Building the library in Java

First thing is book...So what does book  have:

public abstract class Book
{
 String title_;
 String author_;
 float  price_;
 int    category_, subCategory_;

 public Book (String title, String author, double price, int category, int subCategory)
 {

title_ = title;
  author_ = author;
  price_ = price;
  category_ = category;
  subCategory_ = subCategory;
 }

 
 public String toString()
 {
  return "Title:"title_+"--Author:"+author_+"--price:"+price_+"--Category:"+category_+"--SubCategory:"+subCategory_;
 }
}