Accessing an object class variable in ArrayList (java)

Question

So, let's say i have a class to describe a book

   public class Book {
     String name;
     int pages;
     String auother;
     boolean available;
     String rentername;
  }

Now, i have set an array list to contain the unknown numbers of books i wish to have/add/delete during the running time. The thing is, when i try to accesses a certain book in the arraylist by an index i get an error.

enter code here

ArrayList Books = new ArrayList();
Book bk1 = new Book();
Books.add(bk1);
System.out.println(Books[0]. --->>> won't give me accesses to the 'Book' class variables (name, pages...)

So, how can i make it to the class variables? Thanks.

MadProgrammer · Accepted Answer · 2014-12-20 09:04:59Z

12

ArrayList is an implementation of a java.util.List collection backed by a dynamic array.

This means three things.

Firstly, you don't need to worry about resizing, allocating or coping elements into the array, the ArrayList will take care of it for you.

Secondly, any method that wants a List doesn't care about the implementation (you could pass a LinkedList instead) which decouples the code (meaning you can change the implementation without adversely effecting those who rely on it, because of the contract made through the List interface)

Thirdly, to interact with the contents of the List, you need to use one or more of the interfaces access methods.

You already know about add, but to obtaining an object, you need to use get which;

Returns the element at the specified position in this list

You might also be interested in

There are other methods, of course, but these are the immediately useful.

I'd also have a read through the Collections Trail

UPDATED example

public class Test {

    public static void main(String[] args) {
        new Test();
    }

    public Test() {

        List<Book> listOfBooks = new ArrayList<Book>();
        Book bk1 = new Book();
        listOfBooks.add(bk1);

        System.out.println("           bk1 = " + bk1);
        System.out.println("listOfBooks(0) = " + listOfBooks.get(0));

    }

    public class Book {

        String name;
        int pages;
        String auother;
        boolean available;
        String rentername;

        @Override
        public String toString() {
            return "Book: name = " + name + "; pages = " + pages + "; available = " + available + "; rentername = " + rentername + "; hashCode = " + hashCode();
        }

    }
}

Which outputs

           bk1 = Book: name = null; pages = 0; available = false; rentername = null; hashCode = 1137551390
listOfBooks(0) = Book: name = null; pages = 0; available = false; rentername = null; hashCode = 1137551390

edited Dec 20, 2014 at 9:04

answered Oct 9, 2012 at 22:06

MadProgrammer

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2 Comments

user3921 Over a year ago

When i use the get methood, i can't get accsess to the book object varibable (name, pages etc...i can only view the generic object methood such as qeuals/get class/ HashCode...

MadProgrammer Over a year ago

The first problem you have is you are using a vanellia version of List, which accepts any type of Object, but will only return a Object. You can can fix this by 1. casting the result back to a Book (Book book = (Book)Books.get(0)), but this dangerous and should be avoided where possible. 2. Take advantage of Generics, this basically restricts the List to only dealing with certain types of Objects. I've update my post with an example, but you might want to have a read through the Generics Trail for more information

Baz · Accepted Answer · 2012-10-09 22:03:16Z

6

Use the get(int index) method of List to get the entry you want:

System.out.println(Books.get(0));

You cannot access the entries of a List with the notation used for arrays.

BTW: Please stick to the Java Naming Conventions

edited Oct 9, 2012 at 22:03

answered Oct 9, 2012 at 21:56

Baz

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4 Comments

user3921 Over a year ago

When i use the get methood, i can't get accsess to the book object varibable (name, pages etc...i can only view the generic object methood such as qeuals/get class/ HashCode...

Baz Over a year ago

@user3921 Of course you can't. They are neither public, nor do you have getter/setter methods. Change/Add those and you will be able to access them.

user3921 Over a year ago

Hey, im sorry my friend, but adding public didn't solve it, i even attached a photo for you guys to see...i45.tinypic.com/wtswj.jpg

Baz Over a year ago

@user3921 That's because you use an untyped ArrayList. Use ArrayList<Book> wherever your ArrayList contains Books. Otherwise Java and Eclipse cannot know that the ArrayList contains Books and consequently cannot access the attributes. (you can make them private again afterwards).

Jigar Joshi · Accepted Answer · 2012-10-09 21:56:44Z

2

You need to create public setters and getters method to access fields, and to access book instance you need to

books.get(index)

answered Oct 9, 2012 at 21:56

Jigar Joshi

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2 Comments

jpm Over a year ago

Or, for a pure data class, it's probably fine to make the fields public. Accessors and mutators would just be boilerplate for such a class unless they did bounds checking or other validation.

Jigar Joshi Over a year ago

accessor can be used in future also there are many frameworks which may look for accessor if used against them

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