Java中Arrays.sort()和Collections.sort()

時間  2019-12-09
標籤 java arrays.sort arrays sort collections.sort collections 欄目 Java 简体版
原文   原文鏈接

1.簡單示例

sort方法的使用很是的簡單明瞭，下面的例子中，先定義一個比較Dog大小的Comparator，而後將其實例對象做爲參數傳給sort方法，經過此示例，你應該可以快速掌握Arrays.sort()的使用方法。java

import java.util.Arrays;
import java.util.Comparator;
class Dog{
    int size;
    public Dog(int s){
        size = s;  
    }
}
  
class DogSizeComparator implements Comparator<Dog>{
    @Override
    public int compare(Dog o1, Dog o2) {
        return o1.size - o2.size;  
    }
}

public class ArraySort {
    public static void main(String[] args) {
        Dog d1 = new Dog(2);
        Dog d2 = new Dog(1);
        Dog d3 = new Dog(3);
        Dog[] dogArray = {d1, d2, d3};
        printDogs(dogArray);
        
        //或者採用匿名內部類
        /**
        Arrays.sort(dogArray, new Comparator<Dog>(){
            @Override
            public int compare(Dog o1, Dog o2) {
                return o1.size - o2.size;  
            }
        });
        */
        Arrays.sort(dogArray, new DogSizeComparator());
        printDogs(dogArray);
    }

    public static void printDogs(Dog[] dogs)
        for(Dog d: dogs) System.out.print(d.size + " " );  
        System.out.println();
    }
}

2.使用策略模式

這是策略模式(Strategy pattern)的一個完美又簡潔的示例，值得一提的是爲何這種場景下適合使用策略模式。
整體來講，策略模式容許在程序執行時選擇不一樣的算法。好比在排序時，傳入不一樣的比較器(Comparator)，就採用不一樣的算法。
根據上面的例子，假設你想要根據Dog的重量來進行排序，能夠像下面這樣，建立一個新的比較器來進行排序：算法

class Dog{
    int size;
    int weight;

    public Dog(int s,int w){
        size = s;
        weight = w;
    }
}

class DogSizeComparator implements Comparator<Dog>{

    @Override
    public int compare(Dogo1,Dogo2){
        return o1.size - o2.size;
    }
}

class DogWeightComparator implements Comparator<Dog>{

    @Override
    public int compare(Dogo1,Dogo2){
        return o1.weight - o2.weight;
    }
}

public class ArraySort{

    public static void main(String[] args){
        Dog d1 = new Dog(2,50);
        Dog d2 = new Dog(1,30);
        Dog d3 = new Dog(3,40);
    
        Dog[] dogArray = {d1,d2,d3};
        printDogs(dogArray);
    
        Arrays.sort(dogArray, new DogSizeComparator());
        printDogs(dogArray);
    
        Arrays.sort(dogArray,new DogWeightComparator());
        printDogs(dogArray);
    }

    public static void printDogs(Dog[]dogs){
        for(Dog d : dogs) System.out.print("size=" + d.size + "weight=" + d.weight + "");
        System.out.println();
    }
}

Comparator是一個接口，因此sort方法中能夠傳入任意實現了此接口的類的實例，這就是策略模式的主要思想。ide

3.爲什麼使用"super"

若是使用「Comparator < T > c」那是很簡單易懂的，可是sort的第2個參數裏面的< ? super T >意味着比較器所接受的類型能夠是T或者它的超類，爲何是超類呢？ 答案是：這容許使用同一個比較器對不一樣的子類對象進行比較。在下面的示例中很明顯地演示了這一點：this

import java.util.Arrays;
import java.util.Comparator;
 
class Animal{
    int size;
}

class Dog extends Animal{
    public Dog(int s){
        size = s;
    }
}
 
class Cat extends Animal{
    public Cat(int s){
        size= s;
    }
}
 
class AnimalSizeComparator implements Comparator<Animal>{
    @Override
    public int compare(Animal o1, Animal o2) {
        return o1.size - o2.size;
    }
    //in this way, all sub classes of Animal can use this comparator.
}
 
public class ArraySort {
 
    public static void main(String[] args) {
        Dog d1 = new Dog(2);
        Dog d2 = new Dog(1);
        Dog d3 = new Dog(3);
        Dog[] dogArray = {d1, d2, d3};
        printDogs(dogArray);
         
        Arrays.sort(dogArray, new AnimalSizeComparator());
        printDogs(dogArray);
        
        System.out.println();
         
        //when you have an array of Cat, same Comparator can be used. 
        Cat c1 = new Cat(2);
        Cat c2 = new Cat(1);
        Cat c3 = new Cat(3);
         
        Cat[] catArray = {c1, c2, c3};
        printDogs(catArray);
        
        Arrays.sort(catArray, new AnimalSizeComparator());
        printDogs(catArray);
    }
 
    public static void printDogs(Animal[] animals){
        for(Animal a: animals) System.out.print("size="+a.size + " ");

        System.out.println();
    }
}

4.Collections sort(List< T >,Comparator<? super T>) 用法

public class Student implements Comparable<Student> {

    String name;
    int age;

    public Student(String name, int age) {
       this.name = name;
       this.age = age;
    }

    @Override
    public String toString() {
        return name + ":" + age;
    }

    @Override
    public int compareTo(Student o) {
        return Comparators.NAME.compare(this, o);
    }


    public static class Comparators {

        public static Comparator<Student> NAME = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.name.compareTo(o2.name);
            }
        };
        public static Comparator<Student> AGE = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                return o1.age - o2.age;
            }
        };
        public static Comparator<Student> NAMEANDAGE = new Comparator<Student>() {
            @Override
            public int compare(Student o1, Student o2) {
                int i = o1.name.compareTo(o2.name);
                if (i == 0) {
                    i = o1.age - o2.age;
                }
                return i;
            }
        };
    }
}
//用法
List<Student> studentList = new LinkedList<>();
Collections.sort(studentList, Student.Comparators.AGE);

5.Sorting HashMap by values（HashMap值排序）

package leetcode;

/**
 * Created by slyuan on 17-4-18.
 */
import java.util.Collections;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.LinkedList;
import java.util.List;
import java.util.Map;
import java.util.Map.Entry;

public class SortMapByValue {
    public static boolean ASC = true;
    public static boolean DESC = false;

    public static void main(String[] args) {

        // Creating dummy unsorted map
        Map<String, Integer> unsortMap = new HashMap<String, Integer>();
        unsortMap.put("B", 55);
        unsortMap.put("A", 80);
        unsortMap.put("D", 20);
        unsortMap.put("C", 70);

        System.out.println("Before sorting......");
        printMap(unsortMap);

        System.out.println("After sorting ascending order......");
        Map<String, Integer> sortedMapAsc = sortByComparator(unsortMap, ASC);
        printMap(sortedMapAsc);


        System.out.println("After sorting descindeng order......");
        Map<String, Integer> sortedMapDesc = sortByComparator(unsortMap, DESC);
        printMap(sortedMapDesc);

    }

    private static Map<String, Integer> sortByComparator(Map<String, Integer> unsortMap, final boolean order) {

        List<Entry<String, Integer>> list = new LinkedList<Entry<String, Integer>>(unsortMap.entrySet());

        // Sorting the list based on values
        Collections.sort(list, new Comparator<Entry<String, Integer>>() {
            public int compare(Entry<String, Integer> o1, Entry<String, Integer> o2) {
                if (order) {
                    return o1.getValue().compareTo(o2.getValue());
                } else {
                    return o2.getValue().compareTo(o1.getValue());
                }
            }
        });

        // Maintaining insertion order with the help of LinkedList
        Map<String, Integer> sortedMap = new LinkedHashMap<String, Integer>();
        for (Entry<String, Integer> entry : list) {
            sortedMap.put(entry.getKey(), entry.getValue());
        }

        return sortedMap;
    }

    public static void printMap(Map<String, Integer> map) {
        for (Entry<String, Integer> entry : map.entrySet()) {
            System.out.println("Key : " + entry.getKey() + " Value : "+ entry.getValue());
        }
    }
}
//另外一種實現方式
    public static <K, V extends Comparable<? super V>> Map<K, V> sortByValue( Map<K, V> map ) {
        List<Map.Entry<K, V>> list = new LinkedList<Map.Entry<K, V>>( map.entrySet() );
        Collections.sort( list, new Comparator<Map.Entry<K, V>>() {
            public int compare( Map.Entry<K, V> o1, Map.Entry<K, V> o2 ) {
                return (o1.getValue()).compareTo( o2.getValue() );
            }
        } );

        Map<K, V> result = new LinkedHashMap<K, V>();
        for (Map.Entry<K, V> entry : list) {
            result.put( entry.getKey(), entry.getValue() );
        }
        return result;
    }

5.Sorting HashMap by key

Using the TreeMap you can sort the Map， 默認狀況下，TreeMap對key進行升序排序code

Map<String, String> unsortMap = new HashMap<String, String>();        
Map<String, String> treeMap = new TreeMap<String, String>(unsortMap);
for (String str : treeMap.keySet()) {
    System.out.println(str);
}

注意：正常狀況下Map是不可使用Collections.sort()方法進行排序的，不過能夠將Map轉換成list以後再進行排序。對象

List<Entry<String,Integer>> list = new LinkedList<>(unsortMap.entrySet());
Collections.sort(list, new Comparator<Entry<String, Integer>>() {
    @Override
    public int compare(Entry<String, Integer> e1, Entry<String, Integer> e2) {
        return e1.getKey().compareTo(e2.getKey());
    }
});
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