List vs. ArrayList in Java: When Should I Use Which?

Type List vs Type ArrayList in Java [duplicate]

In Java programming, when working with collections, you may come across two similar yet distinct type declarations:

(1) List<?> myList = new ArrayList<?>();
(2) ArrayList<?> myList = new ArrayList<?>();

Why Use Type List?

While both approaches utilize the wildcard character ?, the preferred choice is to declare myList as a List, rather than ArrayList. This is because a List reference allows for a higher level of flexibility and extensibility.

Advantages of Type List:

• Code reusability: It enables swapping different implementations of the List interface, such as LinkedList or Vector, without modifying the rest of the code.
• Future-proofing: If a newer, more efficient implementation of List becomes available in the future, you can seamlessly switch to it without breaking existing code.
• Consistency: It follows the principle of "programming to interfaces" and promotes a more cohesive software architecture.

When to Use Type ArrayList?

While List is generally preferable, there are certain scenarios where you may consider using ArrayList instead:

• Strong assurance of implementation: If you are absolutely certain that an ArrayList will always be used and never need to be changed, you could opt for this declaration. However, this approach limits flexibility and makes any potential future code modifications more difficult.

Examples

Consider the following example:

public class Example {

    public static void main(String[] args) {
        // Initialize using type List
        List<?> list = new ArrayList<>();

        // Add elements to the list
        list.add("One");
        list.add("Two");

        // Iterate over the list (can only access elements using Object type)
        for (Object element : list) {
            System.out.println(element);
        }

        // Downcast to ArrayList (not recommended)
        ArrayList<?> arrayList = (ArrayList<?>) list;

        // Access ArrayList-specific methods
        System.out.println("ArrayList size: " + arrayList.size());
    }
}

In this example, using List allows us to add and iterate over elements, but we cannot access ArrayList-specific methods. To do so, we would need to explicitly cast list to an ArrayList, which is generally not recommended as it introduces potential casting errors.

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