I am reading a book Effective Java which has the following example. In the below example author copies the reference of objects present in the ObjectOutputStream by the following line
byte[] ref = {0x71, 0, 0x7e, 0, 5}; // Ref #5
Why does this reference point to the date object present in the ObjectOutputStream? what is stored in a reference?
import java.io.ByteArrayInputStream;
import java.io.ByteArrayOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.util.Date;
final class Period {
private final Date start;
private final Date end;
/**
* @param start the beginning of the period
* @param end the end of the period; must not precede start * @throws IllegalArgumentException
* if start is after end
* @throws NullPointerException if start or end is null
*/
public Period(Date start, Date end) {
this.start = new Date(start.getTime());
this.end = new Date(end.getTime());
if (this.start.compareTo(this.end) > 0)
throw new IllegalArgumentException(start + " after " + end);
}
public Date start() {
return new Date(start.getTime());
}
public Date end() {
return new Date(end.getTime());
}
public String toString() {
return start + " - " + end;
}
// Remainder omitted
}
public class MutablePeriod {
// A period instance
public final Period period;
// period's start field, to which we shouldn't have access
public final Date start;
// period's end field, to which we shouldn't have access
public final Date end;
public MutablePeriod() {
try {
ByteArrayOutputStream bos = new ByteArrayOutputStream();
ObjectOutputStream out = new ObjectOutputStream(bos);
// Serialize a valid Period instance
out.writeObject(new Period(new Date(), new Date()));
/*
* Append rogue "previous object refs" for internal * Date fields in Period. For
* details, see "Java Object Serialization Specification," Section 6.4.
*/
byte[] ref = {0x71, 0, 0x7e, 0, 5}; // Ref #5
bos.write(ref); // The start field
ref[4] = 4; // Ref#4
bos.write(ref); // The end field
// Deserialize Period and "stolen" Date references
ObjectInputStream in =
new ObjectInputStream(new ByteArrayInputStream(bos.toByteArray()));
period = (Period) in.readObject();
start = (Date) in.readObject();
end = (Date) in.readObject();
} catch (Exception e) {
throw new AssertionError(e);
}
}
}
