Thread: [Asking] Convert String to Byte Array

I'm not sure I understand - the String is 32 characters long. Not sure how you expect the bytes of the information of the String any less than 32. That being said, read the API for the getBytes methods of String, there are ways to specify a Charset that differs from your default OS Charset.

You have a string of length 32. Each character of that string is 1 byte (8 bits) per character - hence an array of 32 bytes (based upon the encoding you are using - the default for your system and most likely some type of ASCII character code). I do not know of an encoding which packs the characters as 2 characters/byte. What is wrong with the current encoding?

Declare an array of bytes. The length of the byte array is equal to the String length()/2.

byte [] byteArray = new byte[strKey.length()/2];

Make a loop that goes through the string two chars at a time:

    for (int i = 0; i < strKey.length(); i+= 2)

Inside the loop

1. 
   Extract two chars at position i and position i+1 in the String
   
   Set char1 = the character of strKey at index value i
   Set char2 = the character of strKey at index value i+1
   
   
2. 
   Convert each char to an integer with the value of that character, radix 16
   int1 = Character.digit(char1,16)
   Similarly for int2
   
   
3. 
   Convert the two hex values to an 8-bit byte
   intValue = int1*16+int2
   
   
4. 
   set byteArray[i/2] equal to the value of intValue, cast to a byte.

Cheers!

Z

Hi Zaphod....

Thanks for your code, so the full code perhaps like this below

public static void convertStr2Byte() {
        String strKey = "BDE540BD7E91EDACA3B0125FE004F52C";
        byte[] byteArray = new byte[strKey.length()/2];
 
        for(int i = 0 ; i < strKey.length(); i+=2) {
            char ch1 = strKey.charAt(i);
            char ch2 = strKey.charAt(i+1);
 
            int int1 = Character.digit(ch1, 16);
            int int2 = Character.digit(ch2, 16);
 
            int value = int1 * 16 + int2;
            byteArray[i/2] = (byte) value;
            System.out.println("byteArray["+i+"] "+byteArray[i/2]);
        }
    }

thanks to copeg to explain more details about byte array

As a matter of conventional programming style, I think it would be better to let the convertStr2Byte() method convert and return the array of bytes and leave printing for the calling function:

public class TestStr2Byte {
    public static void main(String [] args) {
 
        String strKey = "BDE540BD7E91EDACA3B0125FE004F52C";
        byte [] bytes = convertStr2Bytes(strKey);
 
        System.out.printf("The String    : %s\n", strKey);
        System.out.print("The hex bytes : ");
 
        for (int i = 0; i < bytes.length; i++) {
            System.out.printf("%02X ", bytes[i]);
        }
        System.out.println();
 
    } // End main
 
    // The function should do one thing: Convert.
    public static byte [] convertStr2Bytes(String str) {
        byte [] byteArray = new byte[str.length()/2];
 
        //
        // Put your code to convert.  Don't print
        //
        return byteArray;
    }// End convertStr2Bytes

Or some such thing.

Output:

The String    : BDE540BD7E91EDACA3B0125FE004F52C
The hex bytes : BD E5 40 BD 7E 91 ED AC A3 B0 12 5F E0 04 F5 2C

Cheers!

Z

I may have missed the full question when reading your posts as the word 'hex' was never used until Zaphod's posts rang a bell in my head...so your string is hexadecimal? An alternative to the method above is to loop over each hex value (every 2 chars of the string) and use Integer.parseInt with a radix of 16 to get the byte value