Java中Collections.sort()排序详解

 

第一种：Comparable 排序接口

若一个类实现了Comparable接口，就意味着“该类支持排序”。 假设“有一个List列表(或数组)，里面的元素是实现了Comparable接口的类”，则该List列表(或数组)可以通过 Collections.sort（或 Arrays.sort）进行排序。

此外，“实现Comparable接口的类的对象”可以用作“有序映射(如TreeMap)”中的键或“有序集合(TreeSet)”中的元素，而不需要指定比较器。

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());  

    }  

 

}

List< A> list = new ArrayList< A>();

· 类A实现接口Comparable，并实现compareTo()方法

· 调用Collections.sort(lists)即可实现排序

第二种：Comparator比较器接口

我们若需要控制某个类的次序，而该类本身不支持排序(即没有实现Comparable接口)；我们可以建立一个“比较器”来进行排序。这个“比较器”只需要实现Comparator接口即可。

Collections.sort(list, new PriceComparator())

参数一：需要排序的list

参数二：比较器，实现Comparator接口的类，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。

Comparator是个接口，可重写compare()及equals()这两个方法,用于比较功能；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。

两种方法示例

package com.jabberchina.test;  

 

import java.util.ArrayList;  

import java.util.Collections;  

import java.util.Comparator;  

import java.util.List;  

 

public class SortTest {  

    static 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());

        }

    }

    static 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;

        }

    }

 public static void main(String[] args) {

 

        //第一种方法示例：

        List<String> lists = new ArrayList<String>();

        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);

 

        //第一种方法示例：

        List<A> listA = new ArrayList<A>();

        A a1 = new A();

        a1.setName("a1");

        a1.setOrder(1);

        A a2 = new A();

        a2.setName("a2");

        a2.setOrder(2);

        listA.add(a1);

        listA.add(a2);

        //list中的对象A实现Comparable接口

        Collections.sort(listA);

 

        //第二种方法示例：

        List<B> listB = new ArrayList<B>();

        B b1 = new B();

        b1.setName("b1");

        b1.setOrder("a");

        B b2 = new B();

        b2.setName("b2");

        b2.setOrder("b");

        listB.add(b1);

        listB.add(b2);

        //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(listA);

        System.out.println(listB);

    }

}  

 

打印的结果为：  

[2, 5, 9]

[name is a1 order is 1, name is a2 order is 2]

[name is b1 order is a, name is b2 order is b]

再例举一个方法一的示例：

方法二是一个策略模式，可以灵活替换比较器，实现不同排序

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, 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  

        }  

    }

}