Fix #24 - Limit the extensibility of classes and methods

src/main/java/dev/samstevens/totp/exceptions/CodeGenerationException.java

@@ -1,6 +1,6 @@
 package dev.samstevens.totp.exceptions;
 
-public class CodeGenerationException extends Exception {
+public final class CodeGenerationException extends Exception {
     public CodeGenerationException(String message, Throwable cause) {
         super(message, cause);
     }

src/main/java/dev/samstevens/totp/exceptions/QrGenerationException.java

@@ -1,6 +1,6 @@
 package dev.samstevens.totp.exceptions;
 
-public class QrGenerationException extends Exception {
+public final class QrGenerationException extends Exception {
     public QrGenerationException(String message, Throwable cause) {
         super(message, cause);
     }

src/main/java/dev/samstevens/totp/exceptions/TimeProviderException.java

@@ -1,6 +1,6 @@
 package dev.samstevens.totp.exceptions;
 
-public class TimeProviderException extends RuntimeException {
+public final class TimeProviderException extends RuntimeException {
     public TimeProviderException(String message, Throwable cause) {
         super(message, cause);
     }

src/main/java/dev/samstevens/totp/qr/QrData.java

@@ -7,7 +7,7 @@
 import java.nio.charset.StandardCharsets;
 
 @SuppressWarnings("WeakerAccess")
-public class QrData {
+public final class QrData {
 
     private final String type;
     private final String label;

src/main/java/dev/samstevens/totp/time/NtpTimeProvider.java

@@ -6,7 +6,7 @@
 import java.net.InetAddress;
 import java.net.UnknownHostException;
 
-public class NtpTimeProvider implements TimeProvider {
+public final class NtpTimeProvider implements TimeProvider {
 
     private final NTPUDPClient client;
     private final InetAddress ntpHost;

src/main/java/dev/samstevens/totp/time/SystemTimeProvider.java

@@ -3,7 +3,7 @@
 import dev.samstevens.totp.exceptions.TimeProviderException;
 import java.time.Instant;
 
-public class SystemTimeProvider implements TimeProvider {
+public final class SystemTimeProvider implements TimeProvider {
     @Override
     public long getTime() throws TimeProviderException {
         return Instant.now().getEpochSecond();

src/main/java/dev/samstevens/totp/util/Utils.java

@@ -2,7 +2,7 @@
 
 import org.apache.commons.codec.binary.Base64;
 
-public class Utils {
+public final class Utils {
     private static Base64 base64Codec = new Base64();
 
     // Class not meant to be instantiated