java Comparable和Comaprator的对比

Comparable使一个class具备不同实例间进行比较的行为。这些对象的集合，可作为Collections.sort或Arrays.sort的参数

Comparator可以看成一种算法的实现，将算法和数据分离。实现Comparator的可以作为Collections.sort或Arrays.sort的参数

都可以多条件排序。类似于order by field1,field2
都是接口

import java.util.Arrays;
import java.util.Comparator;

public class UserComparator implements Comparator<User> {
    /**
     *
     * @param source
     * @param target
     * @return 负数：升序；0：顺序不变；正数：降序
     */
    @Override
    public int compare(User source, User target) {
        int result = source.getAge() - target.getAge();
        System.out.println(result);
        return result;
    }

    public static void main(String[] args) {
        User[] users = new User[]{new User("a", 30), new User("b", 20)};
        Arrays.sort(users, new UserComparator());
        for (int i = 0; i < users.length; i++) {
            User user = users[i];
            System.out.println(user.getId() + " " + user.getAge());
        }
    }
}

class User {

    private String id;
    private int age;

    public User(String id, int age) {
        this.id = id;
        this.age = age;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

}

import java.util.Arrays;

public class User implements Comparable<User> {

    private String id;
    private int age;

    public User(String id, int age) {
        this.id = id;
        this.age = age;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public String getId() {
        return id;
    }

    public void setId(String id) {
        this.id = id;
    }

    @Override
    public int compareTo(User target) {
        int result = this.age - target.getAge();
        System.out.println(result);
        return result;
    }

    public static void main(String[] args) {
        User[] users = new User[]{new User("a", 30), new User("b", 20)};
        Arrays.sort(users);
        for (int i = 0; i < users.length; i++) {
            User user = users[i];
            System.out.println(user.getId() + " " + user.getAge());
        }
    }
}

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 -;//由高到底排序
        else if(this.score<o.score)
            return ;
        else{
            if(this.age>o.age)
                return ;//由底到高排序
            else if(this.age<o.age)
                return -;
            else
                return ;
        }
    }
}

public class ComparableDemo01 {

    /**
     * @param args
     */
    public static void main(String[] args) {
        // TODO Auto-generated method stub
        Student stu[]={new Student("zhangsan",,90.0f),
                new Student("lisi",,90.0f),
                new Student("wangwu",,99.0f),
                new Student("sunliu",,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。

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 -;
        else if(o1.getScore()<o2.getScore())
            return ;
        else{
            if(o1.getAge()>o2.getAge())
                return ;
            else if(o1.getAge()<o2.getAge())
                return -;
            else
                return ;
        }
    }

}

public class ComparableDemo02 {

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

        Student stu[]={new Student("zhangsan",,90.0f),
                new Student("lisi",,90.0f),
                new Student("wangwu",,99.0f),
                new Student("sunliu",,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。