How to resolve this case " has no instantiable subtypes"

Question

This is my code for class Offer, when i compile this part of code i got a error.. please help me. Thx u guy

import java.io.Serializable;

public class Offer implements Comparable<Object>, Serializable {

    private static final long serialVersionUID = 1L;
    private Product product;
    private RegisteredUser user;
    private float amount;

    //stati dell'offerta
    private OfferState bestOffer;
    private OfferState overtakenOffer;
    private OfferState winnerOffer;
    private OfferState loserOffer;

    //stato attuale dell'offerta
    private OfferState state;

    public Offer(){}

    public Offer(Product product, RegisteredUser user, float amount)
    {
        this.user = user;
        this.amount = amount;
        this.product = product;

        bestOffer = new BestOffer(this);
        overtakenOffer = new OvertakenOffer(this);
        winnerOffer = new WinnerOffer(this);
        loserOffer = new LoserOffer(this);

        state = bestOffer;
    }


    public RegisteredUser getUser() {
        return user;
    }


    public float getAmount() {
        return amount;
    }


    public void setAmount(float amount) {
        this.amount = amount;
    }

    @Override
    public int compareTo(Object arg0) {

        return Float.compare(amount, ((Offer) arg0).getAmount());
    }


    public OfferState getBestOffer() {
        return bestOffer;
    }


    public OfferState getOvertakenOffer() {
        return overtakenOffer;
    }


    public OfferState getWinnerOffer() {
        return winnerOffer;
    }


    public OfferState getLoserOffer() {
        return loserOffer;
    }

    public OfferState getState() {
        return state;
    }

    public void setState(OfferState state) {
        this.state = state;
    }

    public void reciveOffer()
    {
        try
        {
            state.reciveOffer();
        }
        catch (UnsupportedOperationException e) {
        }
    }

    public void deadline()
    {
        try
        {
            state.deadline();
        }
        catch (UnsupportedOperationException e) {

        }
    }

    public Product getProduct() {
        return product;
    }

}

Error gwt

Answer 1

All the members like Product, RegisteredUser, OfferState have to be serializable too or marked transient. I case they aren't you have to make them. There is a nice example here: Serialization - readObject writeObject overrides