Expr and ExprProb are now serializable objects

src/runtime/java/org/grammaticalframework/pgf/Expr.java

@@ -1,6 +1,13 @@
 package org.grammaticalframework.pgf;
 
-public class Expr {
+import java.io.IOException;
+import java.io.ObjectInputStream;
+import java.io.ObjectOutputStream;
+import java.io.Serializable;
+
+public class Expr implements Serializable {
+		private static final long serialVersionUID = 1148602474802492674L;
+	
 		private Pool pool;
 		private PGF  gr;
 		private long ref;
@@ -18,4 +25,15 @@ public class Expr {
 		public static native Expr readExpr(String s) throws PGFError;
 
 		private static native String showExpr(long ref);
+		
+		private void writeObject(ObjectOutputStream out) throws IOException {
+			out.writeObject(showExpr(ref));
+		}
+		
+		private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException {
+			Expr e = readExpr((String) in.readObject());
+			pool = e.pool;
+			gr   = e.gr;
+			ref  = e.ref;
+		}
 }

src/runtime/java/org/grammaticalframework/pgf/ExprProb.java

@@ -1,6 +1,10 @@
 package org.grammaticalframework.pgf;
 
-public class ExprProb {
+import java.io.Serializable;
+
+public class ExprProb implements Serializable {
+	private static final long serialVersionUID = -3112602244416576742L;
+
 	private Expr expr;
 	private double prob;