How to use the toString method in my runner?

I'm trying to learn Java and now I created this Book class, I used the Generate toString() option in Eclipse and now I want to call the toString method in my runner. How do I call it?

import java.time.LocalDateTime;
import com.library.entity.Publication;
public class Book extends Publication {
public Book(String title, String firstName, String lastName, String publisherName, String address,
LocalDateTime datePublished) {
super(title, firstName, lastName, publisherName, address, datePublished);
// TODO Auto-generated constructor stub
}
@Override
public String toString() {
return "Book [getTitle()=" + getTitle() + ", getDatePublished()=" + getDatePublished() + ", getClass()="
+ getClass() + ", hashCode()=" + hashCode() + ", toString()=" + super.toString() + "]";
}
}

I tried using this as a runner but it gives me an error:

Exception in thread "main" java.lang.NullPointerException: Cannot invoke "com.library.entity.Author.setFirstName(String)" because "this.author" is null
at com.library.entity.Publication.<init>(Publication.java:14)
at com.library.entity.publication.Book.<init>(Book.java:12)
at com.library.runner.BookstoreRunner.main(BookstoreRunner.java:9)

Here is the runner:

package com.library.runner;
import java.time.LocalDateTime;
import com.library.entity.publication.*;
public class BookstoreRunner{
public static void main(String args[]) {
Book book1 = new Book("Generic Title", "K", "L", "K Publishing", "Earth", LocalDateTime.now());
String bookdetails = book1.toString();
System.out.println(bookdetails);
}
}

Here is my Publication class, I hope it helps. I'm really new to java.

package com.library.entity;
import java.time.LocalDateTime;
public class Publication {
private String title;
private Author author;
private Publisher publisher;
private LocalDateTime datePublished;
public Publication(String title, String firstName, String lastName, String publisherName, String address, LocalDateTime datePublished) {
this.title = title;
author.setFirstName(firstName);
author.setLastName(lastName);
publisher.setPublisherName(publisherName);
publisher.setAddress(address);
this.datePublished = datePublished;
}
public String getTitle() {
return title;
}
public void setTitle(String title) {
this.title = title;
}
public LocalDateTime getDatePublished() {
return datePublished;
}
public void setDatePublished(LocalDateTime datePublished) {
this.datePublished = datePublished;
}
}

edit: It now works! My error was using:

private Author author;
private Publisher publisher

instead of

	private final Author author = new Author();
private final Publisher publisher = new Publisher();

You're almost there! You shoul wrap your code inside runner with some method:

public class BookstoreRunner{
public static void main(String... args) {
Book book1 = new Book("Generic Title", "K", "L", "K Publishing", "Earth", 
LocalDateTime.now());
String bookdetails = book1.toString();
System.out.println(bookdetails);
}
}

P.S. Whe you call System.out.println(new Book()) then book.toString() method is called implicitly and no need to invoke it manually.

P.S.1 Publication has a problem. author variable should be initialized before it will be accessed in the constructor:

private final Author author = new Author();
private final Publisher publisher = new Publisher();
public Publication(String title, String firstName, String lastName, String publisherName, String address, LocalDateTime datePublished) {
// ...
}