About Question enthuware.ocpjp.v7.2.1390 :

[Danny Sheridan]

Is it okay to declare throws Exception for these methods?
Doesn't this feature need the specific throws clauses ?

Code: Select all

private void writeObject(ObjectOutputStream out) throws IOException;
private void readObject(ObjectInputStream in) throws IOException, ClassNotFoundException;

[Danny Sheridan]

I think I see where I was mistaken.
In interface Serializable docs it says:

Classes that require special handling during the serialization and deserialization process
must implement special methods with these exact signatures

exact signatures - that's just method name and parameter types and order
But not the throws declaration
So when the JVM checks by reflection for these methods it identifies them by signature.
So any throws that covering the exceptions thrown would do.
Is this on the right track?

[admin]

You are correct. Throws clause becomes important when you override a method.

[zhengye1]

I try to create the program to understand this question, however when I try to run the program I got

Code: Select all

Exception in thread "main" java.io.NotActiveException: not in call to writeObject
	at java.io.ObjectOutputStream.defaultWriteObject(ObjectOutputStream.java:436)
	at ocp.wrongq.Portfolio.writeObject(Portfolio.java:29)
	at ocp.wrongq.Portfolio.main(Portfolio.java:46)

Here is my program:

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// In Portfolio.java
package ocp.wrongq;

import java.io.FileInputStream;
import java.io.FileOutputStream;
import java.io.ObjectInputStream;
import java.io.ObjectOutputStream;
import java.io.Serializable;

class Bond {

	String ticker = "bac";
	double coupon = 8.3;

	@Override
	public String toString() {
		return "Bond [ticker=" + ticker + ", coupon=" + coupon + "]";
	}
}

public class Portfolio extends Bond implements Serializable{
	/**
	 * 
	 */
	private static final long serialVersionUID = 2252868398846453877L;
	String accountName;
	transient Bond bond = new Bond();

	private void writeObject(ObjectOutputStream os) throws Exception {
		os.defaultWriteObject();
		os.writeObject(bond.ticker);
		os.writeDouble(bond.coupon);
	}

	private void readObject(ObjectInputStream os) throws Exception {
		os.defaultReadObject();
		this.bond = new Bond();
		bond.ticker = (String) os.readObject();
		bond.coupon = os.readDouble();
		System.out.println(this.bond);
	}

	public static void main(String[] args) throws Exception{
		Portfolio p = new Portfolio();
		FileOutputStream fos = new FileOutputStream("t.tmp");
		ObjectOutputStream oos = new ObjectOutputStream(fos);
		p.writeObject(oos);
		FileInputStream fis = new FileInputStream("t.tmp");
		ObjectInputStream ios = new ObjectInputStream(fis);		
		p.readObject(ios);
	}
}

What did I do wrong?

[admin]

You need to change p.writeObject(oos); to oos.writeObject(p);
and p.readObject(ios); to ios.readObject(p);

[zhengye1]

But if we did this change, how to invoke that two private method?

[admin]

This should be helpful: https://stackoverflow.com/questions/746 ... sient-vari

[bortnikov_sa@mail.ru]

Hellow. Is it ok to use: os.writeObject(accountName) and accountName = (String) os.readObject() instead os.defaultWriteObject() and os.defaultReadObject() respectively?

[admin]

No, both the methods have different purpose. defaultReadObject can only be called from within the implementation of readObject. Please take look at their API description.

[crazymind]

hi, I don't quite understand why this code keep Bond as transient and serialize its fields explicitly?
And if we don't override writeObject(), will writeObject() call defaultWriteObject() internally? Thanks!

[admin]

Because Bond class does not implement Serializable interface. If you try to serialize a Portfolio object, it will fail because it contains a Bond field, which cannot be serialized by the default serialization mechanism. That is why we have to implement our own customized serialization logic for Portfolio class so that information contained in Bond is saved and can be read back later while deserialization.

[radionoise]

Hellow. Is it ok to use: os.writeObject(accountName) and accountName = (String) os.readObject() instead os.defaultWriteObject() and os.defaultReadObject() respectively?

No, both the methods have different purpose. defaultReadObject can only be called from within the implementation of readObject. Please take look at their API description.

I don't see a reason why calling os.writeObject(accountName) instead of os.defaultWriteObject() and this.accountName = (String) os.readObject() instead of os.defaultReadObject() not woking.

Code: Select all

class Bond {
    String ticker = "bac";
    double coupon = 8.3;
}

class Portfolio implements Serializable {
    String accountName;
    transient Bond bond = new Bond();

    private void writeObject(ObjectOutputStream os) throws Exception {
        os.writeObject(accountName);
        os.writeObject(bond.ticker);
        os.writeDouble(bond.coupon);
    }

    private void readObject(ObjectInputStream os) throws Exception {
        this.accountName = (String) os.readObject();
        this.bond = new Bond();
        bond.ticker = (String) os.readObject();
        bond.coupon = os.readDouble();
    }
}