How to Avoid NullPointerException When Accessing Elements from Arrays in Java

Описание: Discover solutions to avoid `NullPointerException` when using `Arrays.asList()` and learn how to correctly populate and access objects in Java lists.
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This video is based on the question https://stackoverflow.com/q/65557157/ asked by the user 'Artium10' ( https://stackoverflow.com/u/7032375/ ) and on the answer https://stackoverflow.com/a/65557532/ provided by the user 'Basil Bourque' ( https://stackoverflow.com/u/642706/ ) at 'Stack Overflow' website. Thanks to these great users and Stackexchange community for their contributions.

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Understanding and Preventing NullPointerException in Java Lists

When programming in Java, encountering a NullPointerException (NPE) can be frustrating and puzzling. One common scenario where this occurs is when accessing elements from a list populated using Arrays.asList(). In this guide, we’ll explore a practical example involving game cells and learn how to properly initialize and access elements in a list to avoid NPE.

The Problem: Accessing Elements from an ArrayList

Consider the following snippet where a Game class attempts to access the 10th element of a list of Cell objects:

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In the getSelectedValue() method, while trying to access the value property of the 10th Cell, an NPE is thrown. Why does this happen? Let’s break it down.

Explanation of the NPE

Understanding Initialization: In the constructor of the Game class, an array of Cell objects is created but not initialized with any actual Cell objects. Effectively, all 80 references in the array point to null:

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Attempted Access of null Reference: When you call cells.get(10), it returns null since there’s no valid Cell object at that index. Attempting to access value from a null reference leads to a NullPointerException, as you are trying to access a field of a nonexistent object.

The Solution: Properly Initializing List Elements

To resolve this issue, you need to create and add actual Cell objects to your ArrayList. Here’s how you can do it:

Step-by-Step Solution

Define the Cell Class: Ensure you have a properly defined Cell class, making any necessary adjustments to field names for clarity:

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Initialize and Populate the List: Instead of using an array of null, we can create Cell objects and add them to the cells list:

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Verify Object Reference before Accessing

Before working with the properties of your Cell, it's good practice to check whether you have a valid reference:

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Conclusion

By following these steps to properly initialize your list and check for null, you can effectively avoid NullPointerException in your Java applications when accessing list elements. Always ensure that whenever you retrieve elements from a list, they are properly instantiated to prevent runtime errors.

Additionally, consider using more descriptive field names instead of value to enhance code clarity. A small change can significantly improve code readability and maintainability.

Happy coding, and may your code always be free of exceptions!