How can I convert hexadecimal into a utf-8 encoded string in Java

Question

I have a hexadecimal string and I am trying to convert it back into a utf-8 encoded string.

Example:

String hexString = "6a6f65";

How do I convert that string above back into "joe"

Answer 1

If you can be sure that the hex string comes from a byte array of a properly UTF-8 encoded string, all you need to do is:

1. Convert the hex string back into a byte array.
2. Convert the byte array further back into a string, with correct encoding of course.

For the first part, there's a range of ways to do it. Just see this question and pick one that suits your needs.

Once you get the byte array back from the hex string, do this:

String s = new String(bytearr, StandardCharsets.UTF_8);

Answer 2

You cannot do so reliably.

Unicode characters may be encoded at any code point from U+0000 to U+10FFFF.

So there is no way for us to know how many characters at a time in your input should be parsed as the hexadecimal number of a Unicode code point.

Substring > code point integer > StringBuilder#appendCodePoint > String

If you know for certain the input should be parsed two characters at a time, use String#substring to retrieve each pair of characters. Parse each pair using Integer.parse.

int codePoint = Integer.parseInt( hexInput ,16 ) ;

Build up your results by using StringBuilder#appendCodePoint.

String hexString = "6a6f65";
StringBuilder builder = new StringBuilder();
for ( int i = 0 ; i < hexString.length() ; i += 2 ) {
    String substring = hexString.substring( i , i + 2 );
    int codePoint = Integer.parseInt( substring , 16 );
    builder.appendCodePoint( codePoint );
}
String result = builder.toString();

See this code run live at IdeOne.com.

result = joe

Caveat: If such inputs are coming from UTF-8 encoded text, this approach is not reliable. Such text may use 1, 2, 3, or 4 octets of data to represent any one character. If your input is indeed UTF-8 encoded text, then you should parse it as such.

Streams

Not that I recommend doing so in this case, but you could use streams.

StringBuilder builder = new StringBuilder();
String input = "6a6f65";
IntStream.iterate( 0 , ( x ) -> x < input.length() , i -> i + 2 ).forEach( i -> builder.appendCodePoint( Integer.parseInt( input.substring( i , i + 2 ) , 16 ) ) );
System.out.println( "builder = " + builder );

builder = joe