Java Byte to Hex String

[Scott Selikoff]

I'm trying to convert a Java byte to a 2-digit Hex string hex string. I can use the System output to get what I want, but I'm having trouble writing a general method.

In short, I'm looking for the equivalent of doing the following:

byte myByte = ...;
System.out.printf("%02X",myByte);

..but with the ability to append the results to a StringBuilder/StringBuffer object. Any suggestions? I searched online a bit but the examples I came across included manually converting byte values which seemed a bit excessive. I'm assuming there's something in the API to simplify this in a one-line call.

[Paul Clapham]

What about Integer.toString(byte, 16) for a start? You'd have to tack on the leading zero for numbers below 16, though.

[Steve Luke]

String.format("%02X", myByte)

Would probably work. Maybe Integer.toHexString(myByte) though I am not sure that does the leading zero...

[Scott Selikoff]

Steve Luke wrote:String.format("%02X", myByte)

Would probably work. Maybe Integer.toHexString(myByte) though I am not sure that does the leading zero...

Thanks Steve, that does work. I knew there was something added in 1.5 to facilitate this but I couldn't remember. Since I need the leading zero, I'll use String.format().

[James Sabre]

This is very very easy. Class java.util.Formatter has an constructor that allows one to specify an Appendable. You can then us the format() method to write values into that Appendable. Both StringBuidler and StringBuffer implement Appendable.

Take a look at my last post in this thread.

[Jesper de Jong]

Thanks, James. Using a Formatter by passing a StringBuilder to it is efficient, because the Formatter directly writes to the StringBuilder, so strings don't have to be copied, which is useful especially if you have to format for example a byte[] to hex characters. I needed this today, here's what I wrote:

public static String bytesToHexString(byte[] bytes) {
    StringBuilder sb = new StringBuilder(bytes.length * 2);

    Formatter formatter = new Formatter(sb);
    for (byte b : bytes) {
        formatter.format("%02x", b);
    }

    return sb.toString();
}

[James Sabre]

I don't like returning a String unless I need a String so if I were doing this I would probably overload the method along the lines of -

    public static Appendable hexEncode(byte buf[], Appendable sb)
    {
        final Formatter formatter = new Formatter(sb);
        for (int i = 0; i < buf.length; i++)
        {
            formatter.format("%02x", buf[i]);
        }
        return sb;
    }

    public static String hexEncode(byte buf[])
    {
        return hexEncode(buf, new StringBuilder()).toString();
    }

As an aside - I'm never sure whether it is more efficient to index an array of primitives or iterate over it. Since the second parameter to the format() method is a varags Object it probably makes no difference her since an Integer or Byte wrapper will be created anyway. I suspect that the cost of having to parse the "%02x" greatly outweighs any cost of creating the wrappers.

[Rob Spoor]

Another way is to do all the hard work yourself:

public static Appendable hexEncode(byte buf[], Appendable sb)   
{   
    final Formatter formatter = new Formatter(sb);   
    for (int i = 0; i < buf.length; i++)   
    {   
        int low = buf[i] & 0xF;
        int high = (buf[i] >> 8) & 0xF;
        sb.append(Character.forDigit(high, 16));
        sb.append(Character.forDigit(low, 16));
    }   
    return sb;   
}

[Scott Selikoff]

Thanks everyone, but I'll stick with the one line solution. No reason to reinvent the wheel!

[Jesper de Jong]

If you just want to format one byte into a pair of hex digits, then String.format("%02X", b); is ofcourse the only thing you need. But our last few posts were about what if you have an array of bytes that you want to convert to a string of hex characters.

[Rob Spoor]

For short term use, String.format should definitely be used. James' and my solutions are minor optimizations that will only make sense if called a lot and/or if you have limited resources. James' solution creates a new Formatter each time, mine doesn't. That's the only difference between the two.

[Jesper de Jong]

Are you sure Rob? Look at line 3 of your last post above I guess you left that in there accidentally.

[Rob Spoor]

Uhm, yes. Guess that's a copy-paste error Unfortunately, the forum doesn't warn about unused variables like Eclipse does