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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");
		System.out.println("TreeMap Entries:");
		Map<String, String> rm = hm.descendingMap();
		System.out.println("Reverse Map Content: ");

class MyCopr implements Comparator<String>{

	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}
<< Previous Program 

List Of All TreeMap Sample Programs:

  1. Basic TreeMap Operations.
  2. How to iterate through TreeMap?
  3. How to copy Map content to another TreeMap?
  4. How to search a key in TreeMap?
  5. How to search a value in TreeMap?
  6. How to get all keys from TreeMap?
  7. How to get entry set from TreeMap?
  8. How to delete all elements from TreeMap?
  9. How to sort keys in TreeMap by using Comparator?
  10. How to sort keys in TreeMap by using Comparator with user define objects?
  11. How to get sorted sub-map from TreeMap?
  12. How to get first key element from TreeMap (Sorted Map)?
  13. How to get last key element from TreeMap (Sorted Map)?
  14. How to reverse sorted keys in a TreeMap?
Knowledge Centre
What is daemon thread?
Daemon thread is a low priority thread. It runs intermittently in the back ground, and takes care of the garbage collection operation for the java runtime system. By calling setDaemon() method is used to create a daemon thread.
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About Author

I'm Nataraja Gootooru, programmer by profession and passionate about technologies. All examples given here are as simple as possible to help beginners. The source code is compiled and tested in my dev environment.

If you come across any mistakes or bugs, please email me to [email protected].

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Reference: Java™ Platform Standard Ed. 7 - API Specification | Java™ Platform Standard Ed. 8 - API Specification | Java is registered trademark of Oracle.
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