java比较器Comparable接口和Comaprator接口

java的比较器有两类，分别是Comparable接口和Comparator接口。

在为对象数组进行排序时，比较器的作用非常明显，首先来讲解Comparable接口。

让需要进行排序的对象实现Comparable接口，重写其中的compareTo(T o)方法，在其中定义排序规则，那么就可以直接调用java.util.Arrays.sort()来排序对象数组，实例如下：

class Student implements Comparable<Student>{
    private String name;
    private int age;
    private float score;
    
    public Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }
    
    public String toString()
    {
        return name+"\t\t"+age+"\t\t"+score;
    }

    @Override
    public int compareTo(Student o) {
        // TODO Auto-generated method stub
        if(this.score>o.score)//score是private的，为什么能够直接调用,这是因为在Student类内部
            return -1;//由高到底排序
        else if(this.score<o.score)
            return 1;
        else{
            if(this.age>o.age)
                return 1;//由底到高排序
            else if(this.age<o.age)
                return -1;
            else
                return 0;
        }
    }
}

public class ComparableDemo01 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student stu[]={new Student("zhangsan",20,90.0f),
                new Student("lisi",22,90.0f),
                new Student("wangwu",20,99.0f),
                new Student("sunliu",22,100.0f)};
        java.util.Arrays.sort(stu);
        for(Student s:stu)
        {
            System.out.println(s);
        }
    }
}

程序运行结果:

sunliu 22 100.0
wangwu 20 99.0
zhangsan 20 90.0
lisi 22 90.0

但是在设计类的时候，往往没有考虑到让类实现Comparable接口，那么我们就需要用到另外的一个比较器接口Comparator。

从上面的实例我们可以发现，compareTo(T o)只有一个参数，而Comparator接口中必须要实现的compare(T o1,T o2)就有两个参数。

代码实例：

package edu.sjtu.ist.comutil;

import java.util.Comparator;

class Student {
    private String name;
    private int age;
    private float score;
    
    public Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    public String getName() {
        return name;
    }
    public void setName(String name) {
        this.name = name;
    }
    public int getAge() {
        return age;
    }
    public void setAge(int age) {
        this.age = age;
    }
    public float getScore() {
        return score;
    }
    public void setScore(float score) {
        this.score = score;
    }

    public String toString()
    {
        return name+"\t\t"+age+"\t\t"+score;
    }

}

class StudentComparator implements Comparator<Student>{

    @Override
    public int compare(Student o1, Student o2) {
        // TODO Auto-generated method stub
        if(o1.getScore()>o2.getScore())
            return -1;
        else if(o1.getScore()<o2.getScore())
            return 1;
        else{
            if(o1.getAge()>o2.getAge())
                return 1;
            else if(o1.getAge()<o2.getAge())
                return -1;
            else 
                return 0;
        }
    }
    
}


public class ComparableDemo02 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub

        Student stu[]={new Student("zhangsan",20,90.0f),
                new Student("lisi",22,90.0f),
                new Student("wangwu",20,99.0f),
                new Student("sunliu",22,100.0f)};
        java.util.Arrays.sort(stu,new StudentComparator());
        for(Student s:stu)
        {
            System.out.println(s);
        }
    }

}

上述程序的运行结果与代码实例1一样。