43: Keyword is now Serializable

The case of Keyword is complicated by the fact that Keyword instances
are controlled.  We use a SerializationProxy which implements
readResolve() by calling newKeyword() so as to maintain the invariant:

    Symbol s1 = ..., s2 = ...;
    Keyword kw1 = Keyword.newKeyword(s1);
    Keyword kw2 = Keyword.newKeyword(s2);

    (s1.equals(s2)) == (kw1 == kw2)

That is, any two keywords are *identical* if and only if they were
created from any two *equal* Symbols.

Note that newKeyword(prefix, name) is exactly equivalent to
newKeyword(newSymbol(prefix, name)).

src/main/java/us/bpsm/edn/Keyword.java

@@ -1,6 +1,11 @@
 // (c) 2012 B Smith-Mannschott -- Distributed under the Eclipse Public License
 package us.bpsm.edn;
 
+import java.io.InvalidObjectException;
+import java.io.ObjectInputStream;
+import java.io.ObjectStreamException;
+import java.io.Serializable;
+
 import static us.bpsm.edn.Symbol.newSymbol;
 
 /**
@@ -19,7 +24,7 @@
  * k.toString()  => ":foo/bar"}
  * </pre>
  */
-public final class Keyword implements Named, Comparable<Keyword> {
+public final class Keyword implements Named, Comparable<Keyword>, Serializable {
     private final Symbol sym;
 
     /** {@inheritDoc} */
@@ -91,4 +96,23 @@ public int compareTo(Keyword o) {
 
     private static final Interner<Symbol, Keyword> INTERNER = new Interner<Symbol, Keyword>();
 
+    private Object writeReplace() {
+        return new SerializationProxy(sym);
+    }
+
+    private void readObject(ObjectInputStream stream)
+      throws InvalidObjectException {
+        throw new InvalidObjectException("only proxy can be serialized");
+    }
+
+    private static class SerializationProxy implements Serializable {
+        private static final long serialVersionUID = 1L;
+        private final Symbol sym;
+        private SerializationProxy(Symbol sym) {
+            this.sym = sym;
+        }
+        private Object readResolve() throws ObjectStreamException {
+            return newKeyword(sym);
+        }
+    }
 }