fix various ObjectOutputStream/ObjectInputStream bugs

classpath/java/util/Vector.java

@@ -10,7 +10,7 @@
 
 package java.util;
 
-public class Vector<T> extends AbstractList<T> {
+public class Vector<T> extends AbstractList<T> implements java.io.Serializable {
   private final ArrayList<T> list;
 
   public Vector(int capacity) {
@@ -81,6 +81,10 @@ public class Vector<T> extends AbstractList<T> {
     remove(index);
   }
 
+  public synchronized void removeAllElements() {
+    list.clear();
+  }
+
   public synchronized boolean remove(Object element) {
     return list.remove(element);
   }