Complete Example of java.io.Externalizable Implementation

/**
 * Invoice (cached value).
 */
public final class Invoice implements Externalizable {

   private int invoiceID;
   private Date invoiceDate = null;
   private int invoiceNumber;
   private int customerID;

   public Invoice(final int invoiceID, final int invoiceNumber, final Date invoiceDate, int customerID) {
      this.invoiceID = invoiceID;
      this.invoiceNumber = invoiceNumber;
      this.invoiceDate = (Date) invoiceDate.clone();
      this.customerID = customerID;
   }

   /**
    * No-argument constructor required by {@link Externalizable}.
    */
   public Invoice() {
   }

   public int getInvoiceID() {
      return invoiceID;
   }

   public Date getInvoiceDate() {
      return (Date) invoiceDate.clone();
   }

   public void setInvoiceDate(final Date invoiceDate) {
      this.invoiceDate = (Date) invoiceDate.clone();
   }

   public int getInvoiceNumber() {
      return invoiceNumber;
   }

   public void setInvoiceNumber(final int invoiceNumber) {
      this.invoiceNumber = invoiceNumber;
   }
   public void writeExternal(final ObjectOutput out) throws IOException {

      out.writeObject(invoiceDate);
      out.writeInt(invoiceID);
      out.writeInt(invoiceNumber);
      out.writeInt(customerID);
   }

   public void readExternal(final ObjectInput in) throws IOException, ClassNotFoundException {
      invoiceDate = (Date) in.readObject();
      invoiceID = in.readInt();
      invoiceNumber = in.readInt();
      customerID = in.readInt();
   }

   public boolean equals(final Object o) {
      if (this == o) return true;
      if (o == null || getClass() != o.getClass()) return false;
      final Invoice invoice = (Invoice) o;
      if (invoiceID != invoice.invoiceID) return false;
      if (invoiceNumber != invoice.invoiceNumber) return false;
      if (customerID != invoice.customerID) return false;
      if (invoiceDate != null ? !invoiceDate.equals(invoice.invoiceDate) : invoice.invoiceDate != null) return false;
      return true;
   }

   public int hashCode() {
      int result = invoiceID;
      result = 31 * result + (invoiceDate != null ? invoiceDate.hashCode() : 0);
      result = 31 * result + invoiceNumber;
      result = 31 * result + customerID;
      return result;
   }
}