Comparable和Comparator的使用

1：对象实现Comparable， 那么对象就具有了比较功能

package comparableAndComparator;

import java.util.Collections;
import java.util.LinkedList;

/**
 * @program: GradleTestUseSubModule
 * @author: Yafei Li
 * @create: 2018-09-10 16:18
 **/
public class Main {
    public static void main(String[] args){
        LinkedList<Person> linkedList = new LinkedList<>();
        linkedList.add(new Person("wangwu"));
        linkedList.add(new Person("lisi"));
        linkedList.add(new Person("zhangsan"));
        linkedList.stream().forEach(person -> {
            System.out.println(person.name);
        });
        Collections.sort(linkedList);
        System.out.println("--------------------------");
        linkedList.stream().forEach(person -> {
            System.out.println(person.name);
        });
    }
}

class Person implements Comparable<Person>{

    public String name;

    public Person(String name) {
        this.name = name;
    }

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

结果：

wangwu
lisi
zhangsan
--------------------------
zhangsan
wangwu
lisi

2：单独定义一个比较器，用于比较Person对象

package comparableAndComparator;

import java.util.Collections;
import java.util.Comparator;
import java.util.LinkedList;

/**
 * @program: GradleTestUseSubModule
 * @author: Yafei Li
 * @create: 2018-09-10 16:18
 **/
public class Main {
    public static void main(String[] args){
        LinkedList<Person> linkedList = new LinkedList<>();
        linkedList.add(new Person("wangwu"));
        linkedList.add(new Person("lisi"));
        linkedList.add(new Person("zhangsan"));
        linkedList.stream().forEach(person -> {
            System.out.println(person.name);
        });
        Collections.sort(linkedList,new PersonComparator());
        System.out.println("--------------------------");
        linkedList.stream().forEach(person -> {
            System.out.println(person.name);
        });
    }
}

class Person{

    public String name;

    public Person(String name) {
        this.name = name;
    }
}

class PersonComparator implements Comparator<Person>{
    public int compare(Person p1, Person p2) {
        return p1.name.compareTo(p2.name);
    }
}

结果：

wangwu
lisi
zhangsan
--------------------------
lisi
wangwu
zhangsan

3：使用比较器对 map进行排序

package com.li.jingdong;

import java.util.Comparator;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.stream.Stream;

/**
 * @program: GradleTestUseSubModule
 * @author: Yafei Li
 * @create: 2018-09-10 10:37
 **/
public class Main5 {

    public static void salaryfrequeny(int num,int[] salaries) throws NoSuchFieldException, IllegalAccessException {
        Map<Integer, Integer> map = new LinkedHashMap<>();
        for (int i = 0; i < num; i++) {
            map.computeIfPresent(salaries[i],(k,v)->{
                return v+1;
            });
            map.putIfAbsent(salaries[i], 1);
        }
        Map<Integer, Integer> sortMap = sortByKey(map);
        System.out.println(sortMap);
    }

    //根据值对map进行排序
    public static <K, V extends Comparable<? super V>> Map<K, V> sortByValue(Map<K, V> map) {
        Map<K, V> result = new LinkedHashMap<>();
        Stream<Map.Entry<K, V>> st = map.entrySet().stream();
        st.sorted(Comparator.comparing(e -> e.getValue())).forEach(e -> result.put(e.getKey(), e.getValue()));
        return result;
    }
    //根据键对map排序
    public static <K extends Comparable,V> Map<K,V> sortByKey(Map<K,V> map) {
        Map<K, V> resultMap = new LinkedHashMap<>();
        Stream<Map.Entry<K, V>> stream = map.entrySet().stream();
        //(a,b)->b.getKey().compareTo(a.getKey())  是一个比较器
        stream.sorted((a,b)->b.getKey().compareTo(a.getKey())).forEach(e->{   //e就是挨个取出map中的Entry,
            System.out.println("key:"+e.getKey()+"  value:"+e.getValue());
            resultMap.put(e.getKey(), e.getValue());
        });
        return resultMap;
    }

    public static void main(String[] args) throws NoSuchFieldException, IllegalAccessException {

        int n=5;
        int[] salary = {1000, 2000, 1000, 3000, 2000};
        salaryfrequeny(n,salary);
    }
}

4：通过反射O(1)的时间获取LinkedHashMap的尾节点

            Map<Integer, Integer> map = new LinkedHashMap<>();
            Field tail = map.getClass().getDeclaredField("tail");
            tail.setAccessible(true);
            Map.Entry<Integer,Integer> entry=(Map.Entry<Integer, Integer>) tail.get(map);
            Integer key = entry.getKey();
            Integer value = entry.getValue();