Люди добрые разъясните причину возникновения исключения

public class Solution {
    public static void main(String[] args) {

        System.out.println(new Solution(4));
        Solution saveObject = new Solution(-8);
        Solution loadObject;
        ObjectOutputStream output;
        ObjectInputStream input;

        try{
            System.out.println(saveObject.toString());
            output = new ObjectOutputStream(new FileOutputStream("test"));
            output.writeObject(saveObject);
            output.close();

            input = new ObjectInputStream(new FileInputStream("test"));
            loadObject = (Solution)input.readObject();
            input.close();

        }catch (FileNotFoundException e){
            System.out.println("File not found");
            e.printStackTrace();
        }
        catch (IOException e){
            System.out.println("error IO");
            e.printStackTrace();
        }
        catch (ClassNotFoundException e){
            System.out.println("Class not found");
            e.printStackTrace();
        }

    }

    transient private final String pattern = "dd MMMM yyyy, EEEE";//константа
    transient private Date currentDate;//создается новая дата при создании объекта
    transient private int temperature;//в водится из потока
    String string;

    public Solution(int temperature) {
        this.currentDate = new Date();
        this.temperature = temperature;

        string = "Today is %s, and the current temperature is %s C";
        SimpleDateFormat format = new SimpleDateFormat(pattern);
        this.string = String.format(string, format.format(currentDate), temperature);
    }

    @Override
    public String toString() {
        return this.string;
    }
}