How to Add the Same Key to an ArrayList of Maps String, String in Java

Learn how to manage keys in an ArrayList of Maps in Java without overwriting previous entries when fetching data from a database.
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Setting Up Your ArrayList of Maps<String, String> in Java

When managing collections of data in Java, particularly when using an ArrayList of Map<String, String>, you may encounter a scenario where you need to add multiple maps with the same keys. In this guide, we'll explore how to correctly add these maps without overwriting the previous entries, ensuring each map retains unique data.

The Problem

You want to create an ArrayList that contains multiple maps with identical keys. For instance, consider you have a structure like this after fetching data from your database:

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Your goal is to populate this list with maps that look like:

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However, it seems you end up with repeated entries like:

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The Solution

Understanding the Issue

The fundamental issue lies in how you're initializing your Map within the loop. Currently, your code is creating only one Map object before the loop:

[[See Video to Reveal this Text or Code Snippet]]

This map is created only once, and each entry in your ArrayList references this single Map. As you modify the Map during each iteration of the loop, you overwrite the same object, leading to multiple identical entries.

Correcting the Code

To avoid overwriting the previous entries, you'll need to create a new Map object inside the loop. Here’s how you can do it:

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Step-by-Step Breakdown

Initialize the ArrayList: Start with your ArrayList that will hold the maps.

Loop through your data: Use a loop to traverse through your fetched data.

Create a new Map for each entry: Inside the loop, initialize a new HashMap object for every entry.

Add key-value pairs: Use put() to add the "firstName" and "lastName" from the current result set to the newly created Map.

Add the map to the list: Finally, push the newly created map onto your ArrayList.

Conclusion

By creating a new Map for each iteration of your loop, you ensure that each entry in your ArrayList is unique. This change will allow you to effectively manage your data, maintaining distinct mappings for each individual entry instead of overwriting them.

With this structure in place, your UI can easily access individual first names and last names by calling:

[[See Video to Reveal this Text or Code Snippet]]

By following these steps, you’ll be able to create an effective and organized list of maps in Java without the worry of data overwriting.