How can you randomize two ArrayLists containing correlated elements while preserving their order?

Preserving Order While Randomizing Multiple ArrayLists Correlated in Content

Introduction

This question tackles a common scenario where two ArrayLists contain related elements, but randomizing one list should automatically randomize the other in the same order. To understand the solution, it's crucial to delve into the problem and explore the underlying concept of parallel randomization.

Understanding the Problem

Given two ArrayLists, fileList and imgList, representing related elements (e.g. text files and their corresponding images), the task is to randomize fileList, thereby automatically randomizing imgList in the same fashion. This ensures that related elements remain paired even after shuffling.

Solution

The following code showcases an effective solution using the Collections.shuffle() method with two Random objects initialized with the same seed:

<code class="java">long seed = System.nanoTime();
Collections.shuffle(fileList, new Random(seed));
Collections.shuffle(imgList, new Random(seed));</code>

Explanation

• Common Seed: Initializing both Random objects with the same seed ensures that the pseudorandom sequence generated by each object is identical. This ensures that both lists are shuffled in the exact same order.
• Separate Collections: By using two Random objects, the ArrayLists remain distinct and can be manipulated independently without affecting the other.
• Randomization Trigger: Once fileList is shuffled, the changes trigger a subsequent shuffle in imgList, preserving the synchronized order between the related elements.

Output

For the provided sample input, the output after randomization will be as follows:

• fileList = {"M4.txt", "M6.txt", "H3.txt", "M5.txt", "H2.txt", "H1.txt"}
• imgList = {"e4.jpg", "e6.jpg", "e3.jpg", "e5.jpg", "e2.jpg", "e1.jpg"}

Notice how the order of elements in imgList mirrors the randomized order of fileList, effectively pairing the related elements.

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