前言

HashMap的儲存是沒有順序的,而是按照key的HashCode實現.
key=手機品牌,value=價格,這裡以這個例子實現按名稱排序和按價格排序.

        Map phone=new HashMap();
        phone.put("Apple",7299);
        phone.put("SAMSUNG",6000);
        phone.put("Meizu",2698);
        phone.put("Xiaomi",2400);
        System.out.println(phone);

直接輸出HashMap得到的是一個無序Map(不是Arraylist那種順序型儲存)

1.按key排序

對名稱進行排序,首先要得到HashMap中鍵的集合(keySet),並轉換為陣列,這樣才能用Arrays.sort()進行排序

        Set set=phone.keySet();
        Object[] arr=set.toArray();
        Arrays.sort(arr);
        for(Object key:arr){
            System.out.println(key);
        }

得到已經排好序的鍵,

最後利用HashMap.get(key)得到鍵對應的值即可

        for
 
(Object key:arr){
            System.out.println(key+": "+phone.get(key));
        }

2.按value排序

對價格進行排序,首先需要得到HashMap中的包含對映關係的檢視(entrySet),
如圖:

將entrySet轉換為List,然後重寫比較器比較即可.這裡可以使用List.sort(comparator),也可以使用Collections.sort(list,comparator)
轉換為list

 List<Map.Entry<String, Integer>
 
> list = new ArrayList<Map.Entry<String, Integer>>(phone.entrySet()); //轉換為list

使用list.sort()排序

       list.sort(new Comparator<Map.Entry<String, Integer>>() {
          @Override
          public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
              return o2.getValue().compareTo(o1.getValue());
          }
      });

使用Collections.sort()排序

        Collections.sort(list, new Comparator<Map.Entry<String, Integer>>() {
           @Override
           public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
               return o2.getValue().compareTo(o1.getValue());
           }
       });

兩種方式輸出結果

 //for迴圈
         for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i).getKey() + ": " + list.get(i).getValue());
        }      
 //for-each迴圈
           for (Map.Entry<String, Integer> mapping : list){
            System.out.println(mapping.getKey()+": "+mapping.getValue());
        }      

輸出結果

完整程式碼如下:

import java.util.*;

//Author:Hibiki last modified in 2018.10.04
public class HashMapSort {

    public static void main(String[] args) {
        Map phone = new HashMap();
        phone.put("Apple", 7299);
        phone.put("SAMSUNG", 6000);
        phone.put("Meizu", 2698);
        phone.put("Xiaomi", 2400);
        //key-sort
        Set set = phone.keySet();
        Object[] arr = set.toArray();
        Arrays.sort(arr);
        for (Object key : arr) {
            System.out.println(key + ": " + phone.get(key));
        }
        System.out.println();
        //value-sort
        List<Map.Entry<String, Integer>> list = new ArrayList<Map.Entry<String, Integer>>(phone.entrySet());
        //list.sort()
        list.sort(new Comparator<Map.Entry<String, Integer>>() {
            @Override
            public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                return o2.getValue().compareTo(o1.getValue());
            }
        });
        //collections.sort()
        Collections.sort(list, new Comparator<Map.Entry<String, Integer>>() {
            @Override
            public int compare(Map.Entry<String, Integer> o1, Map.Entry<String, Integer> o2) {
                return o2.getValue().compareTo(o1.getValue());
            }
        });
        //for
        for (int i = 0; i < list.size(); i++) {
            System.out.println(list.get(i).getKey() + ": " + list.get(i).getValue());
        }
        System.out.println();
        //for-each
        for (Map.Entry<String, Integer> mapping : list) {
            System.out.println(mapping.getKey() + ": " + mapping.getValue());
        }
    }
}

原始碼下載

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參考資料

1.JAVA中Hashmap按key排序
2.Java如何對HashMap按值進行排序
3.Java中Map的entrySet()詳解