Cannot write to file ArrayList with class objects

Question

I've reasearched a lot of websites and I couldn't find answear. I'm trying to write to .txt file my ArrayList which constains class objects. Every time I try to do it I`m getting exception. With reading is the same problem. Here is my code:

public static void write()
{
    try 
    {
        FileOutputStream out = new FileOutputStream("clients.txt");
        ObjectOutputStream oout = new ObjectOutputStream(out);
        oout.writeObject(lista);
        oout.close();
    }
    catch(Exception ioe)
    {
        System.out.println("writing Error!");
        welcome();
    }
}
public static void read()
{

    try
    {
        ObjectInputStream ois = new ObjectInputStream(new FileInputStream("clients.txt"));
        lista = (List<Client>) ois.readObject();
    }
    catch (ClassNotFoundException ex) 
    {
        System.out.println("Koniec pliku");
    }
    catch(IOException ioe)
    {
        System.out.println("Error!");
        welcome();
    }
}

Answer 1

I guess you're looking for the Serializable interface of Java. In order to save objects you're class have to implement it.

The question is: What execatly do you want to save? The content of the list so that you can save it in a file and load it afterwards?

This simple example works for me (for the scenario I mention above):

public class User implements Serializable {
   private static final long serialVersionUID = 1L;

   private String name;
   private int age;

   public User(String name, int ag) {
      this.name = name;
      this.age = age;
   }

   @Override
   public String toString() {
     return (this.name + ' ' + this.age);
   }
}
public class Main {
   private static List<User> l;

   public static void main(String[] args) {
     l = new ArrayList<User>();
     user1 = new User("John", 22);
     user2 = new User("Jo", 33);
     l.add(user1);
     l.add(user2);
     write();
   }

   public static void write() {
      try {
        FileOutputStream fos = new FileOutputStream("testout.txt");
        BufferedOutputStream bos = new BufferedOutputStream(fos);
        ObjectOutputStream oos = new ObjectOutputStream(bos);
        oos.writeObject(l);
        oos.close();
    } catch (Exception ioe) {
        System.out.println("writing Error!");

    }
  }
}

Answer 2

Ok I have changed a bit (not each function just the read and write functionality) and this work. Link to Code . One important thing is that the Scanner class is not serializable. Therefore, you have to make it static for example.