我的 Treeset 只添加了 1 个类对象

Question

我正在尝试将以下书籍对象添加到 TreeSet。但是，当我调试代码时，它说集合的大小为 1，并且只包含添加的第一个对象（book1）。当我注释掉 book1 时，book2 是唯一添加的等等。
为什么 JVm 只能识别一个对象？

代码：

 public static void main(String[] args) {
    Set<Book> bookSet = new TreeSet<Book>();

    Book book1 = new Book("Digital Fortress", "Dan Brown", "St. Martins Press", 1998);
    Book book2 = new Book("Angels and Demons", "Dan Brown", "Pocket Books", 2000);
    Book book3 = new Book("Deception Point", "Dan Brown", "Pocket Books", 2001);
    Book book4 = new Book("The Davinci Code", "Dan Brown", "DoubleDay", 2003);
    Book book5 = new Book("The Lost Symbol", "Dan Brown", "DoubleDay", 2009);
    Book book6 = new Book("Inferno", "Dan Brown", "DoubleDay", 2013);

    bookSet.add(book1);
    bookSet.add(book2);
    bookSet.add(book3);
    bookSet.add(book4);
    bookSet.add(book5);
    bookSet.add(book6);

    System.out.println(bookSet);
}

这里是到目前为止的全部代码（它必须都在同一个类中）：

  import java.util.ArrayList;
  import java.util.Iterator;
  import java.util.List;
  import java.util.Set;
  import java.util.TreeSet;

public class Book implements Comparable<Book> {

String title;
String author;
String publisher;
int publicationYear;

List<String> authorList = new ArrayList<String>();

public Book(String title, String author, String publisher, int publicationYear){
    this.title = title;
    this.author = author;
    this.publisher = publisher;
    this.publicationYear = publicationYear;
}

public String getTitle() {
    return title;
}

public void setTitle(String title) {
    this.title = title;
}

public List<String> getAuthorList() {
    return authorList;
}

public void setAuthorList(List<String> authorList) {
    this.authorList = authorList;
}

public String getPublisher() {
    return publisher;
}

public void setPublisher(String publisher) {
    this.publisher = publisher;
}

public int getPublicationYear() {
    return publicationYear;
}

public void setPublicationYear(int publicationYear) {
    this.publicationYear = publicationYear;
}

public String getAuthor() {
    return author;
}

public void setAuthor(String author) {
    this.author = author;
}


public String toString(){   
    return "Book Title: " + getTitle() + "\nAuthor Name: " + getAuthor() + "\nPublisher: " + getPublisher() + "\nYear of Publication: " + getPublicationYear();  
}

public static void main(String[] args) {
    Set<Book> bookSet = new TreeSet<Book>();

    Book book1 = new Book("Digital Fortress", "Dan Brown", "St. Martins Press", 1998);
    Book book2 = new Book("Angels and Demons", "Dan Brown", "Pocket Books", 2000);
    Book book3 = new Book("Deception Point", "Dan Brown", "Pocket Books", 2001);
    Book book4 = new Book("The Davinci Code", "Dan Brown", "DoubleDay", 2003);
    Book book5 = new Book("The Lost Symbol", "Dan Brown", "DoubleDay", 2009);
    Book book6 = new Book("Inferno", "Dan Brown", "DoubleDay", 2013);

    bookSet.add(book1);
    bookSet.add(book2);
    bookSet.add(book3);
    bookSet.add(book4);
    bookSet.add(book5);
    bookSet.add(book6);

    System.out.println(bookSet);
}

@Override
public int compareTo(Book o) {
    // TODO Auto-generated method stub
    return 0;
}  

}

Answer 1

所有书籍彼此相等（通过返回 0 来证明）。 Set 不能包含重复项，所以这就是为什么只添加一个（第一本书）

@Override
public int compareTo(Book o) {
    // TODO Auto-generated method stub
    return 0;
}  

您需要填写方法的详细信息。负数表示this book 小于o，正数反之亦然

Answer 2

@Override
public int compareTo(Book o) {
    // TODO Auto-generated method stub
    return 0;
}

Comparable#compareTo 返回值说明了什么 -

一个负整数、零或一个正整数，因为这个对象小于 大于、等于或大于指定对象。 documentation

当您将多个对象添加到树集合时，TreeSet 实例使用其compareTo（或compare）方法执行所有元素比较，因此从集合的角度来看，此方法认为相等的两个元素是, 相等。所有对象都等于Book对象compareTo方法（提供）定义，所以TreeSet只是忽略了重复对象。

解决方案： 正确定义 compareTo 方法，记住这件事返回值应该 负整数、零或正整数，因为此对象小于、等于或大于指定对象。

Answer 3

根据您的要求更新您的 compareTo 方法。或者至少更新如下，它将允许在树集中插入所有对象。

public int compareTo(Book o) {
// TODO Auto-generated method stub
return this.compareTo(o);
}