Java Programming

I'm using externalizable for my app; the objects involved have ArrayList(s) of nested objects. All objects involved implement Externalizable. However, the constructor for the objects referenced from the ArrayList(s) are not called; nor is readExternal for such objects. This is causing problems for version control.

Example:

class X implements Externalizable
{
  static final long serialVersionUID = 1L;
  int i;
  ArrayList<Y> list;

  ... constructor, other code ...

  public void writeExternal (ObjectOutput out)
  {
    out.writeLong (serialVersionUID);
    out.writeInt (i);
    out.writeObject (list);
  }

  public void readExternal (ObjectInput in)
  {
    long UID = in.readLong();
    if (UID != serialVersionUID)
      throw new InvalidClassException (UID);
    i  = in.readInt();
    list = (ArrayList<Y>)in.readObject();
  }
}

class Y implements Externalizable
{
  static final long serialVersionUID = 1L;
  int j;

  ... constructor, other code ...

  public void writeExternal (ObjectOutput out)
  {
    out.writeLong (serialVersionUID);
    out.writeInt (j);
  }

  public void readExternal (ObjectInput in)
  {
    long UID = in.readLong();
    if (UID != serialVersionUID)
      throw new InvalidClassException (UID);
    j  = in.readInt();
  }
}

In this example, the constructor nor readExternal are called when X.readExternal executes.

Thanks in advance.