java中Collections.sort排序详解

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本文介绍Java中Comparator接口的应用，通过自定义Book类和Comparator实现类，演示如何根据不同属性对对象列表进行排序。

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Comparator是个接口，可重写compare()及equals()这两个方法,用于比价功能；如果是null的话，就是使用元素的默认顺序，如a,b,c,d,e,f,g，就是a,b,c,d,e,f,g这样，当然数字也是这样的。
compare（a,b）方法:根据第一个参数小于、等于或大于第二个参数分别返回负整数、零或正整数。
equals（obj）方法：仅当指定的对象也是一个 Comparator，并且强行实施与此 Comparator 相同的排序时才返回 true。

Collections.sort(list, new PriceComparator());的第二个参数返回一个int型的值，就相当于一个标志，告诉sort方法按什么顺序来对list进行排序。

具体实现代码方法如下：

Book实体类：

<pre class="java" name="code">import java.text.DecimalFormat;  
import java.text.SimpleDateFormat;  
import java.util.GregorianCalendar;  
import java.util.Iterator;  
import java.util.TreeMap;  
  
 
public class Book implements Comparable {  
    public int id;
    public String name;
    public double price;
    private String author;
    public GregorianCalendar calendar;
  
    public Book() {  
        this(0, "X", 0.0, new GregorianCalendar(), "");  
    }  
  
    public Book(int id, String name, double price, GregorianCalendar calender,  
            String author) {  
        this.id = id;  
        this.name = name;  
        this.price = price;  
        this.calendar = calender;  
        this.author = author;  
    }  
  
   <span style="color:#ff0000;"> // override toString() of parent Object
</span>    public String toString() {  
        String showStr = id + "\t" + name;
        DecimalFormat formatPrice = new DecimalFormat("0.00");// 2 decimal places  
        showStr += "\t" + formatPrice.format(price);
        showStr += "\t" + author;  
        SimpleDateFormat formatDate = new SimpleDateFormat("yyyy.MM.dd");  
        showStr += "\t" + formatDate.format(calendar.getTime());
        return showStr;
    }  
  
    public int compareTo(Object obj) {// method in Comparable
        Book b = (Book) obj;  
        return this.id - b.id; // comparation based on book id. it for default sorting
    }  
  
    public static void main(String[] args) {  
        Book b1 = new Book(10000, "hong lou meng", 150.86, new GregorianCalendar(2009,  
                01, 25), "cao xueqin, gao e ");  
        Book b2 = new Book(10001, "san guo yan yi", 99.68, new GregorianCalendar(2008, 7,  
                8), "luo guanzhong ");  
        Book b3 = new Book(10002, "shui fu zhuan", 100.8, new GregorianCalendar(2009, 6,  
                28), "shi na an ");  
        Book b4 = new Book(10003, "xi you ji", 120.8, new GregorianCalendar(2011, 6,  
                8), "wu cheng en");  
        Book b5 = new Book(10004, "tian long ba bu", 10.4, new GregorianCalendar(2011, 9,  
                23), "sou hu");  
        TreeMap tm = new TreeMap();  
        tm.put(b1, new Integer(255));  
        tm.put(b2, new Integer(122));  
        tm.put(b3, new Integer(688));  
        tm.put(b4, new Integer(453));  
        tm.put(b5, new Integer(40));  
        Iterator it = tm.keySet().iterator();  
        Object key = null, value = null;  
        Book bb = null;  
        while (it.hasNext()) {  
            key = it.next();  
            bb = (Book) key;  
            value = tm.get(key);  
            System.out.println(bb.toString() + "\tkucun:" + tm.get(key));  
        }  
    }  
}

UseComparator class

import java.util.ArrayList;  
import java.util.Collections;  
import java.util.Comparator;  
import java.util.GregorianCalendar;  
import java.util.Iterator;  
import java.util.List;  
  
public class UseComparator {  
    public static void main(String args[]) {  
        List<Book> list = new ArrayList<Book>(); // array list  
        Book b1 = new Book(10000, "hongloumeng", 150.86, new GregorianCalendar(2009,  
                01, 25), "caoxueqin, gaoe ");  
        Book b2 = new Book(10001, "sanguoyanyi", 99.68, new GregorianCalendar(2008, 7,  
                8), "luoguanzhong");  
        Book b3 = new Book(10002, "shuifuzhuan", 100.8, new GregorianCalendar(2009, 6,  
                28), "shinaan ");  
        Book b4 = new Book(10003, "xiyouji", 120.8, new GregorianCalendar(2011, 6,  
                8), "wuchengen");  
        Book b5 = new Book(10004, "tian long ba bu", 10.4, new GregorianCalendar(2011, 9,  
                23), "souhu");  
        list.add(b1);  
        list.add(b2);  
        list.add(b3);  
        list.add(b4);  
        list.add(b5);  
        System.out.println("elements in array list:");  
        myprint(list); // use the default sorting in Book(the sorting method is compareTo()).
                        // in myprint(), it gets string by Book.toString(). 
        Collections.sort(list, new PriceComparator()); // sorting according to price  
        System.out.println("sorting according to price:");  
        myprint(list);  
        Collections.sort(list, new CalendarComparator()); // sorting according to publishing date  
        System.out.println("sorting according to publishing date:");  
        myprint(list);  
    }  
  
    // customized method: print all elements in array list, each line one element
    public static void myprint(List<Book> list) {  
        Iterator it = list.iterator(); // get iterator to work all elements in array list
        while (it.hasNext()) { // if there are elements in iterator, return true
            System.out.println("\t" + it.next());// print this element  
        }  
    }  
  
    // customized comparator: sorting by book price
    static class PriceComparator implements Comparator {  
        public int compare(Object object1, Object object2) {// implement method in interface  
            Book p1 = (Book) object1; // forced type conversion  
            Book p2 = (Book) object2;  
            return new Double(p1.price).compareTo(new Double(p2.price));  
        }  
    }  
  
    // customized comparator: sorting according to publishing date
    static class CalendarComparator implements Comparator {  
        public int compare(Object object1, Object object2) {// implement method in interface  
            Book p1 = (Book) object1; // forced type conversion  
            Book p2 = (Book) object2;  
            return p2.calendar.compareTo(p1.calendar); 
        }  
    }  
}

the result of Book.java:

10000   hong lou meng   150.86 cao xueqin, gao e       2009.02.25      kucun:255
10001   san guo yan yi 99.68   luo guanzhong   2008.08.08      kucun:122
10002   shui fu zhuan   100.80 shi na an       2009.07.28      kucun:688
10003   xi you ji       120.80 wu cheng en     2011.07.08      kucun:453
10004   tian long ba bu 10.40   sou hu 2011.10.23      kucun:40

the result of UseComparator.java:

elements in array list:
        10000   hongloumeng     150.86 caoxueqin, gaoe         2009.02.25
        10001   sanguoyanyi     99.68   luoguanzhong    2008.08.08
        10002   shuifuzhuan     100.80 shinaan         2009.07.28
        10003   xiyouji 120.80 wuchengen       2011.07.08
        10004   tian long ba bu 10.40   souhu   2011.10.23
sorting according to price:
        10004   tian long ba bu 10.40   souhu   2011.10.23
        10001   sanguoyanyi     99.68   luoguanzhong    2008.08.08
        10002   shuifuzhuan     100.80 shinaan         2009.07.28
        10003   xiyouji 120.80 wuchengen       2011.07.08
        10000   hongloumeng     150.86 caoxueqin, gaoe         2009.02.25
sorting according to publishing date:
        10004   tian long ba bu 10.40   souhu   2011.10.23
        10003   xiyouji 120.80 wuchengen       2011.07.08
        10002   shuifuzhuan     100.80 shinaan         2009.07.28
        10000   hongloumeng     150.86 caoxueqin, gaoe         2009.02.25
        10001   sanguoyanyi     99.68   luoguanzhong    2008.08.08