比较器 comparable与comparator用法

comparable

接口 Comparable<T>

类型参数：T - 可以与此对象进行比较的那些对象的类型

public interface Comparable<T>

此接口强行对实现它的每个类的对象进行整体排序。这种排序被称为类的自然排序，类的 compareTo 方法被称为它的自然比较方法。

实现此接口的对象列表（和数组）可以通过 Collections.sort（和 Arrays.sort）进行自动排序。实现此接口的对象可以用作有序映射中的键或有序集合中的元素，无需指定比较器。

负整数、零或正整数，根据此对象是小于、等于还是大于指定对象。

compareTo（对象）
if(this. > ) return -1; //高到低排序

例子：学生分数高到低，年龄低到高排序

package com.ij34;

/**
 * Created by Admin on 2018/3/7.
 */
public class Student implements Comparable<Student>{
    private String name;
    private  int age;
    private  float 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 Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", score=" + score +
                '}';
    }

    @Override
    public int compareTo(Student o) {
        if (this.score > o.score) return -1;
        else if (this.score < o.score) return 1;
         else{
            if (this.age > o.age) return 1;
            else if (this.age < o.age) return -1;
            else return 0;
        }
    }
}

package com.ij34;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

/**
 * Created by Admin on 2018/3/7.
 */
public class ComparableStudentTest {
    public static void main(String[] args) {
        ArrayList<Student> arrays=new ArrayList<Student>();
        Student a=new Student("李白",23,88);
        Student b=new Student("张三",21,80);
        Student c=new Student("李四",22,78);
        Student d=new Student("萧峰",24,77);
        arrays.add(a);
        arrays.add(b);
        arrays.add(c);
        arrays.add(d);
        Collections.sort(arrays,Student::compareTo);
        for(Student s:arrays){
            System.out.println(s);
        }
    }
}

comparator

java.util
接口 Comparator<T>

类型参数：T - 此 Comparator 可以比较的对象类型

强行对某个对象 collection 进行整体排序 的比较函数。可以将 Comparator 传递给 sort 方法（如 Collections.sort 或 Arrays.sort），从而允许在排序顺序上实现精确控制。还可以使用 Comparator 来控制某些数据结构（如有序 set或有序映射）的顺序，或者为那些没有自然顺序的对象 collection 提供排序。

int	compare(T o1, T o2)           比较用来排序的两个参数
boolean	equals(Object obj)           指示某个其他对象是否“等于”此 Comparator

package com.ij34;

/**
 * Created by Admin on 2018/3/7.
 */
public class Student{
    private String name;
    private  int age;
    private  float 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 Student(String name, int age, float score) {
        this.name = name;
        this.age = age;
        this.score = score;
    }

    @Override
    public String toString() {
        return "Student{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", score=" + score +
                '}';
    }
}

package com.ij34;

import java.util.Comparator;

/**
 * Created by Admin on 2018/3/7.
 */
public class Studentcomparator implements Comparator<Student>{
    public int compare(Student o1,Student o2){
        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;
        }
    }
}

package com.ij34;

import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collections;

/**
 * Created by Admin on 2018/3/7.
 */
public class ComparableStudentTest {
    public static void main(String[] args) {
        ArrayList<Student> arrays=new ArrayList<Student>();
        Student a=new Student("李白",23,88);
        Student b=new Student("张三",21,80);
        Student c=new Student("李四",22,78);
        Student d=new Student("萧峰",24,77);
        arrays.add(a);
        arrays.add(b);
        arrays.add(c);
        arrays.add(d);
        Collections.sort(arrays,new Studentcomparator());
        for(Student s:arrays){
            System.out.println(s);
        }
    }
}