How to read file using ObjectInputStream and add objects to HashSet?

Question

I am trying to read file and add all objects into HashSet but I have an error "User cannot be cast to java.util.HashSet". How can I fix it?

private void readFromFile(String file){
    Set<User> users = new HashSet<User>();
    try(FileInputStream fileInputStream = new FileInputStream(file)){
        ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);

        users = (HashSet) objectInputStream.readObject();

        System.out.println(users);
        objectInputStream.close();

    } catch (IOException | ClassNotFoundException e) {
        e.printStackTrace();
    }
}

And how can I use serialize more than one object into file? For example(I am using Scanner and trying to input name and surname), when I do it my file has only one object.

Answer 1

First, as @Nicholas K mentioned,

because you wrote User objects into the stream, then you should read User objects after.

And as you said

When I tried it I had only one object there but my file includes more than three users

You got one object because you read only one object.

Calling

User user = (User) objectInputStream.readObject();

reads only one user (if exists). Calling read() once more and you will get your second object (again, if it exists).

So all you need to do is this:

private void readFromFile(String file){
        Set<User> users = new HashSet<User>();
        try(FileInputStream fileInputStream = new FileInputStream(file)){
            ObjectInputStream objectInputStream = new ObjectInputStream(fileInputStream);

            while (true) {
                User user = (User) objectInputStream.readObject();
                users.add(user);    
            }

        } catch(EOFException e){
            // This exception is raised because the whole file was read.
            // So print the users in the set.

            for (User user : users) {
                System.out.println(user);
            }

        } catch (IOException | ClassNotFoundException e) {
            e.printStackTrace();
        }finally{
            // Never forget to close the stream after you're done
            if (objectInputStream!=null) {
                objectInputStream.close();
            }
    }