java - Class relationship, NullPointerException error -

i have 2 classes of authors , books:

public class authors extends realmobject {     @primarykey     private string url_base;     private realmlist<books> books;     ... getters & setters...     public realmlist<books> getbooks() {         return books;     }     public void setbooks(realmlist<books> books) {         this.books = books;     } }  public class books extends realmobject {       @primarykey      private string url_base;      private authors author;          ... getters & setters...      public authors getauthor() {          return author;      }      public void setauthor(authors author) {          this.author = author;      } } 

perform inserts:

authors author = new authors(); author.seturl_base("url_base")  books book = new books(); book.seturl_base("lala"); book.setauthor(author);  author.getbooks().add(book);     // error comes here  realm.begintransaction(); realm.copytorealmorupdate(author); realm.committransaction(); 

and program gives java.lang.nullpointerexception.

error line : author.getbooks().add (book);

what wrong? ask help.

you haven't set books member author object (as books not initialized null) getbooks() return null.

make sure books initialized before using :

public class authors extends realmobject {     @primarykey     private string url_base;     private realmlist<books> books = new realmlist<books>(); // empty unmanaged books list.      ... getters & setters... 

or use setter method (though initializing on construction more correct in case):

authors author = new authors(); author.seturl_base("url_base") author.setbooks(new realmlist<book>()); 

update:

as @andreyatapin correctly noted in solution above list unmanaged. if want managed list pass required arguments specified in realmlist api whatever choose books list must initialized before start manipulating it