Detailed explanation of examples of Collections.sort sorting in Java

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)); 
    } 
  } 
}

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true 
      System.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); 
    } 
  } 
}