XMLEnocder mess.. (Java)

[Number_25]

Hey guys,

I posted a similar thread earlier on in the month, but there seemed to be some sort of confusion regarding what I was actually asking.

For one of our modules we were asked to create a program which takes some objects and stores them into files using three different methods – Text, XML and Serializable.

I was able to do Serializable and Text, but I seem to be having some confusion with XML.

I have two classes, Books and Authors.

Books are set into the Author using names – so the code looks something akin to this..

Book class -

Instance = private Author who;

public void setAuthor(ArrayList<Author> authors) {  
         this.who = who;
 }

and in the Author class..
.

     public void setBooks(ArrayList<Book> books) {
         this.books = books;
     }
 

     public ArrayList<Book> getBooks() {
         return books;
     }

This is what the XML code looks like..

 public void writeXML(String fn) throws IOException {
 

         XMLEncoder encoder = new XMLEncoder(new BufferedOutputStream(
                 new FileOutputStream(fn)));
         encoder.writeObject(this);
         encoder.close();
 

         }

This is found in the 'model' class which takes user input and stores them into ArrayLists.

However, whenever I save in XML I get..

 <?xml version="1.0" encoding="UTF-8"?>
 <java version="1.7.0_09" class="java.beans.XMLDecoder">
  <object class="Model"/>
 </java>

I hope that's enough information regarding my issue, if any of you can help then I'll forever be thankful!

Thanks

Also, all of the code can be found here.

 

[peterwalkley]

Your setAuthor() method is wrong. Try:

this.who = authors ;

 

[Number_25]

I'm getting a mismatch error when I put that in.

 

[peterwalkley]

Ok ... now I'm looking on a screen big enough to look at your pastebin post, I can see the Book class has two setAuthor() methods. One taking a single Author object and the other (posted above) that takes an array. However, the 'who' member is a single author, so the array version isn't going to make sense. You need to either:

- Remove it. Does your model need to support multiple authors of a single book ?
- Choose one author (e.g. the first one) from the array you're passing in and set 'who' to that
- Change the who member to be an array of authors (and change the single author setter to handle this)


At the moment, the "Book" bean is inconsistent and I think that's stuffing up the reflection/introspection used by the XMLEncoder class.

 

[Number_25]

Ok ... now I'm looking on a screen big enough to look at your pastebin post, I can see the Book class has two setAuthor() methods. One taking a single Author object and the other (posted above) that takes an array. However, the 'who' member is a single author, so the array version isn't going to make sense. You need to either:

- Remove it. Does your model need to support multiple authors of a single book ?
- Choose one author (e.g. the first one) from the array you're passing in and set 'who' to that
- Change the who member to be an array of authors (and change the single author setter to handle this)


At the moment, the "Book" bean is inconsistent and I think that's stuffing up the reflection/introspection used by the XMLEncoder class.

The book can only support one model.

I don't quite understand what you want me to do?

 

[DodgeeRascal]

Your Book class has an Author,

private Author who;

but you have a setter method with an ArrayList as an argument which doesn't make sense. An instance of Book can only ever have 1 Author instance set to it.

The question is, is this setter just a mistake/error or did you intend for a Book to have multiple Authors?

 

[Number_25]

Your Book class has an Author,

private Author who;

but you have a setter method with an ArrayList as an argument which doesn't make sense. An instance of Book can only ever have 1 Author instance set to it.

The question is, is this setter just a mistake/error or did you intend for a Book to have multiple Authors?

I didn't want the book to have many authors.

I don't see where I've done that?

 

[DodgeeRascal]

Ok, then as peterwalkley has suggested, something needs to be done regarding the "public void setAuthor(ArrayList<Author> authors)" method on the Book class.

in your original post you were setting

this.who = who;

which probably wont do anything at all as your are just pointing 'who' at it's self.
setting this.who = authors; wont work either as you are trying to assign a reference to an ArrayList to an Author object (which aren't of the same type).

However, you do already have a valid setter "public void setAuthor(Author who)", so just wondering what the need for overloading was for?

 

[Number_25]

Ok, then as peterwalkley has suggested, something needs to be done regarding the "public void setAuthor(ArrayList<Author> authors)" method on the Book class.

in your original post you were setting

this.who = who;

which probably wont do anything at all as your are just pointing 'who' at it's self.
setting this.who = authors; wont work either as you are trying to assign a reference to an ArrayList to an Author object (which aren't of the same type).

However, you do already have a valid setter "public void setAuthor(Author who)", so just wondering what the need for overloading was for?

So get rid of this.who = who;?

 

[DodgeeRascal]

that whole method yes.

 

[Number_25]

I tried the XML method again and nothing seemed to happen!

 

[csmager]

I know nothing of Java, really... but the method appears to serialize a class with no public properties in it. authors and books are both private members.

I would hazard a guess that if there were public get/set methods for authors and books ArrayLists in Model, the serializer might find something to serialize. It will work by convention.

 

[lordhughes]

Looking at the code, you are attempting to assign an arrayList of authors to a single instance, this will give you a type mismatch.

Your getAuthor() method with the arrayList parameter has a variable called authors, yet you are assigning a variable called who to this.who, in this instance there is no method parameter called who.

	public void setAuthor(ArrayList<Author> [b]authors[/b]) { 
        this.who = [b]who[/b] //who does not exists as a method parameter;
        //who is also a single instance of author, not a list of authors.
        //This is a type mismatch.
	}

It would appear you just copy/pasted the method then modified it this is one reason copy/pasting code is bad, any errors will just get replicated to multiple places.

That being said, your code looks a lot better than some of my early uni code

 

[Number_25]

Thanks for the complement, but I will say that some of the people here at Uni are absolutely amazing at programming - I'm nothing in comparison.

I'll just try my hardest to catch up.

I'll give that a go in a bit.

 

[Number_25]

I'm still getting no output of information into the .xml file!

 

[csmager]

Still got no public properties? Check my earlier post.

I think the setAuthor type mismatch, while incorrect, is a red herring. It will never be called.

 

[Number_25]

If I knew what I was supposed to do, I would have done it mate.

Can you explain it to me like you would to a 8 year old?

 

[csmager]

As I alluded, I've not written any Java, so I'm not totally familiar. However a quick google seems to confirm my guess that XMLEncoder operates by convention - it will serialize/deserialize properties in a class by using getXXX and setXXX methods.

If you look at your Model class, you don't have any property get/set methods, so as far as XMLEncoder is concerned, it's empty. So you might now see why your output just has a line for an object called 'Model' and nothing else.

My guess is that if you added the following methods into Model so it looked like this, you might get a little more:

public class Model implements Serializable 
{
		private books = new ArrayList<Book>();
		private authors = new ArrayList<Author>();
		
                          {...}

		public ArrayList<Book> getBooks() {
			return books;
		}
		
		public void setBooks(ArrayList<Book> books) {
			this.books = books;
		}
		
		public ArrayList<Author> getAuthors() {
			return authors;
		}
		
		public void setAuthors(ArrayList<Author> authors) {
			this.authors = authors;
		}
                          
                          {...}
	}

 

[csmager]

[Ignore]

 

[Number_25]

Aaah, I get it now! You need the gets in the actual Method class!

Thanks!