How to Call Methods of Class from Inside Extended ArrayList in Java

Learn how to effectively call methods from classes using custom `ArrayList` in Java without extending it unnecessarily.
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Handling Method Calls in Extended ArrayLists in Java

When working with Java, you might face situations where you need to call methods from objects of classes you’ve stored in a collection, such as an ArrayList. This post addresses a common issue developers encounter: how to call methods from classes in a custom ArrayList without running into type errors.

The Problem

Imagine you have two classes, A and B, both with a common method getId(). You want to create a custom ArrayList (let's call it ArrayListId) that can handle objects of either type. However, when you attempt to call getId() on an object retrieved from your ArrayList, you receive a type error because the method is not recognized at the Object level.

Your ArrayListId looks something like this:

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The Solution

Understanding Type Safety

Instead of extending ArrayList and running into this issue, a more straightforward approach would be to leverage interfaces. Here’s how you can achieve your goal without creating unnecessary complexities:

Define a Common Interface

First, create an interface that both A and B will implement:

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Then, implement this interface in both classes:

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Use a Typed ArrayList

Instead of using ArrayListId, directly use an ArrayList defined with the common interface as its type:

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Update the doSomething Method

Modify your method to accept a list of CommonInterface objects:

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When to Extend ArrayList

If you still feel the need to create a subclass of ArrayList, ensure that your generic parameter extends CommonInterface:

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This modification guarantees that any object you retrieve from this extended ArrayList will be of type CommonInterface, allowing you to safely call getId().

Conclusion

In conclusion, while you might think extending ArrayList is the solution when working with varied types, it’s often best to rely on Java’s interfaces and generics. By implementing a common interface, you can achieve type safety and clarity without the complexity of subclassing. Embracing these principles will not only prevent errors but also lead to cleaner, more maintainable code.