Java排序：按属性对对象数组进行排序，对象不允许使用Comparable

Question

我有一个类 Library，其中包含 Book 对象的数组，我需要根据 Book 的属性（标题或页码）对数组进行排序。问题是我不允许将 Comparable 类与 Book 一起使用。您建议我如何对图书馆中的书籍进行排序？写我自己的排序？或者有更简单的方法吗？如果您需要代码片段，请询问！

Answer 1

您可以提供

Comparator

来比较您想要的任何类型、

Comparable

或其他类型。

对于您使用的数组和集合

Arrays.sort(array, myComparator);
Collections.sort(list, myComparator);

甚至像 TreeSet 这样的排序集合也可以采用自定义比较器

例如

Collections.sort(books, new Comparator<Book>() {
   public int compare(Book b1, Book b2) {
      return if b1 is greater return +1, if b2 is smaller return -1 otherwise 0
   }
});

Answer 2

如果您可以使用

Comparators

，请为您需要的每种排序类型编写一个，例如书名升序和页码降序。如果第一个参数大于第二个参数，则

compare

的

Comparator

方法必须返回正值；如果第一个参数小于第二个参数，则必须返回负值；如果它们相等，则必须返回零。

import java.util.Comparator;
import java.util.List;
import java.util.Arrays;

class Book{
    String title;
    int pageNumber;

    public Book(String title, int pageNumber){
        this.title = title;
        this.pageNumber = pageNumber;
    }

    String getTitle(){ return title; }
    int getPageNumber(){ return pageNumber; }

    public String toString(){
        return "(" + title + ", " + pageNumber + " pages)";
    }
}

public class Library{

    // These variables are static because you don't need multiple copies
    // for sorting, as they have no intrinsic state.
    static private Comparator<Book> ascTitle;
    static private Comparator<Book> descPageNumber;

    // We initialize static variables inside a static block.
    static {
        ascTitle = new Comparator<Book>(){
            @Override
            public int compare(Book b1, Book b2){
                return b1.getTitle().compareTo(b2.getTitle());
            }
        };

        descPageNumber = new Comparator<Book>(){
            @Override
            public int compare(Book b1, Book b2){
                // Java 7 has an Integer#compare function
                return Integer.compare(b1.getPageNumber(), b2.getPageNumber());
                // For Java < 7, use 
                // Integer.valueOf(n1).compareTo(n2);
                // DO NOT subtract numbers to make a comparison such as n2 - n1.
                // This can cause a negative overflow if the difference is larger 
                // than Integer.MAX_VALUE (e.g., n1 = 2^31 and n2 = -2^31)
            }
        };
    }

    private Book[] books;
    public Book[] getBooks(){ return books; }

    public void sortAscTitle(){
        Arrays.sort(books, ascTitle);
    }

    public void sortDescPageNumber(){
        Arrays.sort(books, descPageNumber);
    }

    public Library(Book[] books){
        this.books = books;
    }

    public static void main(String[] args){
        Library library = new Library( new Book[]{
            new Book("1984", 123), 
            new Book("I, Robot", 152), 
            new Book("Harry Potter and the Philosopher's Stone", 267),
            new Book("Harry Potter and the Goblet of Fire", 759),
            new Book("The Bible", 1623)
        });

        library.sortAscTitle();
        System.out.println(Arrays.toString(library.getBooks()));

        library.sortDescPageNumber();
        System.out.println(Arrays.toString(library.getBooks()));
    }
}

Answer 3

将其粘贴到您的图书馆中：

java.util.Collections.sort(bookList, bookComparator);

Answer 4

扩展@PeterLawrey对Java 8的答案，您现在可以使用Lambda表达式而不是

Comparable<T>

委托：

Collections.sort(books, (firstBook, secondBook -> b1 is greater return +1, 
                                                  if b2 is smaller return -1 otherwise 0));