Exception in thread "main" java.lang.ClassCastException [duplicate]

Question

import java.io.*;

class rak implements Serializable{

    int i;
}

public class Main {

    public static void main(String[] args) throws Exception {
        // write your code herer
        rak r = new rak();
        r.i = 9;

        File f = new File("da.txt");
        FileOutputStream f1 = new FileOutputStream(f);
        ObjectOutputStream oos = new ObjectOutputStream(f1);
        oos.writeObject("value of  i is" + r.i);

        FileInputStream f0 = new FileInputStream(f);
        ObjectInputStream f9 = new ObjectInputStream(f0);
        rak r1 = new rak();
        r1 = (rak) f9.readObject();

        System.out.println(r1.i);

    }
}

Please consider asking a real question, and describing which exact errors you get and from which lines. — Arnaud
– Arnaud, Commented Jul 18, 2018 at 13:08

davidxxx · Accepted Answer · 2018-07-18 13:09:01Z

2

You serialize a String :

rak r = new rak();
...
oos.writeObject("value of  i is" + r.i);

And you cast the deserialization result into a rak object :

r1 = (rak) f9.readObject();

Whereas the ClassCastException : a String is not a rak.

If you want to deserialize a rak, serialize it and not only one of its fields such as :

oos.writeObject(r);

answered Jul 18, 2018 at 13:09

davidxxx

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Comments

Nikolas · Accepted Answer · 2018-07-18 13:11:14Z

0

At this line:

oos.writeObject("value of  i is" + r.i);

You serialize the String "value of i is9" literally instead of the object itself. Thus the only possible cast is to the String again.

String string = (String) f9.readObject();

To fix this issue, serialize the whole object:

rak r = new rak();
r.i = 9;
// ...
oos.writeObject(r);

And the result of the last line would be correct:

// ...
rak r1 =  (rak) f9.readObject();
System.out.println(r1.i);          // prints 9

answered Jul 18, 2018 at 13:11

Nikolas

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4 Comments

Nikolas Over a year ago

Please, mark the best answer as accepted to let others know this issue has been resolved.

rakshit ks Over a year ago

The same thing worked for my friend who used netbeans ide for debugging.... How is that possible?

rakshit ks Over a year ago

And how to mark the best answer...I am new to stack overflow...I am newbie

Nikolas Over a year ago

You choose the best one and click to the “mark” symbol on the left side of the answer. It will change color to green. meta.stackexchange.com/questions/5234/…

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