import java.math.BigInteger;
public class Main {
public static void main(String[] args) {
final int RADIX = 16;
String str = new String("I'm so cool.");
// String to HexString (String -> byte[] -> BigInteger -> HexString)
String hexStr = (new BigInteger(str.getBytes())).toString(RADIX);
// HexString to String (HexString -> BigInteger -> byte[] -> String)
String s = new String((new BigInteger(hexStr, RADIX)).toByteArray());
}
}