创建继承对象的数组

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..

Answer 1

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++;
           }
      }