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Converting Integer-String HashMap to String-Array-Integer HashMap in Java

Last Updated : 29 Dec, 2023
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Efficient Conversion of a HashMap<Integer, String> into another HashMap<String, Integer[]> involves organizing integer keys based on their corresponding string values. Here’s a concise and efficient Java method to accomplish this transformation, focusing on clear and straightforward implementation.

Step-by-Step Implementation of Conversion

The method follows a step-by-step procedure using HashMaps and Lists.

  • First, it gathers integer keys together by looking at their matching string values in a temporary HashMap.
  • Then, it turns these grouped keys into arrays, creating a fresh HashMap where strings are linked to arrays of integers.


// Java program for converting a 
// HashMap<Integer, String> to HashMap<String, Integer[]>.
import java.util.*;
// Driver class
public class HashMapConversion {
    // Main Method
    public static void main(String[] args){
        // Create an input HashMap with integer keys and
        // string values
        HashMap<Integer, String> inputMap = new HashMap<>();
        inputMap.put(1, "a");
        inputMap.put(2, "b");
        inputMap.put(3, "a");
        inputMap.put(4, "k");
        inputMap.put(5, "fg");
        inputMap.put(7, "aa");
        inputMap.put(9, "b");
        inputMap.put(10, "vc");
        // Create an output HashMap to store strings as keys
        // and lists of associated integer keys
        HashMap<String, List<Integer>> outputMap= new HashMap<>();
        // Iterate through each entry in the input map
        for (Map.Entry<Integer, String> entry:inputMap.entrySet()) {
           // Get the integer key
            Integer key= entry.getKey();
           // Get the string value
            String value= entry.getValue();
            // Check if the output map already contains the
            // string value as a key
            if (outputMap.containsKey(value)) {
                // If it exists, add the current integer key
                // to its associated list
            else {
                // If it doesn't exist, create a new list
                // for the string value and add the current
                // integer key
                List<Integer> list = new ArrayList<>();
                outputMap.put(value, list);
        // Create a final HashMap to store strings as keys
        // and arrays of associated integer keys
        HashMap<String, Integer[]> finalMap= new HashMap<>();
        // Iterate through each entry in the output map
        // (which has string keys and list values)
        for (Map.Entry<String, List<Integer> > entry:outputMap.entrySet()) {
               // Get the string key
            String key = entry.getKey();
               // Get the list of integer values
            List<Integer> values= entry.getValue();
            // Convert the list of integer values to an
            // array and store it in the final map
                         values.toArray(new Integer[0]));
        // Print the final map contents
        for (Map.Entry<String, Integer[]> entry:finalMap.entrySet()) {
            System.out.print("<" + entry.getKey() + ",[");
              // Get the array of integer values
            Integer[] values = entry.getValue(); 
            // Print each integer of array
            for (int i = 0; i < values.length; i++) {
                if (i < values.length - 1) {



Explanation of the above code:

  • This program uses the HashMap where numbers are linked to words and rearranges it.
  • It groups numbers based on the words they’re linked to, creating arrays for each word with their corresponding numbers.
  • For example, the key “a” in the final map has the value [1, 3]. This means that in the input map, the String value “a” was associated with both the Integer keys 1 and 3.


This method provides an efficient way to convert a HashMap with integer keys and string values to a HashMap with string keys and integer arrays. It leverages temporary HashMaps and Lists for intermediate storage while prioritizing clear and straightforward implementation. The code effectively utilizes Java’s built-in data structures and iterators, making it concise and easy to understand.

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