While Conditions for Deserializing Multiple Objects in Java

Question

I am currently preparing for an evaluation for a Job Offer i have. So the company has given some practice assignments and their respective Solutions.

I am facing a bit of dilemma on how to deserialize multiple objects from a Serialized file.

Initially when i was solving on my own, after studying the class definitions for FileInputStream and ObjectInputStream i figured the best way to achieve that would be.

        FileInputStream fis = new FileInputStream(fileName);            
        ObjectInputStream ois = new ObjectInputStream(fis);
        while(fis.available()!=0)
        {
            obj = (Show)ois.readObject();
            lst.add(obj);
        }
        ois.close();
        fis.close();

But later when i searched , i didn't find anyone suggesting this, Though this worked perfectly for me. The Companies' Solution to the Assignment is

         in=new ObjectInputStream(new FileInputStream(fileName));
         Show s=null;
         while((s=(Show)in.readObject())!=null){

            list.add(s);
         }

But when i try to use the latter solution in another program it gives me an Exception

        FileInputStream fis = new FileInputStream(fileNameChannel);
        ObjectInputStream ois = new ObjectInputStream(fis);

        while((obj=(Channel) ois.readObject())!=null){
            list.add(obj);
        }
        fis.close();
        ois.close();

Exception:

java.io.EOFException
    at java.io.ObjectInputStream$BlockDataInputStream.peekByte(Unknown Source)
    at java.io.ObjectInputStream.readObject0(Unknown Source)
    at java.io.ObjectInputStream.readObject(Unknown Source)
    at com.psl.util.SetTopBoxManagementSystemImpl.populateByChannelCategory(SetTopBoxManagementSystemImpl.java:29)
    at com.psl.main.Client.main(Client.java:18)

To find a proper answer i tried searching over StackExchange and found people were actually suggesting in catching the exception and Handling it in combination with using While(true). - Link

What i couldn't understand is

Why shouldn't I use fis.available()>0 ? or rather why doesn't anyone?
What might be the possible reasons that the same implementation is resulting in an EOFException in one program but not in the other? Since both are extracting all the serialized objects present in the Serialized file.
Why should I consider using while(true) and catching EOFException, if it is possible for me to implement in a way that no Exception would occur as long as everything is All right? It just seems like a shortcut for achieving. If something does go wrong(EOF wise) , how would we tackle that?

It is possible for available to return 0 even though we haven't read all the data we want too. I think they may be writing a null object to indicate the end of file. — BevynQ
– BevynQ, Commented Jun 8, 2015 at 23:42

user207421 · Accepted Answer · 2015-06-09 00:23:38Z

2

readObject() doesn't return null at end of stream. It can do that any time. It returns null if you wrote a null. So you can't use that.
available() == 0 is not a test for end of stream: see the Javadoc. So you can't use that.
readObject() throws EOFException at end of stream. So use that.

To answer your questions:

Why shouldn't I use fis.available()>0 ?

Because the number of bytes available to be read without blocking has nothing to do with the end of the stream.

or rather why doesn't anyone?

Because it doesn't work.

What might be the possible reasons that the same implementation is resulting in an EOFException in one program but not in the other?

One of them is being sent a null to indicate end of stream. This is poor practice, as it prevents you using null for anything else. An out of band value is required. Indeed it isn't clear that the null does mean end of stream here. You might be prematurely stopping reading.

Why should I consider using while(true) and catching EOFException

Because that's the way the API is designed.

if it is possible for me to implement in a way that no Exception would occur as long as everything is All right?

But everything won't be all right forever. You will get truncated files, dropped connections, ...

It just seems like a shortcut for achieving.

This is meaningless.

If something does go wrong(EOF wise) , how would we tackle that?

Catch the exception.

answered Jun 9, 2015 at 0:23

user207421

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2 Comments

jaco0646 Over a year ago

To be fair, the way the API is designed... sucks. It's a bit like those old, locked questions on SO.

This API exists because it has historical significance, but it is not considered a good, on-topic API for this language, so please do not use it as evidence that you should write similar APIs. This API and its implementations are frozen and cannot be changed.

user207421 Over a year ago

@jaco0646 Rubbish. There is no other way it could possibly have been designed. An out-of band value is required, and an exception is the only way to provide that when an object is being returned. And don't use code formatting for text that isnt code. If it's intended to be a quotation, please provide a proper citation. And please provide a citation for your claim that 'this API and its implementations are frozen'.

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