java - method for adding objects in array of object -

i'm working on class called library. class has 2 attributes: books[] array of type book , nbrbooks of type int.

i'm looking build method called addbook book parameter of type book returns true if book added. , returns false if parameter book null. far have this:

public boolean addbook(book book) throws livreinvalideexception {     boolean = true;     getnbrbooks();     if (book == null) {         = false;         throw new livreinvalideexception();     }        books [nbrbooks] = book;     return a; } 

my professor gave class test check if our code working , doesn't seem work. i'm wondering why...

thanks help!

public boolean addbook(book book){    if( book == null) {    return false;}     books[nbrbooks] = book;     nbrbooks++;    return true;} 

this super simple hope helps bit!