Book.java

package library;

public class Book {

	private String title;
	private String isbn;
	private String author;
	private String category;
	
	public String getTitle() {
		return title;
	}

	public String getIsbn() {
		return isbn;
	}

	public String getAuthor() {
		return author;
	}

	public String getCategory() {
		return category;
	}

	public Book(String title, String isbn, String author, String category) {
		this.title = title;
		this.isbn = isbn;
		this.author = author;
		this.category = category;
	}
}

Library.java

package library;

import java.util.HashMap;
import java.util.Map;

public class Library {

	Map<String, Book> booksByIsbn = new HashMap<String, Book>();

	public void add(Book book) {
		booksByIsbn.put(book.getIsbn(), book);
	}
}

LibraryApp.java

package library;

public class LibraryApp {

	public static void main(String[] args) {
		
		Library myLibrary = new Library();
		
		String title = "The Great Gatsby";
		String isbn = "178139685X";
		String author = "F. Scott Fitzgerald";
		String category = "Classics";
		
		Book gatsby = new Book(title, isbn, author, category);
		
		myLibrary.add(gatsby);
	}
}