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。

import java.util.ArrayList;
import java.util.Collections;
import java.util.Comparator;
import java.util.List;

public class StudentComparator implements Comparator<Object>
{
    @Override
    public int compare(Object arg0, Object arg1)
    {
        return ((Student)arg0).getId() - ((Student)arg1).getId();
    }
    
    public static void main(String[] args)
    {
        Student stu1 = new Student(11, "A");
        Student stu2 = new Student(33, "B");
        Student stu3 = new Student(22, "C");
        Student stu4 = new Student(55, "D");
        List<Student> stuList = new ArrayList<Student>(4);
        stuList.add(stu1);
        stuList.add(stu2);
        stuList.add(stu3);
        stuList.add(stu4);
        Collections.sort(stuList, new StudentComparator());
        System.out.println(stuList);
    }
}

class Student
{
    private int id;
    
    private String name;
    
    public Student(int id, String name)
    {
        this.id = id;
        this.name = name;
    }
    
    public int getId()
    {
        return id;
    }
    
    public void setId(int id)
    {
        this.id = id;
    }
    
    public String getName()
    {
        return name;
    }
    
    public void setName(String name)
    {
        this.name = name;
    }
    
    public String toString()
    {
        return this.getId() + this.getName();
    }
}

输出：[11A, 22C, 33B, 55D]

2、在需要排序的类上直接实现Comparable接口

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class Student implements Comparable<Object>
{
    private int id;
    
    private String name;
    
    public Student(int id, String name)
    {
        this.id = id;
        this.name = name;
    }
    
    public int getId()
    {
        return id;
    }
    
    public void setId(int id)
    {
        this.id = id;
    }
    
    public String getName()
    {
        return name;
    }
    
    public void setName(String name)
    {
        this.name = name;
    }
    
    public String toString()
    {
        return this.getId() + this.getName();
    }
    
    @Override
    public int compareTo(Object o)
    {
        return this.getId() - ((Student)o).getId();
    }
    
    public static void main(String[] args)
    {
        Student stu1 = new Student(11, "A");
        Student stu2 = new Student(33, "B");
        Student stu3 = new Student(22, "C");
        Student stu4 = new Student(55, "D");
        List<Student> stuList = new ArrayList<Student>(4);
        stuList.add(stu1);
        stuList.add(stu2);
        stuList.add(stu3);
        stuList.add(stu4);
        Collections.sort(stuList);
        System.out.println(stuList);
    }
    
}

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。