Can't compile this code! can't tell why. Error message is below. Thanks

Question

import java.util.*;
public class Library {
        private Collection books;                                               //Defining a collection of books here
        private Iterator bookIter;                                              //We want to iterate the collection of books here
        //Constructor to help us initialize our collection to hold elements of a newly created collection
        public Library() {
                books = new ArrayList();                                        //Our collection holds a the newly created collection of type ArrayList (providing storage for books)
                books.add(new Book("War and Crime"));
                books.add(new Book("The Redeemers"));
        public void display() {
                bookIter = books.iterator();
                while (bookIter.hasNext()) {
                        Book aBook = (Book)bookIter.next();
                        System.out.println("Title : " + aBook.ret_title());
        public static void main(String args[]) {
                display();
class Book {
        private String title;
        Book(String b_title) {
                this.title = b_title;
        public String ret_title() {
                return this.title;
}THIS IS THE ERROR:
"Library.java" 105 lines, 3597 characters
clio:~/javamachine/learning/topics/utils/Collection (Lord G.) % javac Library.java
Library.java:91: non-static method display() cannot be referenced from a static context
display();
^
1 error
Thanks for helping!
BR,
G-Afric

Answer 1

Hi folkenf,
I tried what you recommended already but the result was even more timidating.
public static void display()ERROR OUTPUT NOW IS:
Library.java:83: non-static variable bookIter cannot be referenced from a static context
bookIter = books.iterator();
^
Library.java:83: non-static variable books cannot be referenced from a static context
bookIter = books.iterator();
^
Library.java:84: non-static variable bookIter cannot be referenced from a static context
while (bookIter.hasNext()) {
^
Library.java:85: non-static variable bookIter cannot be referenced from a static context
Book aBook = (Book)bookIter.next();
^
4 errors
I fixed this error by making books and bookIter static but now main() complains
of null pointer as shown below:
Runtime error:
Exception in thread "main" java.lang.NullPointerException
at Library.main(Library.java:89)
Thanks!
BR,
G-Afric