This means that byte is 8 bits and char is 16 bits, containing UTF-16. Now UTF-16 encodes Unicode text - where the characters ("code points") are numbered into the 3 byte range - in a format sometimes needing two 2-byte chars. All in all you cannot use arbitrary chars. And the implicit or explicit conversion always is costly.

Better use ByteArrayInputStream and ByteArrayOutputStream which can be used to collect a variable number of bytes and then retrieve the byte[].

If still the wish exists, you can convert from a 1 byte encoding like ISO-8859-1.

String s = "blah\u00d0";
String s = "blah" + '\u00d0';
String s = "blah" + ((char)0x00d0); // < 0x100 still in safe range
byte[] b = s.getBytes("ISO-8859-1");
s = new String(b, "ISO-8859-1");

The other difference with C is that \u0000 is a normal character in a String.

Daniel Griffin · Accepted Answer · 2016-04-15 15:04:46Z

0

How about this:

String s = "Hello";
s += (char)((byte)0xD0 & 0xFF);

s == "HelloÐ"

Integer.toHexString(s.charAt(s.length()-1) & 0xff) == "d0"

To specify encoding while getting the bytes you can do s.getBytes("UTF-8"); if you need a specific encoding to send over the network or wherever

And you can create a new String from encoded bytes like this String s = new String(utfByteArray, "UTF-8");

edited Apr 15, 2016 at 15:04

answered Apr 15, 2016 at 14:29

Daniel Griffin

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2 Comments

Rahul Chawre Over a year ago

Thanks, but I'm basically trying to get D0 in byte[] array not -48, basically I need to transfer D0 as single byte over serial port. I tried as you said but still getting 3f

Daniel Griffin Over a year ago

Just updated my answer (I'm new to this site idk if it tells you)

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