A sample program to change bin to hex and hex to bin.

ChangeBinHex.java

public class ChangeBinHex {
    public static void main(String[] args) {
        String str = "z";
        String hexStr = binToHexString(str.getBytes());
        System.out.println(hexStr);  // 7a (0111 1100)

        byte[] bytes = hexStrToBin(hexStr);
        for (byte b : bytes) {
            System.out.print(b + " ");  // 122 (0111 1010) : Ascii Code of "z"
        }
        System.out.println();

        String orgStr = new String(bytes);
        System.out.println(orgStr); // z
    }

    private static String binToHexString(byte[] bytes) {
        StringBuilder hexString = new StringBuilder();
        for (byte b : bytes) {
            // I want to change as follows.
            // 11000000 -> 00000000 00000000 00000000 11000000 ->C0
            // If no "0xFF", b is treated as int and changed as follows.
            // 11000000 -> 11111111 11111111 11111111 11000000 -> FF FF FF C0
            String s = Integer.toHexString(0xFF & b);
            if (s.length() == 1) {
                hexString.append('0');
            }
            hexString.append(s);
        }
        return hexString.toString();
    }

    private static byte[] hexStrToBin(String hexStr) {
        byte[] bin = new byte[hexStr.length() / 2];
        try {
            for (int i = 0; i < bin.length; i++) {
                // Change a hex string to a numeric value(binary value).
                // Ex. "3f" (‭0011 0011  ‭0110 0110‬‬)  ->  63 (0011 1111)
                bin[i] = (byte) Integer.parseInt(hexStr.substring(i * 2, i * 2 + 2), 16);
            }
        } catch (NumberFormatException e) {
            System.err.println("Failed to change hex string to binary value");
            System.exit(1);
        }
        return bin;
    }
}