list.add() does not add data to ArrayList

Question

I want to add data to ArrayList object. In my code addBook() method will be show Input Dialogue Box and pass this string to isbn variable. Now there is need to add isbn variable data to ArrayList which is reside in BookInfoNew() constructor but list object not found in addBook() method. (list.add(isbn))

kindly help me.

import java.util.*;
import javax.swing.JOptionPane;

public class BookInfoNew {
    private static String isbn;
    private static String bookName;
    private static String authorName;
    public static int totalBooks = 0;

    //default constructor
    public BookInfoNew() {
        List<String> list = new ArrayList<String>(); //create ArrayList
    }

    //Parameterized constructor
    public void BookInfoNew(String x, String y, String z) {
        isbn = x;
        bookName = y;
        authorName = z;
    }

    //add book method
    public void addBook() {
        String isbn = JOptionPane.showInputDialog("Enter ISBN");

        //add books data to ArrayList
        list.add(isbn);
    }
}

because list is a local variable, which only exists in your non parameter constructor. — SomeJavaGuy
– SomeJavaGuy, Commented Nov 19, 2015 at 8:04
your question title is erroneous , since your addBook() doesn't even have access to list — Ramanlfc
– Ramanlfc, Commented Nov 19, 2015 at 8:10

jiaweizhang · Accepted Answer · 2015-11-19 08:18:37Z

8

This is an issue with scope. You cannot access your list object within the addBook() object. So, you have to either make list a parameter to addBook() or you can make it a global variable.

This code fixes it using a global variable:

import java.util.*;
import javax.swing.JOptionPane;

public class BookInfoNew {
    private String isbn;
    private String bookName;
    private String authorName;
    public int totalBooks = 0;

    // global list variable here which you can use in your methods
    private List<String> list;

    //default constructor
    public BookInfoNew() {
        list = new ArrayList<String>(); //create ArrayList
    }

    //Parameterized constructor - constructor has no return type
    public BookInfoNew(String x, String y, String z) {
        isbn = x;
        bookName = y;
        authorName = z;
    }

    //add book method
    public void addBook() {
        String isbn = JOptionPane.showInputDialog("Enter ISBN");

        //add books data to ArrayList
        list.add(isbn);
    }
}

edited Nov 19, 2015 at 8:18

answered Nov 19, 2015 at 8:04

jiaweizhang

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7 Comments

jiaweizhang Over a year ago

Your code has been edited. As @manu asked, why are your other variables static?

Zohaib Siddique Over a year ago

Now list.add(isbn) is not working. it says list.add(isbn) method is undefined.

Zohaib Siddique Over a year ago

its works well when list.add(isbn) will in BookInfoNew() constructor

jiaweizhang Over a year ago

If you want to run addBook() every time you create a new BookInfoNew object, you can just write addBook() in the constructor.

jiaweizhang Over a year ago

And maybe you should remove the String in front of isbn in the addBook() method. That way you would be updating the instance variable isbn and not a local variable.

|

user3056857 · Accepted Answer · 2015-11-19 08:11:34Z

1

You should rewrite your code a little bit like that:

...
List<String> list = null;
public BookInfoNew() {
    list = new ArrayList<String>(); //create ArrayList
}
...

and it should be ok.

answered Nov 19, 2015 at 8:11

user3056857

1134 bronze badges

1 Comment

jiaweizhang Over a year ago

No need for = null;

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