Comparable和Comparator接口的使用

一、Comparable接口，需要某一个类实现该接口，接口中复写comparaTo（<T> o）方法，自己写比较规则，示例代码如下

Person类，实现了Comparable接口，复写了comparaTo方法

package cn.gao.day1019;

public class Person implements Comparable<Person> {
    private String name;
    private int age;
    private double weight;
    private double height;

    public Person() {
    }

    public Person(String name, int age, double weight, double height) {
        this.name = name;
        this.age = age;
        this.weight = weight;
        this.height = height;
    }

    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 double getWeight() {
        return weight;
    }

    public void setWeight(double weight) {
        this.weight = weight;
    }

    public double getHeight() {
        return height;
    }

    public void setHeight(double height) {
        this.height = height;
    }

    @Override
    public String toString() {
        return "Person{" +
                "name='" + name + '\'' +
                ", age=" + age +
                ", weight=" + weight +
                ", height=" + height +
                '}';
    }

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

 在测试类中，如果要排序该类型的数组，可以直接调用Arrays.sort()方法，即可实现按自己写的规则排序。

package cn.gao.day1019;

import java.util.Arrays;

public class PersonDemo1 {
    public static void main(String[] args) {
        Person[] per = {new Person("zhangsan",20,75.5,165.5),
                new Person("lisi",25,88.5,193.4),
                new Person("wangwu",18,60.5,175.7),
                new Person("zhaoliu",23,72.4,181.2),
                new Person("chenqi",16,53.5,162.5)};
        Arrays.sort(per);
        for (Person person : per) {
            System.out.println(person);
        }
    }
}

打印结果如下：可以看到按照comparaTo()中的柜子，按照年龄的升序进行了排列。

Person{name='chenqi', age=16, weight=53.5, height=162.5}

Person{name='wangwu', age=18, weight=60.5, height=175.7}

Person{name='zhangsan', age=20, weight=75.5, height=165.5}

Person{name='zhaoliu', age=23, weight=72.4, height=181.2}

Person{name='lisi', age=25, weight=88.5, height=193.4}

二、Comparator需要根据不同属性的比较，分别写不同的实现类。

Person类不变，还是上面的原样，按Age比较的类如下

package cn.gao.day1019;

import java.util.Comparator;

/**
 * 实现按age排序的类
 */
public class PersonSortByAge implements Comparator<Person> {
    @Override
    public int compare(Person o1, Person o2) {
        if (o1.getAge() < o2.getAge()) {
            return -1;
        } else if (o1.getAge() > o2.getAge()) {
            return 1;
        }
        return 0;
    }
}

按Height比较的类如下

package cn.gao.day1019;

import java.util.Comparator;

/**
 * 实现按height比较的类
 */
public class PersonSortByHeight implements Comparator<Person> {
    @Override
    public int compare(Person o1, Person o2) {
        if (o1.getHeight() < o2.getHeight()) {
            return -1;
        } else if (o1.getHeight() > o2.getHeight()) {
            return 1;
        }
        return 0;
    }
}

按Weight比较的类如下

package cn.gao.day1019;

import java.util.Comparator;

/**
 * 实现按weight比较的类
 */
public class PersonSortByWeight implements Comparator<Person> {
    @Override
    public int compare(Person o1, Person o2) {
        if (o1.getWeight() < o2.getWeight()) {
            return -1;
        } else if (o1.getWeight() > o2.getWeight()) {
            return 1;
        }
        return 0;
    }
}

测试类如下

package cn.gao.day1019;

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

public class PersonDemo2 {
    public static void main(String[] args) {
        Person[] per = {new Person("zhangsan", 20, 75.5, 165.5),
                new Person("lisi", 25, 88.5, 193.4),
                new Person("wangwu", 18, 60.5, 175.7),
                new Person("zhaoliu", 23, 72.4, 181.2),
                new Person("chenqi", 16, 53.5, 162.5)};
        System.out.println("按age排序");
        Arrays.sort(per, new PersonSortByAge());
        for (Person person : per) {
            System.out.println(person);
        }
        System.out.println("按height排序");
        Arrays.sort(per, new PersonSortByHeight());
        for (Person person : per) {
            System.out.println(person);
        }
        System.out.println("按weight排序");
        Arrays.sort(per, new PersonSortByWeight());
        for (Person person : per) {
            System.out.println(person);
        }
    }
}

执行结果如下：

按age排序
Person{name='chenqi', age=16, weight=53.5, height=162.5}
Person{name='wangwu', age=18, weight=60.5, height=175.7}
Person{name='zhangsan', age=20, weight=75.5, height=165.5}
Person{name='zhaoliu', age=23, weight=72.4, height=181.2}
Person{name='lisi', age=25, weight=88.5, height=193.4}
按height排序
Person{name='chenqi', age=16, weight=53.5, height=162.5}
Person{name='zhangsan', age=20, weight=75.5, height=165.5}
Person{name='wangwu', age=18, weight=60.5, height=175.7}
Person{name='zhaoliu', age=23, weight=72.4, height=181.2}
Person{name='lisi', age=25, weight=88.5, height=193.4}
按weight排序
Person{name='chenqi', age=16, weight=53.5, height=162.5}
Person{name='wangwu', age=18, weight=60.5, height=175.7}
Person{name='zhaoliu', age=23, weight=72.4, height=181.2}
Person{name='zhangsan', age=20, weight=75.5, height=165.5}
Person{name='lisi', age=25, weight=88.5, height=193.4}

 可以看出，结果已经分别按age、height、weight进行了排序

总结：Comparable接口，实现该接口后，需要实现接口中的comparaTo方法，自身实现了排序，就可以和基本数据类型数组一样，使用Arrays.sort（）方法进行排序了，传递参数为要排序的数组对象。只能定义一种比较规则，如果比较规则发生改变，那么就需要改动comparaTo()方法内的内容。

Comparator接口，该接口是单独的一个类去实现，即提前准备好不同参数的比较方法，有几套方法，就提前写接个比较的类，也可以后期扩展比较的类。排序时，也调用Arrays.sort()，但是传递的参数除了数组对象外，还要传递一个比较器对象。

Comparable接口较为单一，如果比较的参数固定，不存在多种情况，那么采用这种方法即可，只需要类里面实现comparaTo()方法，调用时直接传递待排序数组即可。

Comparable接口是预定义几种排序的类，同时后期可以根据业务扩展更多的排序类，扩展性强，调用时除了待排序的方法，还要传递排序类对象。