java基础(12)-- 深入理解Collections.sort()

本文转载自：http://blog.youkuaiyun.com/tjcyjd/article/details/6804690
http://blog.youkuaiyun.com/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]