Serializable Class

[BramVandenbon]

I have a class that needs to implement the "Serializable" interface. I am doing this because I am saving an instance of my class to a binary file. And in order to do that it needs to be Serializable.

The name of the class is "Animation". It is a collection of images.
And has a lot of functions to make an animation of these images. I am not going to mention those here though. Because they have nothing to do with my question.

My class looks a bit like this

public class Animation(){
    ArrayList files; // a collection of Strings (the path of the imagefile)
    ArrayList images; // a collection of BufferedImages
    
    public Animation(){ //a constructor
    }

    public void addFile(String file){ //function to add new Images
        images.add(loadimages(file));
        files.add(file);
    }
}

now what functions do I need to add to make it Serializable ?
I added the implements Serializable on top of my class and then I tried the following functions ...

    private void writeObject(java.io.ObjectOutputStream out)
    throws IOException{
        out.writeObject(files);
    }
    private void readObject(java.io.ObjectInputStream in)
    throws IOException, ClassNotFoundException{
        String[] imported_files = (String[])in.readObject();
        setImageArray(imported_files);        
    }
    private void setImageArray(String[] imported_files){
        images.clear();
        files.clear();
        for(int i=0;i<imported_files.length;i++){
            images.add(loadimages(imported_files[i]));
            files.add(imported_files[i]);
        }
    }

When I run this I get an error at the following line:
String[] imported_files = (String[])in.readObject();
It says that the casting to String[] is not possible. I tried the same thing with int[] in another project and there it worked fine. a bit strange.

I only get this error when i am trying to read my instance from a file. Writing to the file works without errors. (Allthough I can not be sure it works because I have never been able to load it again).

Why do I get this error and how can I fix it?
Thank you in advance