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I want to put a string value to an array byte without any conversion. the string length is always 14 and it's in fact an hexadecimal value presenting the date and time. so every 2 chars in this string is a byte so my byte array is a 7 bytes length. i didn't find a way to do so. the string.getBytes() method is doing a conversion. i tryed this method also https://stackoverflow.com/a/11208685/3343174 but after debugging it's showing a different values from the first string.

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  • Provide an example and what you have tried. Commented Jul 31, 2015 at 10:34
  • You will need to design a loop that iterates over the characters of your String. Once you have some code ready and if you run into trouble, then the community will help you fix it. Commented Jul 31, 2015 at 10:34
  • i tryed string.getBytes(); and i tryed another method that i found in this answer: stackoverflow.com/a/11208685/3343174 but all of them are doing a conversion !! Commented Jul 31, 2015 at 10:38
  • @SaHa what to you mean by conversion? Surely that is exactly what you want to do. Otherwise, for example, A has a byte value of 65. You understand that the numeric value of A as a hexadecimal digit is different to the value of A is a char? Commented Jul 31, 2015 at 10:41
  • docs.oracle.com/javase/7/docs/api/java/lang/… ?? You can split your String into pieces of 2 and use this to get the corresponding byte-value. Then put it in an array. Commented Jul 31, 2015 at 10:41

2 Answers 2

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You can get a substring of two chars of your string like this:

String yourString = "07df";
String firstByteAsString = yourString.substring(0,2); // start - end(excl.)
                      // = "07"

You can get a byte from this like this:

byte b = Byte.parseByte( firstByteAsString, 16 );

I am sure, you'll manage to use this to get your desired functionality.

EDIT As Peter points out: Values > 127 are a Problem. So you'll actually have to use 

byte b = (byte) (Integer.parseInt( firstByteAsString, 16) & 0xFF);

But his answer is much simpler and doing all this for you. You should go for BigInteger and accept Peter's answer.

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11 Comments

It's generating this exception : Caused by: java.lang.NumberFormatException: Invalid int: "" at this instruction : result[i] = Byte.parseByte(firstByteAsString ,16);
Can you update the question with your new code and complete stacktrace of the error, please?
It think he is trying to say that df is > 127 and cannot be parsed as a byte.
See Peter's answer. It's shorter and better.
@PeterLawrey I thought he wanted to "stay" unsigned but I think, I misunderstood what was simply a matter of "print-out" interpretation of the value.
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Lets assume you want to convert a hexidecimal string into byte[]. e.g. you have 14 characters in hexidecimal and you want to convert this to bytes where two character represent each byte.

for example the first four chars in my string are '07df' i want that the 2 first bytes in my byte array contain respectively '07' and 'df'

String base16 = "07df07df07df07";
byte[] bytes = new BigInteger(base16, 16).toByteArray();
if (bytes[0] == 0)
    bytes = Arrays.copyOfRange(bytes, 1, bytes.length);
System.out.println(Arrays.toString(bytes));

prints

[7, -33, 7, -33, 7, -33, 7]

I want to get [07, df, 07, df, 07, df, 07]

This is a matter of formatting the byte[] in the form you want. (byte) 0xDF is -33 Numbers are just numbers, they don't remember the format you used to create them, they just have one default format and if you don't like, it you can add your own.

StringBuilder sb = new StringBuilder();
String sep = "[";
for (byte b : bytes) {
    sb.append(sep).append(String.format("%02x", b & 0xFF));
    sep = ", ";
}
sb.append("]");
System.out.println(sb);

prints

[07, df, 07, df, 07, df, 07]
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8 Comments

i want to get [07, df, 07, df, 07, df, 07] another thing i tryed the answer of @Fildor but the problem is that the value df is out of range and it's generating the exception : Caused by: java.lang.NumberFormatException: Value out of range for byte: "f0"
@SaHa (byte) 0xdf is -33 if you can change the way it is printed if you want hexidecimal.
@SaHa all numbers are just numbers. They are not decimal, they are not hexadecimal. They are binary numbers. If you want to print a number then you need to decide on a format when it is printed.
@BoristheSpider ok i get it. so if i pass the "-33" value is the same if i pass "df" ?? i'm developing an application that use Bluetooth low energy and i must pass a byte[] parameter to the method so i can send a value.
i resolve the problem. even when i pass value converted the other device will know it!! they are all the same :)
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