Merging two object arrays, Java

Question

I am trying to merge two object arrays of a type "BookCollection" into one BookCollection array. I start by making a new blank BookCollection with a size equivalent to the sum of the other two BookCollections.

The code works with the first "collection1":

public static BookCollection merge(BookCollection collection1, BookCollection collection2) {
BookCollection mergedCollection = new BookCollection(collection1.getSize() + collection2.getSize());
int collectionSize = 0;
// Add books from collection1 to mergedCollection.
// This loop works, 2/21/15, 3:30
for (int c1 = 0; c1 < collection1.getSize(); c1++) {
  Book bookCopy = new Book(collection1.objectAt(c1));
  mergedCollection.addBook(bookCopy);
  collectionSize += 1;
}

Now I run into trouble. My intention is to loop through the next BookCollection (collection2) and if the book exists, just update price and stock. If it doesn't exist, I want to add the book to the BookCollection.

// Loop through collection2
for (int c2 = 0; c2 < collection2.getSize(); c2++) {
  // Create a copy of the current book.
  Book bookCopy = new Book(collection2.objectAt(c2));
  String copyIsbn = bookCopy.getIsbn();
  // Loop through mergedCollection
  for (int m = 0; m < mergedCollection.getSize(); m++) {

    Book mergedBook = new Book(mergedCollection.objectAt(m));
    String mergedIsbn = mergedBook.getIsbn();

    // If the current book is already in mergedCollection.
    // NOT GETTING THROUGH THE LOGIC HERE.
    if (copyIsbn.equals(mergedIsbn)) {
      // Variables to update values in mergedCollections
      double price = bookCopy.getPrice();
      int stock = bookCopy.getStock();
      // If the mergedBook's price is larger than the collection's, change it.
      if (mergedBook.getPrice() > price) {
        mergedBook.setPrice(price);
      }
      // Update stock in mergedCollection
      mergedCollection.changeStock(mergedBook.getIsbn(), bookCopy.getStock() + mergedBook.getStock());    

    // If the current book is not in mergedCollection.      
    } else {
      mergedCollection.addBook(bookCopy);          
      collectionSize += 1;
    }          

  }
}
return mergedCollection;

}

I have tried to comment this well, and there is a comment in all caps where I believe the problem is. Now I also think it's pertinent to know that addBook() as used here:

mergedCollection.addBook(bookCopy);

basically says, if I parse through the array, and get a non-null return, then I can add the book.

Now, when I run this code, the compiler tells me the book I'm trying to add exists and stops. So I know that a) addBook is find a non-null value, and b)this little bit of logic:

if (copyIsbn.equals(mergedIsbn))

is not working as I want it to.

Why doesn't my program catch Book objects that occur twice and update the information?

Because you want a Set, specifically the TreeSet's addAll method. — hd1
– hd1, Commented Feb 22, 2015 at 0:41
I don't want a Set as I would like to learn why this method is not working. Thank you for introducing me to that idea and I will try to implement it with this once I figure out why my approach is not working. — camerow
– camerow, Commented Feb 22, 2015 at 0:44
"not working as I want it to" This isn't a useful description of the problem. What happens? — Radiodef
– Radiodef, Commented Feb 22, 2015 at 0:54
merge is supposed to merge the two BookCollections into a new BookCollection. If there is a duplicate occurrence of a Book object in the new BookCollection, then I just want to update the stock, and if the price in the second collection is lower than in the first, I want to update that to the lower price — camerow
– camerow, Commented Feb 22, 2015 at 0:55

Kartic · Accepted Answer · 2015-02-22 01:27:43Z

1

I would suggest to use Set instead. Let me show you an example :

Lets assume your Book class is something like :

class Book
{
    private String bookNo;
    private int price;
    private int stock;

    public Book(String bookNo, int price, int stock) {
        this.bookNo = bookNo;
        this.price = price;
        this.stock = stock;
    }

    public String getBookNo() {
        return bookNo;
    }
    public int getPrice() {
        return price;
    }
    public int getStock() {
        return stock;
    }

    @Override
    public boolean equals(Object obj) {
        if (obj != null && obj instanceof Book) {
            if(this.bookNo.equals(((Book) obj).bookNo)) {
                return true;
            }
        }
        return false;
    }

    @Override
    public int hashCode() {
        return this.bookNo.hashCode();
    }
}

Now to achive what you want, you can do something like :

    Set<Book> hs1 = new HashSet<Book>();
    Set<Book> hs2 = new HashSet<Book>();

    hs1.add(new Book("book1", 10, 10));
    hs1.add(new Book("book2", 20, 20));

    hs2.add(new Book("book3", 30, 30));
    hs2.add(new Book("book1", 40, 40));

    hs2.addAll(hs1);

hs2 have the desired data that you want.

answered Feb 22, 2015 at 1:27

Kartic

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3 Comments

hd1 Over a year ago

Please see my comment +1d

Kartic Over a year ago

Yes, I completely agree with you.

camerow Over a year ago

This is a great answer, thank you. As I posted earlier I figured out a way to make my method work, but this is much more elegant and an awesome thing to know. Will be using this in the future for sure.

Gregory Basior · Accepted Answer · 2015-02-22 00:47:17Z

0

If your goal is to update the object, instead of creating a new instance:

   Book mergedBook = new Book(mergedCollection.objectAt(m));

use the existing instance already in the collection:

 Book mergedBook = mergedCollection.objectAt(m);

Then any updates made to mergedBook will be persistent to the object that is in the mergedCollection.

answered Feb 22, 2015 at 0:47

Gregory Basior

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Comments

camerow · Accepted Answer · 2015-02-22 01:20:09Z

0

Ok, I got this figured out. I was always running:

mergedCollection.addBook(bookCopy);

All I had to do was create a:

found = false;

variable and imbed my .addBook function into that.

answered Feb 22, 2015 at 1:20

camerow

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