Question

I am trying to seriliaze an Object having a reference to another Object as instance variable. I follow the common way, nevertheless I cannot retrive in the main() method the second Object (which is non Serializable). Here is my Code:

 public class Car {

String type;
int speed;

public Car(String s, int v){
    type = s;
    speed = v;
}

}

public class Employee implements Serializable {

String name;
int Id;
transient Car car;

public Employee(String s, int i, Car c){
    name = s;
    Id = i;
    car = c;
}

private void writeObject(ObjectOutputStream os){
    try {
        os.defaultWriteObject();
        os.writeObject(car);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

private void readObject(ObjectInputStream is){
    try {
        is.defaultReadObject();
        car = (Car)is.readObject();
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

}

In the main method:

public static void main(String[] args) {

    Car c = new Car("noType", 100);
    Employee e = new Employee("Aris", 1, c);

    try {
        FileOutputStream fo = new FileOutputStream("save.txt");
        ObjectOutputStream out = new ObjectOutputStream(fo);
        out.writeObject(e);
        out.close();
    } catch (IOException e1) {
        e1.printStackTrace();
    }


    try {
        FileInputStream fi = new FileInputStream("save.txt");
        ObjectInputStream in = new ObjectInputStream(fi);
        Employee emp = (Employee) in.readObject();     // Comment
        System.out.println(emp.car.speed);
        in.close();
    }catch (ClassNotFoundException e1) {
        e1.printStackTrace();
    } 
    catch (IOException e1) {
        e1.printStackTrace();
    }
}

On the line “Comment” in the second try block it throws a nullPointerException. It cannot save the Car object this way. What should I do to get over it? Ofcource I want necessarily the Car class to remain non-seriliazable. If prefer to save the instrance variables of the Car object and to recreate it by the aid of them and its constructor, how can I save (and retrieve) the String attribute?

UPDATE:

When trying to save only the int instance variable, it works. The relevant methods are this time:

private void writeObject(ObjectOutputStream os){
    try {
        os.defaultWriteObject();
        os.writeInt(car.speed);
    } catch (IOException e) {
        e.printStackTrace();
    }
}

private void readObject(ObjectInputStream is){
    try {
        is.defaultReadObject();
        car = new Car("theNew", is.readInt());
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}

But my question is still how (or if) I can save the entire object. After all I did not manage to save and retrieve the String. WriteString(), ReadString() methods
do not exist.

Answer 1

The line

car = (Car)is.readObject();

is wrong.
Car is not Serializable.

Solutions:

1. make car serializable, and remove the line

2. Use a MyCar object which is Seriazable only for the purpose of ser/ deser, and create car from MyCar, see point 3.

3. create a new Car : car = new Car(); but where to get the data from? (see point 2)

4. seialize the fields of car, one by one using os.writeObject() and for primitives e.g os.writeInt();

5. use your own custom serialization, using DataOutputStream

Update answer to your update question

String is an object, you serialize it with os.writeObject(car.type);
See also ObjectOutputStream
Code using solution 4:

private void writeObject(ObjectOutputStream os){
    try {
        os.defaultWriteObject();
        os.writeObject(car.type);
        os.writeInt(car.speed);
    } catch (IOException e) {
        e.printStackTrace();
    }
}


private void readObject(ObjectInputStream is){
    try {
        is.defaultReadObject();
        String type = (String) is.readObject();
        int speed = is.readInt();
        this.car = new Car(type, speed);
    } catch (ClassNotFoundException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    }
}