How to Randomize Two Related ArrayLists in Java?

Question

How can I synchronize the randomization of two related ArrayLists in Java?

String [] file = {"H1.txt","H2.txt","H3.txt","M4.txt","M5.txt","M6.txt"};
ArrayList<String> fileList = new ArrayList<String>(Arrays.asList(file));

String [] img = {"e1.jpg","e2.jpg","e3.jpg","e4.jpg","e5.jpg","e6.jpg"};
ArrayList<String> imgList = new ArrayList<String>(Arrays.asList(img));

// Randomizing files
Collections.shuffle(fileList);

Answer

To randomize two ArrayLists that are related to one another while preserving their association, you can create a method that combines both lists into a data structure, randomizes it, and then extracts the lists back. This ensures that the relationship between elements in both lists remains intact after the randomization process.

import java.util.ArrayList;
import java.util.Collections;
import java.util.List;
import java.util.AbstractMap;
import java.util.Map;
import java.util.Arrays;

public class RandomizeLists {
    public static void main(String[] args) {
        String [] file = {"H1.txt","H2.txt","H3.txt","M4.txt","M5.txt","M6.txt"};
        ArrayList<String> fileList = new ArrayList<>(Arrays.asList(file));

        String [] img = {"e1.jpg","e2.jpg","e3.jpg","e4.jpg","e5.jpg","e6.jpg"};
        ArrayList<String> imgList = new ArrayList<>(Arrays.asList(img));

        // Combine the lists into a List of Map.Entry
        List<Map.Entry<String, String>> combinedList = new ArrayList<>();
        for (int i = 0; i < fileList.size(); i++) {
            combinedList.add(new AbstractMap.SimpleEntry<>(fileList.get(i), imgList.get(i)));
        }

        // Shuffle the combined list
        Collections.shuffle(combinedList);

        // Extract back to separate lists
        for (int i = 0; i < combinedList.size(); i++) {
            fileList.set(i, combinedList.get(i).getKey());
            imgList.set(i, combinedList.get(i).getValue());
        }

        // Check results
        System.out.println("Randomized fileList: " + fileList);
        System.out.println("Corresponding imgList: " + imgList);
    }
}

Causes

The need to maintain the relationship between two lists when performing randomization.
Element correspondence between two lists based on their indices.

Solutions

Combine both lists into a single data structure such as a List of pairs (e.g., using a Map.Entry or a custom class).
Randomize the combined list using Collections.shuffle() method.
Separate the lists back into their original individual lists after shuffling.

Common Mistakes

Mistake: Not using a proper data structure to maintain relationships while randomizing.

Solution: Use a List of pairs (or a Map) to keep the related elements together during randomization.

Mistake: Shuffling one list without handling the other results in losing the correlation.

Solution: Always shuffle the combined list that contains elements of both ArrayLists together.

Helpers

Java randomize ArrayList
synchronize two ArrayLists Java
randomize related lists Java
Java Collections shuffle
maintain relationship between lists