serialization - When to use flush() in java?

Question

import java.io. * ;
public class Ser {

    public static void main(String args[]) {

        try {
            John myObj = new John("Sachin", "Cricket");
            System.out.println(myObj);
            FileOutputStream fos = new FileOutputStream("FileName");
            ObjectOutputStream oos = new ObjectOutputStream(fos);
            oos.writeObject(myObj);
            oos.flush();
            oos.close();
        } catch (Exception e) {
            System.out.println("Expection" + e);
            System.exit(0);
        }

        try {
            John myObj2;
            FileInputStream fis = new FileInputStream("FileName");
            ObjectInputStream ois = new ObjectInputStream(fis);
            myObj2 = (John) ois.readObject();
            ois.close();
            System.out.println("New Object" + myObj2);
        } catch (Exception e) {
            System.out.println("Expection" + e);
            System.exit(0);
        }

    }
}

class John implements Serializable {

    private String name;
    private String department;

    public John(String name, String department) {
        this.name = name;
        this.department = department;
    }

    public String toString() {
        return "Name" + name + " " + "Department" + department;

    }

}

I have a few questions in the above example.

1. When to use flush method and why do we use it?
2. What does close method carry a score here?
3. myObj2 = (John) ois.readObject(); ... please correct me if i am wrong, i am reading the file object and storing into another object and typecasting the file object.
4. What are the alternatives of Serialization or persisting the data in Java. I don't want the data to get into file as byte stream.

Answer 1

1. When to use flush method and why do we use it?

This is used when there needs to be synchronous sending

For example you have a duplex (2-way) connection and you just sent a message and now need to wait for the reply on it, without flush a buffered outputstream might hold it until the buffer fills up (deadlock)

2. myObj2 = (John) ois.readObject(); Please correct me if I am wrong, I am reading the file object and storing into another object and typecasting the file object.

WRONG! You read some data from the stream typecast it into a John (which will throw if the object read wasn't a John)

3. What are the alternatives of Serialization or persisting the data in Java. I don't want the data to get into file as byte stream.

You can write it as text (write out the fields as text on a line) and provide a method to parse it into an object