Comparator篇

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

public class UserComparator {

    public static void main(String[] args) {

        /**
         * Comparator是一个专用的比较器，当这个对象不支持自比较或者自比较函数不能满足要求时，可写一个比较器来完成两个对象之间大小的比较。
         * Comparator体现了一种策略模式(strategy design pattern)，就是不改变对象自身，而用一个策略对象(strategy object)来改变它的行为。
         *
         */

        // 一组数字需要对其进行正序排序,java8之前我们通常的做法是:
        Integer[] values1 = new Integer[]{1, 0, 3, 5, 7, 8, 5, 4, 3, 7, 9, 56, 3, 23};
        Arrays.sort(values1, new Comparator<Integer>() {
            @Override
            public int compare(Integer value1, Integer value2) {
                return value1 - value2;
            }
        });
        // 而在java8中,由于新增lambda表达式,我们可以很简洁的使用如下的方式:
        Integer[] values2 = new Integer[]{1, 0, 3, 5, 7, 8, 5, 4, 3, 7, 9, 56, 3, 23};
        Arrays.sort(values2, (x, y) -> x - y);
        // jdk甚至为了排序做了一些定制的方法,比如上面的代码可以写成如下的方式,更加让人容易理解:
        Integer[] values3 = new Integer[]{1, 0, 3, 5, 7, 8, 5, 4, 3, 7, 9, 56, 3, 23};
        Arrays.sort(values3, Comparator.comparingInt(x -> x));


        // reversed():这个方法是用来生成一个逆序器,比如我们开始需要得到一个正序的排序序列,然后又想得到一个反转的排序序列,就可以使用该方法
        Integer[] values = new Integer[]{1, 0, 3, 5, 7, 8, 5, 4, 3, 7, 9, 56, 3, 23};
        Comparator<Integer> comparator = Comparator.comparingInt(x -> x);
        Arrays.sort(values, comparator);
        System.out.println(JSON.toJSONString(values));  // [0,1,3,3,3,4,5,5,7,7,8,9,23,56]
        Arrays.sort(values, comparator.reversed());
        System.out.println(JSON.toJSONString(values));  // [56,23,9,8,7,7,5,5,4,3,3,3,1,0]


        // thenComparing(Comparator<? super T> other):该方法是在原有的比较器上再加入一个比较器,比如我们有一些用户,在对其进行排序的时候,
        // 我们首先对他的名字长度进行排序,然后再对他的年龄进行排序
        Person[] persons = new Person[]{
                new Person(22, "zhangsan"),
                new Person(23, "lisi"),
                new Person(23, "wangwu"),
                new Person(22, "wangwu"),
                new Person(22, "lisi"),
                new Person(23, "zhangsan")
        };
        System.out.println(JSON.toJSONString(persons));
        Comparator<Person> comparator = Comparator.comparingInt(x -> x.getName().length());
        Arrays.sort(persons, comparator);
        System.out.println(JSON.toJSONString(persons));
        comparator = comparator.thenComparing(Comparator.comparingInt(Person::getAge));
        Arrays.sort(persons, comparator);
        System.out.println(JSON.toJSONString(persons));
        // 输出结果如下：
//        [{"age":22,"name":"zhangsan"},{"age":23,"name":"lisi"},{"age":23,"name":"wangwu"},{"age":22,"name":"wangwu"},{"age":22,"name":"lisi"},{"age":23,"name":"zhangsan"}]
//        [{"age":23,"name":"lisi"},{"age":22,"name":"lisi"},{"age":23,"name":"wangwu"},{"age":22,"name":"wangwu"},{"age":22,"name":"zhangsan"},{"age":23,"name":"zhangsan"}]
//        [{"age":22,"name":"lisi"},{"age":23,"name":"lisi"},{"age":22,"name":"wangwu"},{"age":23,"name":"wangwu"},{"age":22,"name":"zhangsan"},{"age":23,"name":"zhangsan"}]

        // 简化写法如下：
        Comparator<Person> comparator = Comparator.comparing(Person::getName).thenComparing(Person::getAge);
        Arrays.sort(persons, comparator);
    }
}