java Comparable和Comaprator的對比

 

Comparable使一個class具備不同例項間進行比較的行為。這些物件的集合，可作為Collections.sort或Arrays.sort的引數

Comparator可以看成一種演算法的實現，將演算法和資料分離。實現Comparator的可以作為Collections.sort或Arrays.sort的引數

都可以多條件排序。類似於order by field1,field2
都是介面

 

 

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

public class UserComparator implements Comparator<User> {
    /**
     *
     * @param source
     * @param target
     * @return 負數：升序；0：順序不變；正數：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());
        }
    }
}

 

例項如下：

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，為什麼能夠直接呼叫,這是因為在Student類內部
            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);
        }
    }
}

程式執行結果:

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

但是在設計類的時候，往往沒有考慮到讓類實現Comparable介面，那麼我們就需要用到另外的一個比較器介面Comparator。

從上面的例項我們可以發現，compareTo(T o)只有一個引數，而Comparator介面中必須要實現的compare(T o1,T o2)就有兩個引數。

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);
        }
    }

}

上述程式的執行結果與程式碼例項1一樣。