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For example I have 3 books: Booknumber (int), Booktitle (string), Booklanguage (string), Bookprice (int).

Now, I want to have an array called books[3][4]. I'm getting the data I set via setBooknumber like this:
Book1.getBooknumber(), Book1.getBooktitle(),...,Book3.getBookprice().

How do I realize this: books[3][4] array.

I can't call it String books[][] = new String [3][4]. Because I can't get Booknumber (int) into it. I don't want Booknumber to be String neither Bookprice. How do I realize it, please?

To further elaborate it. I have 2 classes: book and bookUI.

book

public class book{
String Booktitle, Booklanguage;
int Booknumber, Bookprice;

//constructor

//get

//set
}

bookUI

public class bookUI
{
 public static void main(String arg[])
 {
   book book1 = new book();
   book book2 = new book();
   book book3 = new book();

   book1.setBooktitle();
   ...
   book3.setBookprice();

   //Here I want to have books[3][4] Array. And gettin the data via book1.get...book3.get into the array
 }
}
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6
  • Why has it to be an array of arrays? Commented Jan 6, 2010 at 19:55
  • 1
    Because it's what I'm trying to get working. Doesn't matter how ugly it is or how stupid. It's my own goal and I want to do it so I can understand why it didn't work, how it works, and LATER ON how to do it different. Commented Jan 6, 2010 at 19:59
  • 1
    OK then the simple solution is: There is no way to access class properties in this array[0] style. (correct me if I'm wrong). Arrays are also typed so the only solution is to create an array of arrays of Objects which is bad as you have to know what object type you are actually dealing with (String or Integer). OOP is there to make life easier for you just accept it. Commented Jan 6, 2010 at 20:14
  • 1
    -1: If you are not willing to accept that some things are not possible than nobody can help you. It is like you gave us water and asked us to cook pasta without pasta and a pot (and in time for dinner). Commented Jan 6, 2010 at 20:29
  • Felix is correct. Arbitrary objects are not arrays and you can't use array syntax on them. That just isn't Java. Commented Jan 6, 2010 at 20:29

5 Answers 5

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35 
public class Book
{
    public int number;
    public String title;
    public String language;
    public int price;

    // Add constructor, get, set, as needed.
}

then declare your array as:

Book[] books = new Book[3];

EDIT: In response to O.P.'s confusion, Book should be an object, not an array. Each book should be created on it's own (via a properly designed constructor) and then added to the array. In fact, I wouldn't use an array, but an ArrayList. In other words, you are trying to force data into containers that aren't suitable for the task at hand.

I would venture that 50% of programming is choosing the right data structure for your data. Algorithms naturally follow if there is a good choice of structure.

When properly done, you get your UI class to look like: Edit: Generics added to the following code snippet.

...
ArrayList<Book> myLibrary = new ArrayList<Book>();
myLibrary.add(new Book(1, "Thinking In Java", "English", 4999));
myLibrary.add(new Book(2, "Hacking for Fun and Profit", "English", 1099);

etc.

now you can use the Collections interface and do something like:

int total = 0;
for (Book b : myLibrary)
{
   total += b.price;
   System.out.println(b); // Assuming a valid toString in the Book class
}
System.out.println("The total value of your library is " + total);
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5 Comments

I second this comment. You should strongly consider creating a custom type for this, rather than sticking it in a multidimensional array with a generic Object type. Constant casting is a sign of bad design.
I don't understand it. Can you please explain further?
He's saying a book should not be an array. It's an aggregate type, and should be represented by a class.
"Each book should be created on it's own (via a properly designed constructor) and then added to the array." THAT'S WHAT I WANT ;). Excatly what I want. I want create 3 objects (3 books), set all the data, and give it to an array. Then I want to call via book[0][0] the booknumber for example.
I cannot understand why you insist on book[0][0] that much. That is not how it works in Java. If you want to do it like this you have to choose e.g. PHP.
4

Notice the repetition of Book in Booknumber (int), Booktitle (string), Booklanguage (string), Bookprice (int)- it screams for a class type.

class Book {
  int number;
  String title;
  String language;
  int price;
}

Now you can simply have:

Book[] books = new Books[3];

If you want arrays, you can declare it as object array an insert Integer and String into it:

Object books[3][4]
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1 Comment

Now that it's been edited, my comment looks a bit moronic. +1 for the edit regarding the repetition of "book" in the field names. That's a smell that does indeed scream for refactoring.
3

@NoCanDo: You cannot create an array with different data types because java only supports variables with a specific data type or object. When you are creating an array, you are pulling together an assortment of similar variables -- almost like an extended variable. All of the variables must be of the same type therefore. Java cannot differentiate the data type of your variable unless you tell it what it is. Ex: int tells all your variables declared to it are of data type int. What you could do is create 3 arrays with corresponding information.

int bookNumber[] = {1, 2, 3, 4, 5};
int bookName[] = {nameOfBook1, nameOfBook2, nameOfBook3, nameOfBook4, nameOfBook5} // etc.. etc..

Now, a single index number gives you all the info for that book. Ex: All of your arrays with index number 0 ([0]) have information for book 1.

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Comments

1

use object type ie Object books[3][4];

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0

Why not create a class Book with properties: Number, Title, and Price. Then store them in a single dimensional array? That way instead of calling

Book[i][j]

..to get your books title, call

Book[i].Title

Seems to me like it would be a bit more manageable and code friendly.

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6 Comments

Because it's not what I want.
Why is it a problem to do it this way? Besides being probably the only way the code is way more easier to understand.
Becaus I still don't want it and it's not what I'm trying to understand. I'm selflearning Java. It doesn't help me when folks are trowing hunderds of different codesolutions. I want to stay within MY given "codeframe", so I can understand it and THEN move to something different.
@NoCanDo: If you're self-learning, then you should listen to the people who know what they're doing when they tell you you're going about something the wrong way. And people are not throwing hundreds of different solutions at you — they're suggesting the same thing, which is to make a Book class.
Yea, well that might be. But often it's like this analogy: one is learning basic geometry and folks start bombarding him/her with advanced differential calculus. Whereas he/she has no idea about differential calculus. Not in this case right here, but quite often the case on stackoverflow. I mean all the solutions work if I implement them, but those weren't what I was looking for. If it doesn't work, then it doesn't work. I'd appreciate it if folks could explain a beginner WHY and HOW it doesn't work, and not immediately start throwing other, quite better, solutions. It doesn't help at all.
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