Program: How to reverse sorted keys in a TreeMap?

Below example shows how to reverse keys from TreeMap based on sorting. To reverse keys, you have to sort the TreeMap based on user defined objects by using comparator object. You can include you own custom sorting logic with compare method. By passing comparator object to the TreeMap, you can sort the keys based on the logic provided inside the compare method. Once the TreeMap keys are in sorting order, you can call descendingMap() method to reverse the Map object.

package com.java2novice.treemap;

import java.util.Comparator;
import java.util.Map;
import java.util.TreeMap;

public class MyReverseOrderMap {

	public static void main(String a[]){
		//the treemap sorts by key
		TreeMap<String, String> hm = new TreeMap<String, String>(new MyCopr());
		//add key-value pair to TreeMap
		hm.put("java", "language");
		hm.put("computer", "machine");
		hm.put("india","country");
		hm.put("mango","fruit");
		hm.put("game","cricket");
		System.out.println("TreeMap Entries:");
		System.out.println(hm);
		Map<String, String> rm = hm.descendingMap();
		System.out.println("Reverse Map Content: ");
		System.out.println(rm);
	}
}

class MyCopr implements Comparator<String>{

	@Override
	public int compare(String str1, String str2) {
		return str1.compareTo(str2);
	}
	
}

TreeMap Entries:
{computer=machine, game=cricket, india=country, java=language, mango=fruit}
Reverse Map Content: 
{mango=fruit, java=language, india=country, game=cricket, computer=machine}