private(?) writeObject/readObject

[Roely de Vries]

Question about writeObject and readObject,

 
import java.io.*;
 
class B {
private int i;

B (int i) {this.i = i;}
public int getI(){ return i;}
}
 
 
class A  implements Serializable {
transient B b = new B(10);

 private void writeObject(ObjectOutputStream os){
 try {
os.defaultWriteObject();
os.writeInt(b.getI());
} catch (Exception e) {e.printStackTrace();}
}

private void readObject(ObjectInputStream ois) {
try {ois.defaultReadObject();
    b = new B(ois.readInt());
} catch (Exception e){e.printStackTrace();}
}
}  
 
 
 
public class TestSer {
public static void main (String[] args){
        A a = new A();
try {
FileOutputStream fs = new FileOutputStream("test");
ObjectOutputStream os = new ObjectOutputStream(fs);
os.writeObject(a);  //invokes private a.writeObject!
os.close();
}
catch ( Exception e) {
e.printStackTrace();
}

try {
FileInputStream fi = new FileInputStream("test");
ObjectInputStream ois = new ObjectInputStream(fi);
a = (A)ois.readObject(); //invokes private a.readObject!
 
System.out.println(a.b.getI());
ois.close();
}
catch (Exception e){
e.printStackTrace();
}
}
}
 

In this code fragment b cannot be serialized,therefore it is marked as transient in A.

A is serialized an deserialized by TestSer. Because A has the private methods writeObject and readObject instead of os.defaultWriteObject and ois.defaultWriteObject.

The question is, how is this possible?
writeObject and readObject are private methods of A, so how can they be invoked by the serialization proces?


(edited: added

tags)
[ January 30, 2007: Message edited by: Barry Gaunt ]

[Henry Wong]

The question is, how is this possible?
writeObject and readObject are private methods of A, so how can they be invoked by the serialization proces?

Other questions that you may also ask include:

- How does the defaultReadObject() method of the stream object access the private instance variables?

- How come the readObject() method doesn't call the super.readObject() method? Don't the parent class have special serialization needs too?

- How come all readObject() and writeObject() methods are private?

Serialization is one area where the "basic rules of Java" does not apply. You need to learn it independently -- without thinking in java.

Henry