Question

I am trying to save an object to a file with the class below but it gives me java.io.NotSerializableException.

import java.io.FileOutputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;
import java.util.Calendar;

class Saver {

  Calendar cal = Calendar.getInstance();

  public void save(ArrayList<Product> products) {
    for (int i = 0; i < products.size(); i++) {
      try { 
        FileOutputStream saveFile = new FileOutputStream(
        "/" + products.get(i) + ".product"
        );

        ObjectOutputStream oos = new ObjectOutputStream(saveFile);
        oos.writeObject(products.get(i));
      } catch(Exception ex) {
        System.out.println(ex); 
      }
    }
  }

}

Whereas the class Product looks like this:

class Product {

  private String title;
  private int id;
  private double price;

  public Product(String title, int id, double price) {
    this.title = title;
    this.id = id;
    this.price = price;
  }

  public String getTitle() {
    return title;
  }

  public int getId() {
    return id;
  } 

  public double getPrice() {
    return price;
  }

  public void setTitle(String title) {
    this.title = title;
  }

  public void setId(int id) {
    this.id = id;
  }

  public void setPrice(double price) {
    this.price = price;
  }

}

what am I doing wrong and how can I store the objects in a file then?
Thanks in advance!

Answer 1

As the exception says, the class (Product in this case) isn’t serializable. You can probably just get away with this change:

class Product implements Serializable {
     // Code as before
}

but you should also consider declaring a serialVersionUID field for versioning purposes. Read the docs for Serializable and the serialization tutorial for a lot more detail. You don’t need to write any new members – it’s just a marker interface – but there are other methods you can add to give more control over serialization and deserialization.

You should also consider alternative serialization approaches – personally I like Google Protocol Buffers, but I’m biased 🙂 Consider what versioning you’ll need, and any cross-platform support.

Additionally:

• You’re using Product.toString() implicitly in the filename, but that won’t give you anything useful. What do you want the filename to be based on?
• You’re not doing anything with cal – how do you want the current date/time to be stored?