java学习(十六)：对象的自定义比较，Comparator和Comparable

通过两个例子实现对象的自定义排序

1、实现Comparator接口。

 1 import java.util.ArrayList;
 2 import java.util.Collections;
 3 import java.util.Comparator;
 4 import java.util.List;
 5 
 6 public class StudentComparator implements Comparator<Object>
 7 {
 8     @Override
 9     public int compare(Object arg0, Object arg1)
10     {
11         return ((Student)arg0).getId() - ((Student)arg1).getId();
12     }
13     
14     public static void main(String[] args)
15     {
16         Student stu1 = new Student(11, "A");
17         Student stu2 = new Student(33, "B");
18         Student stu3 = new Student(22, "C");
19         Student stu4 = new Student(55, "D");
20         List<Student> stuList = new ArrayList<Student>(4);
21         stuList.add(stu1);
22         stuList.add(stu2);
23         stuList.add(stu3);
24         stuList.add(stu4);
25         Collections.sort(stuList, new StudentComparator());
26         System.out.println(stuList);
27     }
28 }
29 
30 class Student
31 {
32     private int id;
33     
34     private String name;
35     
36     public Student(int id, String name)
37     {
38         this.id = id;
39         this.name = name;
40     }
41     
42     public int getId()
43     {
44         return id;
45     }
46     
47     public void setId(int id)
48     {
49         this.id = id;
50     }
51     
52     public String getName()
53     {
54         return name;
55     }
56     
57     public void setName(String name)
58     {
59         this.name = name;
60     }
61     
62     public String toString()
63     {
64         return this.getId() + this.getName();
65     }
66 }

输出：[11A, 22C, 33B, 55D]

2、在需要排序的类上直接实现Comparable接口

 1 import java.util.ArrayList;
 2 import java.util.Collections;
 3 import java.util.List;
 4 
 5 public class Student implements Comparable<Object>
 6 {
 7     private int id;
 8     
 9     private String name;
10     
11     public Student(int id, String name)
12     {
13         this.id = id;
14         this.name = name;
15     }
16     
17     public int getId()
18     {
19         return id;
20     }
21     
22     public void setId(int id)
23     {
24         this.id = id;
25     }
26     
27     public String getName()
28     {
29         return name;
30     }
31     
32     public void setName(String name)
33     {
34         this.name = name;
35     }
36     
37     public String toString()
38     {
39         return this.getId() + this.getName();
40     }
41     
42     @Override
43     public int compareTo(Object o)
44     {
45         return this.getId() - ((Student)o).getId();
46     }
47     
48     public static void main(String[] args)
49     {
50         Student stu1 = new Student(11, "A");
51         Student stu2 = new Student(33, "B");
52         Student stu3 = new Student(22, "C");
53         Student stu4 = new Student(55, "D");
54         List<Student> stuList = new ArrayList<Student>(4);
55         stuList.add(stu1);
56         stuList.add(stu2);
57         stuList.add(stu3);
58         stuList.add(stu4);
59         Collections.sort(stuList);
60         System.out.println(stuList);
61     }
62     
63 }

同样输出：[11A, 22C, 33B, 55D]

3、一个类实现了Comparable接口则表明这个类自身对象之间是可以相互比较的，这个类对象组成的集合就可以直接使用sort方法排序。

　　Comparator可以看成一种算法的实现，将算法和数据分离。Comparator也可以在下面两种环境下使用：

　　（1）类的设计师没有考虑到比较问题而没有实现Comparable，可以通过Comparator来实现排序而不必改变对象本身。

　　（2）可以使用多种排序标准，比如定义升序类、降序类等。