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：
package com.tjcyjd.comparator;import java.text.DecimalFormat;import java.text.SimpleDateFormat;import java.util.GregorianCalendar;import java.util.Iterator;import java.util.TreeMap;/** * 书实体类 * * @author yjd * */public class Book implements Comparable { // 定义名为Book的类，默认继承自Object类public int id;// 编号public String name;// 名称public double price; // 价格private String author;// 作者public GregorianCalendar calendar;// 出版日期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;}// 重写继承自父类Object的方法，满足Book类信息描述的要求public String toString() {String showStr = id + "\t" + name; // 定义显示类信息的字符串DecimalFormat formatPrice = new DecimalFormat("0.00");// 格式化价格到小数点后两位showStr += "\t" + formatPrice.format(price);// 格式化价格showStr += "\t" + author;SimpleDateFormat formatDate = new SimpleDateFormat("yyyy年MM月dd日");showStr += "\t" + formatDate.format(calendar.getTime()); // 格式化时间return showStr; // 返回类信息字符串}public int compareTo(Object obj) {// Comparable接口中的方法Book b = (Book) obj;return this.id – b.id; // 按书的id比较大小，用于默认排序}public static void main(String[] args) {Book b1 = new Book(10000, "红楼梦", 150.86, new GregorianCalendar(2009,01, 25), "曹雪芹、高鄂");Book b2 = new Book(10001, "三国演义", 99.68, new GregorianCalendar(2008, 7,8), "罗贯中 ");Book b3 = new Book(10002, "水浒传", 100.8, new GregorianCalendar(2009, 6,28), "施耐庵 ");Book b4 = new Book(10003, "西游记", 120.8, new GregorianCalendar(2011, 6,8), "吴承恩");Book b5 = new Book(10004, "天龙八部", 10.4, new GregorianCalendar(2011, 9,23), "搜狐");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() + "\t库存：" + tm.get(key));}}}自定义比较器和测试类：package com.tjcyjd.comparator;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>(); // 数组序列Book b1 = new Book(10000, "红楼梦", 150.86, new GregorianCalendar(2009,01, 25), "曹雪芹、高鄂");Book b2 = new Book(10001, "三国演义", 99.68, new GregorianCalendar(2008, 7,8), "罗贯中 ");Book b3 = new Book(10002, "水浒传", 100.8, new GregorianCalendar(2009, 6,28), "施耐庵 ");Book b4 = new Book(10003, "西游记", 120.8, new GregorianCalendar(2011, 6,8), "吴承恩");Book b5 = new Book(10004, "天龙八部", 10.4, new GregorianCalendar(2011, 9,23), "搜狐");list.add(b1);list.add(b2);list.add(b3);list.add(b4);list.add(b5);// Collections.sort(list); //没有默认比较器，不能排序System.out.println("数组序列中的元素:");myprint(list);Collections.sort(list, new PriceComparator()); // 根据价格排序System.out.println("按书的价格排序:");myprint(list);Collections.sort(list, new CalendarComparator()); // 根据时间排序System.out.println("按书的出版时间排序:");myprint(list);}// 自定义方法：分行打印输出list中的元素public static void myprint(List<Book> list) {Iterator it = list.iterator(); // 得到迭代器，用于遍历list中的所有元素while (it.hasNext()) {// 如果迭代器中有元素，则返回trueSystem.out.println("\t" + it.next());// 显示该元素}}// 自定义比较器：按书的价格排序static class PriceComparator implements Comparator {public int compare(Object object1, Object object2) {// 实现接口中的方法Book p1 = (Book) object1; // 强制转换Book p2 = (Book) object2;return new Double(p1.price).compareTo(new Double(p2.price));}}// 自定义比较器：按书出版时间来排序static class CalendarComparator implements Comparator {public int compare(Object object1, Object object2) {// 实现接口中的方法Book p1 = (Book) object1; // 强制转换Book p2 = (Book) object2;return p2.calendar.compareTo(p1.calendar);}}}
自己选择的路，跪着也要把它走完。
