public class Book {
  private static int idCounter = 0;
  private String title;
  private String authors;
  private int id;
  
  public Book(String title, String authors) {
    if (title == null || authors == null) {
      throw new IllegalArgumentException("title or authors is null");
    }
    this.title = title;
    this.authors = authors;
    this.id = Book.idCounter++;
  }

  public String getTitle() {
    return this.title;
  }

  public String getAuthors() {
    return this.authors;
  }

  public int getId() {
    return this.id;
  }

  @Override
  public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((authors == null) ? 0 : authors.hashCode());
    result = prime * result + id;
    result = prime * result + ((title == null) ? 0 : title.hashCode());
    return result;
  }

  @Override
  public boolean equals(Object obj) {
    if (this == obj) {
      return true;
    }
    if (obj == null) {
      return false;
    }
    if (getClass() != obj.getClass()) {
      return false;
    }
    Book other = (Book) obj;
    if (id != other.id) {
      return false;
    }
    return true;
  }

  @Override
  public String toString() {
    return this.getTitle() + " by " + this.getAuthors();
  }
}