refactor: add final modifier for classes

Author: oldratlee

library/src/main/java/com/alibaba/dcm/DnsCache.java

@@ -19,7 +19,7 @@
 @Immutable
 @ParametersAreNonnullByDefault
 @ReturnValuesAreNonnullByDefault
-public class DnsCache implements Serializable {
+public final class DnsCache implements Serializable {
     private static final long serialVersionUID = -8614746635950970028L;
 
     private final List<DnsCacheEntry> cache;

library/src/main/java/com/alibaba/dcm/DnsCacheManipulator.java

@@ -31,7 +31,7 @@
  */
 @ParametersAreNonnullByDefault
 @ReturnValuesAreNonnullByDefault
-public class DnsCacheManipulator {
+public final class DnsCacheManipulator {
     /**
      * Set a <b>never expired</b> dns cache entry.
      *

library/src/main/java/com/alibaba/dcm/DnsCacheManipulatorException.java

@@ -6,7 +6,7 @@
  * @author Jerry Lee (oldratlee at gmail dot com)
  * @see DnsCacheManipulator
  */
-public class DnsCacheManipulatorException extends RuntimeException {
+public final class DnsCacheManipulatorException extends RuntimeException {
     private static final long serialVersionUID = -7843069964883320844L;
 
     /**

library/src/main/java/com/alibaba/dcm/internal/IPAddressUtil.java

@@ -38,7 +38,7 @@
 
 @SuppressWarnings({"RedundantIfStatement", "PointlessBitwiseExpression"})
 @SuppressFBWarnings({"SF_SWITCH_FALLTHROUGH", "SF_SWITCH_NO_DEFAULT"})
-class IPAddressUtil {
+final class IPAddressUtil {
     private static final int INADDR4SZ = 4;
     private static final int INADDR16SZ = 16;
     private static final int INT16SZ = 2;