In a previous article, scifluent@gmail.com said:

Serializing is a "shallow copy", just like cloning.  It doesn't serialize
the objects in the ArrayList, just the ArrayList itself.

Serialisation of ArrayLists should work...

But you haven't given enough information to know what you are doing
wrong. Start with something simple that works, and work back to find
where your problem is introduced.

Here is some working *complete* code to start from:

import java.io.*;
import java.util.*;
class Write {
    public static void main(String[] args) throws Exception {
        List<Float> data = new ArrayList<Float>(
            Arrays.asList(1.0f, 2.0f, 3.0f)
        );
        ObjectOutputStream out = new ObjectOutputStream(
            new FileOutputStream("data.ser")
        );
        out.writeObject(data);
        out.close();
    }
}

import java.io.*;
import java.util.*;
class Read {
    public static void main(String[] args) throws Exception {
        ObjectInputStream in = new ObjectInputStream(
            new FileInputStream("data.ser")
        );
        List<Float> data = (List<Float>)in.readObject();
        System.out.println(data);
    }
}

Tom Hawtin

...

Perhaps an issue with the (de)serialization code for the content?

I tried the following, with a String and an Integer in my ArrayList, and
it works, with output:

java.lang.String xxx
java.lang.Integer 3

import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.IOException;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.ArrayList;

public class SerialTest {

  public static void main(String[] args) throws IOException,
ClassNotFoundException {
    // Create an ArrayList with some serializable content
    ArrayList l1 = new ArrayList();
    l1.add("xxx");
    l1.add(new Integer(3));

    // Serialize l1 into a byte[]
    ByteArrayOutputStream outBytes = new ByteArrayOutputStream();
    ObjectOutputStream out = new ObjectOutputStream(outBytes);
    out.writeObject(l1);
    out.close();
    byte[] data = outBytes.toByteArray();

    // Deserialize from byte[] into l2
    ByteArrayInputStream inBytes = new ByteArrayInputStream(data);
    ObjectInputStream in = new ObjectInputStream(inBytes);
    ArrayList l2 = (ArrayList)in.readObject();

    // Print the content of the deserialized list
    for(Object o: l2){
      if(o == null){
        System.out.println("null");
      }else{
        System.out.printf("%s %s%n",           
          o.getClass().getName(),o.toString());
      }
    }
  }
}