Creating an array of inherited objects

Question

i'm trying to create a method that adds a certain object into an array of inherited objects.

public class Biblio {
Biblio[] Tab; static int i=0;
Biblio();
void insert(Biblio O){Tab[i]=O;i++;}}

in the main class, i created 3 objects of classes that extend from each others: means Document extends from Biblio, Article extends from Document, Book extends from Article.

public class TestBiblio {
public static void main(String[] args) {
    Document A= new Document();
    Article B= new Article();
    Book C= new Book();
    Biblio D= new Biblio();
    D.insert(A);
    D.insert(B);
    D.insert(C);}}

Once i run the code, i get Exception in thread "main" java.lang.NullPointerException error. i'm a beginner in java, i couldn't find out hat went wrong..

you should do something like: Biblio[] Tab = new Biblio[specify_count]; and your constructor doesn't has a body — Sarthak Mittal
– Sarthak Mittal, Commented Dec 3, 2014 at 19:04
can you post the full stack trace? Are all classes in the same package/folder? — Mechkov
– Mechkov, Commented Dec 3, 2014 at 19:05
yes it worked, i had to add Tab = new Biblio[5]; in the constructor. — EatingTooMuch
– EatingTooMuch, Commented Dec 8, 2014 at 12:54

committedandroider · Accepted Answer · 2014-12-03 19:04:55Z

4

You never initialized the array that you used to insert. When you do Tab[i], you are dereferencing a null pointer. Have something like

    public class Biblio {
           Biblio[] Tab; 
           static int i=0;
           public Biblio() {
                  Tab = new Biblio[5];
           }
          void insert(Biblio O){
                 Tab[i]=O;i++;
           }
      }

answered Dec 3, 2014 at 19:04

committedandroider

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