java基础(12)-- 深入理解Collections.sort()

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本文转载自：http://blog.csdn.net/tjcyjd/article/details/6804690
http://blog.csdn.net/janronehoo/article/details/8746447 尊重原创

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：

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：

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);          }      }  }

/** * @author guwh * @version 创建时间：2011-11-3 上午10:49:36 * 类说明 */   package com.jabberchina.test;  import java.util.ArrayList;  import java.util.Collections;  import java.util.Comparator;  import java.util.List;  public class SortTest {      public static void main(String[] args) {          List<String> lists = new ArrayList<String>();          List<A> list = new ArrayList<A>();          List<B> listB = new ArrayList<B>();          lists.add("5");          lists.add("2");          lists.add("9");          //lists中的对象String 本身含有compareTo方法，所以可以直接调用sort方法，按自然顺序排序，即升序排序          Collections.sort(lists);          A aa = new A();          aa.setName("aa");          aa.setOrder(1);          A bb = new A();          bb.setName("bb");          bb.setOrder(2);          list.add(bb);          list.add(aa);          //list中的对象A实现Comparable接口          Collections.sort(list);          B ab = new B();          ab.setName("ab");          ab.setOrder("1");          B ba = new B();          ba.setName("ba");          ba.setOrder("2");          listB.add(ba);          listB.add(ab);          //根据Collections.sort重载方法来实现          Collections.sort(listB,new Comparator<B>(){              @Override              public int compare(B b1, B b2) {                  return b1.getOrder().compareTo(b2.getOrder());              }          });          System.out.println(lists);          System.out.println(list);          System.out.println(listB);      }  }  class A implements Comparable<A>{      private String name;      private Integer order;      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }      public Integer getOrder() {          return order;      }      public void setOrder(Integer order) {          this.order = order;      }      @Override      public String toString() {          return "name is "+name+" order is "+order;      }      @Override      public int compareTo(A a) {          return this.order.compareTo(a.getOrder());      }  }  class B{      private String name;      private String order;      public String getName() {          return name;      }      public void setName(String name) {          this.name = name;      }      public String getOrder() {          return order;      }      public void setOrder(String order) {          this.order = order;      }      @Override      public String toString() {          return "name is "+name+" order is "+order;      }  }  打印的结果为：   [2, 5, 9]  [name is aa order is 1, name is bb order is 2]  [name is ab order is 1, name is ba order is 2]

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