reduces size of serialised form by 15-20 percent

This trims some overhead from the serialised form. The sample IBAN used in the
unit test drops from 101 bytes to 82 bytes. The drop gets smaller the longer
the IBAN, but this is a tidy percentage in all cases.

Relates to #23.

src/main/java/nl/garvelink/iban/IBAN.java

@@ -15,8 +15,11 @@
  */
 package nl.garvelink.iban;
 
+import java.io.Externalizable;
 import java.io.IOException;
 import java.io.InvalidObjectException;
+import java.io.ObjectInput;
+import java.io.ObjectOutput;
 import java.io.ObjectStreamException;
 import java.io.Serializable;
 import java.util.Comparator;
@@ -312,7 +315,7 @@ private void readObjectNoData()
      *
      * There should be no need to ever use IBAN.Memento in your code.
      */
-    static final class Memento implements Serializable {
+    static final class Memento implements Externalizable {
         private static final long serialVersionUID = 1L;
         private String value;
 
@@ -325,6 +328,22 @@ public Memento() {
             this.value = value;
         }
 
+        @Override
+        public void writeExternal(ObjectOutput out) throws IOException {
+            out.writeLong(serialVersionUID);
+            out.writeUTF(this.value);
+        }
+
+        @Override
+        public void readExternal(ObjectInput in) throws IOException, ClassNotFoundException {
+            long serialUID = in.readLong();
+            if (serialUID == 1L) {
+                this.value = in.readUTF();
+            } else {
+                throw new InvalidObjectException("Unsupported serial version: " + serialUID);
+            }
+        }
+
         private Object readResolve() throws ObjectStreamException {
             try {
                 return IBAN.parse(this.value);

src/test/java/nl/garvelink/iban/IBANTest.java

@@ -164,7 +164,7 @@ public void testSerializationRoundTrip() throws IOException, ClassNotFoundExcept
     public void testSerializedFormCompatibility() throws IOException, ClassNotFoundException {
         // This was manually sampled from the preceding test. This test is to ensure that the serialised form remains
         // stable as the library evolves.
-        String serializedForm = "rO0ABXNyAB5ubC5nYXJ2ZWxpbmsuaWJhbi5JQkFOJE1lbWVudG8AAAAAAAAAAQIAAUwABXZhbHVldAASTGphdmEvbGFuZy9TdHJpbmc7eHB0ABJOTDkxQUJOQTA0MTcxNjQzMDA=";
+        String serializedForm = "rO0ABXNyAB5ubC5nYXJ2ZWxpbmsuaWJhbi5JQkFOJE1lbWVudG8AAAAAAAAAAQwAAHhwdxwAAAAAAAAAAQASTkw5MUFCTkEwNDE3MTY0MzAweA==";
         ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(Base64.getDecoder().decode(serializedForm)));
         IBAN copy = (IBAN) ois.readObject();
         assertThat(copy.toPlainString(), is(equalTo(VALID_IBAN)));
@@ -173,7 +173,7 @@ public void testSerializedFormCompatibility() throws IOException, ClassNotFoundE
     @Test
     public void testDeserializationPerformsValidation() throws IOException, ClassNotFoundException {
         // This is the same base64 blob as above, altered to put in a garbage IBAN value.
-        String serializedForm = "rO0ABXNyAB5ubC5nYXJ2ZWxpbmsuaWJhbi5JQkFOJE1lbWVudG8AAAAAAAAAAQIAAUwABXZhbHVldAASTGphdmEvbGFuZy9TdHJpbmc7eHB0ABJOTDkxQUJORTA0MTcxNjQzMDA=";
+        String serializedForm = "rO0ABXNyAB5ubC5nYXJ2ZWxpbmsuaWJhbi5JQkFOJE1lbWVudG8AAAAAAAAAAQwAAHhwdxwAAAAAAAAAAQASTkw5MUFCTkEwNEE3MTY0MzAweA==";
         ObjectInputStream ois = new ObjectInputStream(new ByteArrayInputStream(Base64.getDecoder().decode(serializedForm)));
         try {
             IBAN copy = (IBAN) ois.readObject();