Java Sorting: sort an array of objects by property, object not allowed to use Comparable

You can provide a Comparator for comparing any type you wish, Comparable or otherwise.

For Arrays and Collections you use

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

Even sorted collections like TreeSet can take a custom Comparator

e.g.

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   }});

If you can use Comparators, write one for each type of sorting you need, e.g., ascending for book title and descending for page number. The compare method of a Comparator must return positive if the first argument is larger than the second, negative if the first is smaller and zero if they are equal.

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()));    }}

Stick this in your Library:

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