Java中的Comparator使用

Comparator 与 Comparable

比较器，一般用于排序的时候。即如果某个没有实现Comparable接口，这时可以通过Comparator来实现类的外在排序器。它与 Comparable 不同，它一般是用于类自身，即类的内在排序。

Comparator 比较灵活点，可以通过需求，按指定的排序要求来进行排序。而实现Comparable则是Collections.sort(list)时，默认的排序。这里可以通过一个简单的例子来说明。

比如，Person类，默认情况下，自身是可以按年龄来排序的。如:

Comparable

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

public class T3 {

	public static void main(String[] args) {
		List<Person> persons = new ArrayList<Person>();
		Random r = new Random();
		Person p = null;
		for (int i = 0; i < 5; i++) {
			p = new Person();
			p.setAge(r.nextInt(100));
			p.setName("name" + i);
			persons.add(p);
		}
		System.out.println("排序之前：");
		printInfo(persons);
		System.out.println("排序之后");
		Collections.sort(persons);
		printInfo(persons);
	}

	public static void printInfo(List<Person> list) {
		for (Person person : list) {
			System.out.println(" name = " + person.getName() + ", age = " + person.getAge());
		}
	}

}

class Person implements Comparable<Person> {

	private int age;
	private String name;

	public int getAge() {
		return age;
	}

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

	public String getName() {
		return name;
	}

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

	@Override
	public int compareTo(Person o) {
		return this.age - o.age;
	}

}

结果为：

排序之前：
 name = name0, age = 71
 name = name1, age = 64
 name = name2, age = 9
 name = name3, age = 52
 name = name4, age = 22
排序之后
 name = name2, age = 9
 name = name4, age = 22
 name = name3, age = 52
 name = name1, age = 64
 name = name0, age = 71

使用Comparator

改为Comparator，则可以动态按需排序。只需要构造相应

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

public class T3 {

	public static void main(String[] args) {
		List<Person> persons = new ArrayList<Person>();
		Random r = new Random();
		Person p = null;
		for (int i = 0; i < 5; i++) {
			p = new Person();
			p.setAge(r.nextInt(100));
			p.setName("name" + i);
			persons.add(p);
		}
		System.out.println("排序之前：");
		printInfo(persons);
		System.out.println("排序之后");
		Collections.sort(persons, new PersonComparator());
		printInfo(persons);
	}

	public static void printInfo(List<Person> list) {
		for (Person person : list) {
			System.out.println(" name = " + person.getName() + ", age = " + person.getAge());
		}
	}

}

class PersonComparator implements Comparator<Person> {

	@Override
	public int compare(Person o1, Person o2) {
		return o1.getAge() - o2.getAge();
	}

}

class Person {

	private int age;
	private String name;

	public int getAge() {
		return age;
	}

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

	public String getName() {
		return name;
	}

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

}


结果：

排序之前：
 name = name0, age = 80
 name = name1, age = 16
 name = name2, age = 12
 name = name3, age = 22
 name = name4, age = 22
排序之后
 name = name2, age = 12
 name = name1, age = 16
 name = name3, age = 22
 name = name4, age = 22
 name = name0, age = 80

升序，降序

Comparator

升序

@Override
public int compare(Person o1, Person o2) {
	return o1.getAge() - o2.getAge();
}

降序

@Override
public int compare(Person o1, Person o2) {
	return o2.getAge() - o1.getAge();
}

Comparable

升序

@Override
public int compareTo(Person o) {
	return this.age - o.age;
}

降序

@Override
public int compareTo(Person o) {
	return o.age - this.age;
}

Contents

Comparator 与 Comparable

Comparable

使用Comparator

升序，降序

Comparator

Comparable