Java Array Object compile issue

Question

The following code get's an error when compiled. I know it's missing something but I just can't figure out what.

public class Books {
    String title;
    String author;

    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Books[] myBooks = new Books[3];

        myBooks[0].title = "Learn Java";
        myBooks[1].title = "Java Development";
        myBooks[2].title = "Java Today!";
        myBooks[0].author = "S";
        myBooks[1].author = "Mr. S";
        myBooks[2].author = "SN";

        int x = 0;
        while (x < myBooks.length) {
            x = x + 1;
            System.out.print(myBooks[x].title);
            System.out.print(" by ");
            System.out.println(myBooks[x].author);
        }

    }

}

Books[] myBooks = new Books[3]; myBooks[0] = new Books(); myBooks[1] = new Books(); myBooks[3] = new Books(); .... — Arya, May 25 '14 at 06:10
Do you have a main method? In Java, you need to have a method named main in at least one class. — akrabi, May 25 '14 at 06:11
I get an Exception in thread "main" java.lang.NullPointerException at Books.main(Books.java:11). I'm reading the "What is a null point exception, and how to fix it" question that Makoto suggested. — some random dude, May 25 '14 at 06:14
That's also what I received when I compiled and ran this code. You should reference the duplicate question linked here. — Makoto, May 25 '14 at 06:14
Specifically, you created an empty array and didn't put any `Book` objects in it. — chrylis -cautiouslyoptimistic-, May 25 '14 at 06:36

Aniket Thakur · Answer 1 · 2014-05-25T06:21:01.287

0

Change

    while (x < myBooks.length) {
        x = x + 1;
        System.out.print(myBooks[x].title);
        System.out.print(" by ");
        System.out.println(myBooks[x].author);
    }

to

    while (x < myBooks.length) {
        System.out.print(myBooks[x].title);
        System.out.print(" by ");
        System.out.println(myBooks[x].author);
        x = x + 1;
    }

Consider case when x=2 at srat of the loop. It becomes 3 first and then books[3] will throw Exception.

Other than that you have simply defined an array which is nothing but just references. You need to initialize all of them.

for(Books book : myBooks ) {
   book = new Books();
}

If you are not initializing the array they are just pointing to null and if you perform any operations on it like myBooks[i].title you will get NPE.

edited May 25 '14 at 06:21

answered May 25 '14 at 06:05

Aniket Thakur

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Um...no. That doesn't fix the issue. – Makoto May 25 '14 at 06:05
It may not be the issue you may be getting at compile time but it is an issue and you need to fix it. I see no compile time violations in your code. – Aniket Thakur May 25 '14 at 06:10
I don't disagree that it's a problem, but that block of code isn't even reached in the program's lifecycle (yet). – Makoto May 25 '14 at 06:16
Agreed. @user3638942 you need to initialize your array. Answer updated. – Aniket Thakur May 25 '14 at 06:22

Java Array Object compile issue

1 Answers1