HashMapSortingByKeys.java

package com.dojo.java8.tutorials.hashmap;

import java.util.Map;
import java.util.stream.Collectors;
import java.util.Comparator;
import java.util.HashMap;
import java.util.LinkedHashMap;

public class HashMapSortingByKeys {

	public static void main(String[] args) {
		
		// create the map of primitives
		HashMap<Integer, String> empIdVsName = new HashMap<>();
		empIdVsName.put(99, "Tom");
		empIdVsName.put(5, "Tom");
		empIdVsName.put(2, "Sam");
		empIdVsName.put(3, "Roger");
		empIdVsName.put(7, "Tim");
		//empIdVsName.put(null, "Harry");

		System.out.println("default output: " + empIdVsName);
		
		// asc sort by keys
		// note that you need to create a new linkedhashmap to get desired sorting order
		// sorted api- make use of Map.Entry.comparingByKey
		Map<Integer, String> empIdVsName_sorted = empIdVsName
				.entrySet()
				.stream()
				.sorted(Map.Entry.comparingByKey())
				.collect(Collectors.toMap
						(Map.Entry::getKey, 
						//keymapper
						Map.Entry::getValue,
						//valuemapper
						(oldValue, newValue) -> oldValue, 
						// merge function (optional, based on your data/needs)- 
						//logic that decided what needs to be in case there
						// are two same keys mappd with different values
						LinkedHashMap::new) 
						// map supplier (Map into which the results will be inserted)
						);

		System.out.println("sorted by keys in asc order: " + empIdVsName_sorted);

		// desc sort by keys
		empIdVsName_sorted = empIdVsName
				.entrySet()
				.stream()
				.sorted(Map.Entry.comparingByKey(Comparator.reverseOrder()))
				.collect(Collectors.toMap
								(Map.Entry::getKey, 
								//keymapper
								Map.Entry::getValue,
								//valuemapper
								(oldValue, newValue) -> oldValue, 
								// merge function (optional, based on your data/needs)- 
								//logic that decided what needs to be in case there
								// are two same keys mappd with different values
								LinkedHashMap::new) 
								// map supplier (Map into which the results will be inserted)
								);

		System.out.println("sorted by keys in dec order: " + empIdVsName_sorted);
	}
	
	//default output: {2=Sam, 99=Tom, 3=Roger, 5=Tom, 7=Tim}
	//sorted by keys in asc order: {2=Sam, 3=Roger, 5=Tom, 7=Tim, 99=Tom}
	//sorted by keys in dec order: {99=Tom, 7=Tim, 5=Tom, 3=Roger, 2=Sam}

}