HashMap in Java calculates hash based on the key of the inserted (key, value) pair and store its elements according to that. So essentially HashMap is an unordered collection but you may come across a scenario when you want to sort HashMap in Java.
Sorting on HashMap can be done on either key or value, in this post we’ll see ways to sort HashMap in Java based on its keys or on its values.
Sorting HashMap on keys
If you have to sort HashMap on keys in Java, simplest option is to convert your HashMap to a TreeMap.
TreeMap is sorted according to the natural ordering of its keys, or by a Comparator provided at map creation time, depending on which constructor is used. If you want to sort HashMap according to the natural ordering of its keys you can use the following constructor of the TreeMap class.
TreeMap(Map<? extends K,? extends V> m)- Constructs a new tree map containing the same mappings as the given map, ordered according to the natural ordering of its keys.
import java.util.HashMap;
import java.util.Map;
import java.util.TreeMap;
public class SortMap {
public static void main(String[] args) {
// Creating HashMap
Map<String, String> empMap = new HashMap<String, String>();
// Storing elements
empMap.put("E01", "Jack");
empMap.put("E02", "Amy");
empMap.put("E11", "Harvey");
empMap.put("E45", "Mike");
System.out.println("** Unsorted Map **");
for(Map.Entry<String, String> emp : empMap.entrySet()) {
System.out.println("Key- " + emp.getKey() +
" Value- " + emp.getValue());
}
TreeMap<String, String> sortedEmpMap = new TreeMap<>(empMap);
System.out.println("** Map sorted by keys **");
for(Map.Entry<String, String> emp : sortedEmpMap.entrySet()) {
System.out.println("Key- " + emp.getKey() +
" Value- " + emp.getValue());
}
}
}
Output
** Unsorted Map ** Key- E11 Value- Harvey Key- E02 Value- Amy Key- E01 Value- Jack Key- E45 Value- Mike ** Map sorted by keys ** Key- E01 Value- Jack Key- E02 Value- Amy Key- E11 Value- Harvey Key- E45 Value- Mike
Sorting HashMap on Values
For sorting Java HashMap on values you will have to convert HashMap to a Set and then sort that Set. For sorting a Set you have again two options convert that set to List or to a TreeSet.
If you just want the values of the Map in the sorted order then converting the values of the Map to a TreeSet is the simplest option.
public class SortMap {
public static void main(String[] args) {
// Creating HashMap
Map<String, String> empMap = new HashMap<String, String>();
// Storing elements
empMap.put("E01", "Jack");
empMap.put("E02", "Amy");
empMap.put("E11", "Harvey");
empMap.put("E45", "Mike");
System.out.println("** Unsorted Map **");
for(Map.Entry<String, String> emp : empMap.entrySet()) {
System.out.println("Key- " + emp.getKey() + " Value- " + emp.getValue());
}
// Creating a TreeSet
Set<String> empSet = new TreeSet<String>(empMap.values());
System.out.println("** Sorted Set **");
for(String emp : empSet) {
System.out.println(" Value- " + emp);
}
}
}
Output
** Unsorted Map ** Key- E11 Value- Harvey Key- E02 Value- Amy Key- E01 Value- Jack Key- E45 Value- Mike ** Sorted Set ** Value- Amy Value- Harvey Value- Jack Value- Mike
As you can see the values are sorted, only drawback is now you have a Set instead of a Map as you have only values stored now.
If you want a Map as end result of sorting a HashMap in Java then the process is a bit lengthy.
- Create a TreeSet specifying a Comparator for sorting.
- Convert the entry set of HashMap to a TreeSet by using addAll() method. At this step you have a sorted set containing the Map.entry elements.
- Create a LinkedHashMap to store the sorted values. LinkedHashMap is used here because it maintains insertion order.
- Iterate the TreeSet and put values in the created LinkedHashMap.
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;
public class SortMap {
public static void main(String[] args) {
// Creating HashMap
Map<String, String> empMap = new HashMap<String, String>();
// Storing elements
empMap.put("E01", "Jack");
empMap.put("E02", "Amy");
empMap.put("E11", "Harvey");
empMap.put("E45", "Mike");
System.out.println("** Unsorted Map **");
for(Map.Entry<String, String> emp : empMap.entrySet()) {
System.out.println("Key- " + emp.getKey() + " Value- " + emp.getValue());
}
// Creating a treeset with comparator
Set<Map.Entry<String, String>> empSet = new TreeSet<Map.Entry<String, String>>(new EmpComparator());
// Adding the entry set to a TreeSet
empSet.addAll(empMap.entrySet());
System.out.println("** Sorted Set **");
for(Map.Entry<String, String> emp : empSet) {
System.out.println("Key- " + emp.getKey() + " Value- " + emp.getValue());
}
// Using LinkedHashMap to maintain insertion order
Map<String, String> sortedEmpMap = new LinkedHashMap<>();
// Insert values in LinkedHashMap
for(Map.Entry<String, String> emp : empSet) {
sortedEmpMap.put(emp.getKey(), emp.getValue());
}
System.out.println("** Map Sorted by Values **");
for(Map.Entry<String, String> emp : sortedEmpMap.entrySet()) {
System.out.println("Key- " + emp.getKey() + " Value- " + emp.getValue());
}
}
}
// Comparator class
class EmpComparator implements Comparator<Map.Entry<String, String>>{
@Override
public int compare(Map.Entry<String, String> entry1, Map.Entry<String, String> entry2) {
return entry1.getValue().compareTo(entry2.getValue());
}
}
Output
** Unsorted Map ** Key- E11 Value- Harvey Key- E02 Value- Amy Key- E01 Value- Jack Key- E45 Value- Mike ** Sorted Set ** Key- E02 Value- Amy Key- E11 Value- Harvey Key- E01 Value- Jack Key- E45 Value- Mike ** Map Sorted by Values ** Key- E02 Value- Amy Key- E11 Value- Harvey Key- E01 Value- Jack Key- E45 Value- Mike
Rather than TreeSet you can also use an ArrayList to store Map.Entry elements. In that case you can use
Collections.sort(list, new EmpComparator()) to sort on values.
That's all for the topic How to Sort Java HashMap. If something is missing or you have something to share about the topic please write a comment.
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