How to do a totp in Java

Totp.java

import java.nio.ByteBuffer;
import java.nio.ByteOrder;
import java.security.InvalidKeyException;
import java.security.NoSuchAlgorithmException;
import java.time.Instant;

import javax.crypto.Mac;
import javax.crypto.spec.SecretKeySpec;

import org.apache.commons.codec.binary.Base32;

/**
 * Needs:
 * <pre> {@code
 *       <dependency>
 *           <groupId>commons-codec</groupId>
 *           <artifactId>commons-codec</artifactId>
 *           <version>1.15</version>
 *       </dependency>
 * } </pre>
 */
public class Totp {
    private int timeStepSizeSeconds = 30;
    private int numberOfDigits = 6;

    public String totp(String code) throws NoSuchAlgorithmException, InvalidKeyException {
        return hotp(code, Math.floorDiv(Instant.now().getEpochSecond(), timeStepSizeSeconds), numberOfDigits);
    }

    public static String hotp(String keyString, long counter, int digits) throws NoSuchAlgorithmException, InvalidKeyException {
        byte[] key = new Base32().decode(keyString);
        byte[] counterBytes = ByteBuffer.allocate(8).order(ByteOrder.BIG_ENDIAN).putLong(counter).array();
        SecretKeySpec keySpec = new SecretKeySpec(key, "HmacSHA1");
        Mac mac = Mac.getInstance("HmacSHA1");
        mac.init(keySpec);
        byte[] macValue = mac.doFinal(counterBytes);
        int offset = macValue[macValue.length - 1] & 0x0f;
        int binary = ByteBuffer.wrap(macValue, offset, 4).order(ByteOrder.BIG_ENDIAN).getInt() & 0x7fffffff;
        String otp = Integer.toString(binary);
        return otp.substring(otp.length() - digits);
    }

    public static void main(String[] args) throws InvalidKeyException, NoSuchAlgorithmException {
        String base32Secret = args[0];
        System.out.println(new Totp().totp(base32Secret));
    }
}