I see a couple of issues here:
public Book[] arr = new Book[countElements]; You creating your array of 0 length here. Initializing should be done in the constructor like this
public...
Type: Posts; User: armela
I see a couple of issues here:
public Book[] arr = new Book[countElements]; You creating your array of 0 length here. Initializing should be done in the constructor like this
public...