Can map in java be sorted based on key?

Commonly used sorting methods for map in java: sort by key, sort by value. Recommendation: java video tutorial

1. Sort by key

TreeMap under jdk’s built-in java.util package can meet such needs. Its construction method TreeMap(Comparator comparator) can be sorted by key by passing in our custom comparator.

Default ascending sorting method:

import java.util.Map;
import java.util.Set;
import java.util.TreeMap;
 
public class TEST {
 
    public static void main(String[] args) {
        TEST t = new TEST();
        t.sort();
    }
     
    public void sort(){
        Map<String, String> treeMap = new TreeMap<String, String>();
        treeMap.put("c", "ccccc"); 
        treeMap.put("a", "aaaaa"); 
        treeMap.put("b", "bbbbb"); 
        treeMap.put("d", "ddddd"); 
        Set<String> s = treeMap.keySet();
        for (String key : s) { 
            System.out.println(key+" : "+treeMap.get(key)); 
        } 
    }
}

Output result:

a : aaaaa
b : bbbbb
c : ccccc
d : ddddd

2. Sorting by value

Sorting by value is relatively troublesome. It seems that there is no directly available data structure that can handle similar requirements, so we need to convert it ourselves.

Map itself is very meaningful to sort by value. In many situations, similar requirements will be encountered, and its value can be considered to be some defined rule or weight.

Principle: Place all the elements in the Map to be sorted into a list, and then use a static method of Collections sort(List list, Comparator c)

To sort the list, also use a comparator to define comparison rules. The elements in the sorted list are then loaded into the Map in sequence. In order to definitely ensure that the elements in the Map are in the same order as the elements in the sorted List, the LinkedHashMap data type is used.

Implementation code

public class MapSortDemo {

    public static void main(String[] args) {

        Map<String, String> map = new TreeMap<String, String>();

        map.put("KFC", "kfc");
        map.put("WNBA", "wnba");
        map.put("NBA", "nba");
        map.put("CBA", "cba");

        Map<String, String> resultMap = sortMapByKey(map);    //按Key进行排序
//        Map<String, String> resultMap = sortMapByValue(map); //按Value进行排序

        for (Map.Entry<String, String> entry : resultMap.entrySet()) {
            System.out.println(entry.getKey() + " " + entry.getValue());
        }
    }
    
    /**
     * 使用 Map按value进行排序
     * @param map
     * @return
     */
    public static Map<String, String> sortMapByValue(Map<String, String> oriMap) {
        if (oriMap == null || oriMap.isEmpty()) {
            return null;
        }
        Map<String, String> sortedMap = new LinkedHashMap<String, String>();
        List<Map.Entry<String, String>> entryList = new ArrayList<Map.Entry<String, String>>(
                oriMap.entrySet());
        Collections.sort(entryList, new MapValueComparator());

        Iterator<Map.Entry<String, String>> iter = entryList.iterator();
        Map.Entry<String, String> tmpEntry = null;
        while (iter.hasNext()) {
            tmpEntry = iter.next();
            sortedMap.put(tmpEntry.getKey(), tmpEntry.getValue());
        }
        return sortedMap;
    }
}

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