java之比较器

java中的比较器有两种：

1、实体类实现Comparable接口，并实现其中的compareTo方法

2、在外部定义实现Comparator接口的比较器类，并实现其中的compare方法

Person类源码：

package test;

public class Person implements Comparable<Person> {
    private Integer id;
    private String name;
    private Integer age;

    public Person(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

    @Override
    public int compareTo(Person person) {
        return name.compareTo(person.name);
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

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

按名字进行排序，测试代码：

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ComparableTest {

    public static void main(String[] args) {
        List<Person> list = new ArrayList<Person>();
        list.add(new Person(1, "b", 13));
        list.add(new Person(2, "d", 12));
        list.add(new Person(3, "a", 12));
        list.add(new Person(4, "a", 21));
        Collections.sort(list);
        System.out.println("编号\t姓名\t年龄");
        for (Person p : list) {
            System.out.println(p);
        }
    }
}

打印：

可以看出人员根据名字排序，当名字相等时，不做处理。

若要当名字相等时，年龄按从大到小排序，则修改Person类的compareTo方法，代码如下：

    public int compareTo(Person person) {
        if (name.equals(person.name))
            return -(age.compareTo(person.age));
        return name.compareTo(person.name);
    }

打印：

package test;

public class Person  {
    private Integer id;
    private String name;
    private Integer age;

    public Person(Integer id, String name, Integer age) {
        this.id = id;
        this.name = name;
        this.age = age;
    }

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

    public int getId() {
        return id;
    }

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

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getAge() {
        return age;
    }

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

自定义比较器PersonComparator类，代码如下：

package test;

import java.util.Comparator;

public class PersonComparator implements Comparator<Person> {

    public int compare(Person o1, Person o2) {
        return o1.getName().compareTo(o2.getName());
    }
}

测试代码：

package test;

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;

public class ComparatorTest {
    public static void main(String[] args) {
        List<Person> list = new ArrayList<Person>();
        list.add(new Person(1, "b", 13));
        list.add(new Person(2, "d", 12));
        list.add(new Person(3, "a", 12));
        list.add(new Person(4, "a", 21));
        Collections.sort(list, new PersonComparator());
        System.out.println("编号\t姓名\t年龄");
        for (Person p : list) {
            System.out.println(p);
        }
    }
}

打印：