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

Type List vs Type ArrayList in Java: Understanding the Differences

In Java, both List and ArrayList are widely used for creating collections of objects. However, there are subtle but important distinctions between the two.

Understanding the Question

The question posed asks whether (1) and (2) are interchangeable.

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

The answer is that (1) is generally preferred over (2).

Answer to the Question

The reason (1) is preferred is due to the principle of coding to interfaces and encapsulation. List is an interface, while ArrayList is a specific implementation of that interface.

By declaring myList as List, you can assign any implementation of the List interface (e.g., LinkedList, Vector) to it. This flexibility allows you to choose the most appropriate implementation for your application at runtime.

In contrast, if you declare myList as ArrayList, you are specifically binding it to the ArrayList implementation. This limits your ability to change the implementation later.

Example Use Case

Consider the following example:

List<Integer> myList = new LinkedList<Integer>();

Initially, you may start with a LinkedList implementation of the List interface. However, suppose you encounter a performance bottleneck where a more efficient implementation, such as ArrayList, is better suited.

In this case, you can easily swap the LinkedList for an ArrayList without affecting other parts of the codebase. If you had used ArrayList instead of List, this change would require additional refactoring because your code would be tied specifically to ArrayList.

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