Java CompareTo for objects not Working

Question

public class Book implements Comparable<Book>, Serializable{ 
int numberOfPages; 
boolean hardback; 
String title; 
double price; 

public Book (int numberOfPages,boolean hardback, String title,double price)throws   BookException { 
    setNumberOfPages(numberOfPages); 
    setHardback(hardback); 
    setPrice(price); 
    setTitle(title); 
} 
public int getNumberOfPages(){ 
    return numberOfPages; 
} 
public void setNumberOfPages(int numberOfPages) throws BookException { 
    if(numberOfPages < 1){ 
     throw new BookException("Pages must be a positive integer"); 
    } 
    this.numberOfPages = numberOfPages; 
} 
public boolean isHardback(){ 
    return hardback; 
} 
public void setHardback(boolean hardback) { 
    this.hardback = hardback; 
} 
public String getTitle(){ 
     return title; 
} 
public void setTitle(String title) throws BookException { 
    if (title == null || title.isEmpty()){ 
     throw new BookException("No title must be less than 0 characters"); 
    } 
    this.title = title; 
} 
public double getPrice(){ 
    return price; 
} 
public void setPrice(double price) throws BookException { 
    if (price < 0){ 
     throw new BookException("Price can't be negative"); 
    } 
    this.price = price; 
} 
@Override 
public int compareTo(Book o){ 
    int Before = -1; 
    int Equal = 0; 
    int After = 1; 
    if (this == o) return Equal; 
    if (this.getNumberOfPages() < o.getNumberOfPages()) return Before; 
    if (this.getNumberOfPages() > o.getNumberOfPages()) return After; 
    if (this.getPrice() < o.getPrice()) return After; 
    if (this.getPrice() > o.getPrice()) return Before; 
    int comparison = this.getTitle().compareTo(o.getTitle()); 
    if (comparison != Equal) return comparison; 
    return Equal; 
} 
@Override public boolean equals(Object o) { 
    if (this == o) return true; 
    if (!(o instanceof Book)) return false; 

    Book that = (Book)o; 
    return 
      (this.numberOfPages == that.numberOfPages) && 
        (this.price == that.price) && 
        (this.title.equals(that.title)) 
      ; 
} 
@Override 
public String toString(){ 
    StringBuilder sb = new StringBuilder(); 
     sb.append("Title " + getTitle()); 
     sb.append("\n"); 
     sb.append("Number Of Pages " + getNumberOfPages()); 
     sb.append("\n"); 
     sb.append("Is Hardback " + isHardback()); 
     sb.append("\n"); 
     sb.append("Price " + getPrice()); 
     sb.append("\n"); 
     return sb.toString(); 
    } 
} 

当我运行这段代码时，顺序全部搞乱了，就好像compareTo不起作用。

import java.util.Iterator; 
import java.util.PriorityQueue; 
public class BookQueue { 
public static void main(String[] args)throws BookException { 

     PriorityQueue<Book> pq = new PriorityQueue<Book>(); 
     pq.offer(new Book(22, true,"Cat in the hat", 10)); 
     pq.offer(new Book(32, true,"Green eggs and ham", 12)); 
     pq.offer(new Book(42, true,"Hop on Pop", 11)); 
     pq.offer(new Book(20, true,"Lorax", 19)); 
     pq.offer(new Book(20, false,"Lorax", 19)); 

     Iterator itr = pq.iterator(); 
     System.out.println("\n"); 
     while(itr.hasNext()){ 
      System.out.println(itr.next()); 
     } 


} 

}

这是输出：

应该订购它由最低数量的页则价格然后标题 它开始以及形成20但随后进行到42，然后32和22？

Title Lorax 
Number Of Pages **20** 
Is Hardback true 
Price 19.0 

Title Lorax 
Number Of Pages **20** 
Is Hardback false 
Price 19.0 

Title Hop on Pop 
Number Of Pages **42** 
Is Hardback true 
Price 11.0 

Title Green eggs and ham 
Number Of Pages **32** 
Is Hardback true 
Price 12.0 

Title Cat in the hat 
Number Of Pages **22** 
Is Hardback true 
Price 10.0 

Answer 1

PriorityQueue类不保证顺序如果使用iterator()方法。

的Javadoc明确地定义它here为：

在方法迭代器设置（）的迭代器并不保证遍历优先级队列中的元素的任何特定顺序。如果您需要有序遍历，请考虑使用Arrays.sort（pq.toArray（））。

您只需修改main如下。

import java.util.Arrays; 
import java.util.PriorityQueue; 

public class BookQueue { 
    public static void main(String[] args) throws BookException { 

     PriorityQueue<Book> pq = new PriorityQueue<Book>(); 
     pq.offer(new Book(32, true, "Green eggs and ham", 12)); 
     pq.offer(new Book(42, true, "Hop on Pop", 11)); 
     pq.offer(new Book(20, true, "Lorax", 19)); 
     pq.offer(new Book(20, false, "Lorax", 19)); 
     pq.offer(new Book(22, true, "Cat in the hat", 10)); 

     // Iterator itr = pq.iterator(); 
     // System.out.println("\n"); 
     // while(itr.hasNext()){ 
     // System.out.println(itr.next()); 
     //} 

     Book[] books = new Book[pq.size()]; 
     Arrays.sort(pq.toArray(books)); 
     System.out.println("\n"); 
     for (int i = 0; i < books.length; i++) { 
      System.out.println(books[i]); 
     } 
    } 
} 

希望这会有所帮助。如果您需要不同的排序方法，您也可以尝试执行Comparator并传入Arrays.sort(T[], Comparator)，如Comparator vs Comparable中所述。

Answer 2

使用此compareTo()方法。

@Override 
public int compareTo(Book o){ 
    if (this.getNumberOfPages() != o.getNumberOfPages()) 
     return new Integer(this.getNumberOfPages()).compareTo(o.getNumberOfPages());  
    if (this.getPrice() != o.getPrice()) 
     return new Double(this.getPrice()).compareTo(o.getPrice());  
    if(!this.getTitle().equalsIgnoreCase(o.getTitle())) 
     return this.getTitle().compareToIgnoreCase(o.getTitle()); 
    return 0; 
}