Input/Output - Beginning Java 8 Language Features - page 330

Writing an Object More Than Once to a Stream
The JVM keeps track of object references it writes to the object output stream using the writeObject() method.
Suppose you have a PersonMutable object named john and you use an ObjectOutputStream object oos to write it to a
file as follows:
PersonMutable john = new PersonMutable("John", "Male", 6.7);
oos.writeObject(john);
At this time, Java makes a note that the object john has been written to the stream. You may want to change some
attributes of the john and write it to the stream again as follows:
john.setName("John Jacobs");
john.setHeight(5.9);
oos.writeObject(john);
At this time, Java does not write the john object to the stream. Rather, the JVM back references it to the john
object that you wrote the first time. That is, all changes made to the name and height fields are not written to the
stream separately. Both writes for the john object share the same object in the written stream. When you read the
objects back, both objects will have the same name , gender , and height .
An object is not written more than once to a stream to keep the size of the serialized objects smaller. Listing 7-29
shows this process. The MultipleSerialization class as shown in Listing 7-30, in its serialize() method, writes an
object, changes object's attributes, and serializes the same object again. It reads the objects in its deserialize() method.
The output shows that Java did not write the changes made to the object when it wrote the object the second time.
Listing 7-29. A MutablePerson Class Whose Name and Height Can Be Changed
// MutablePerson.java
package com.jdojo.io;
import java.io.Serializable;
public class MutablePerson implements Serializable {
private String name = "Unknown";
private String gender = "Unknown" ;
private double height = Double.NaN;
public MutablePerson(String name, String gender, double height) {
this.name = name;
this.gender = gender;
this.height = height;
}
public void setName(String name) {
this.name = name;
}
public String getName() {
return name;
}