Is an arrayList in Java an array or a list?

arraylist, arrays, java, list

Solution

You ask:

is an arrayList an array or a list, or both?

It is a List through inheritance, and it is composed with an array.

It implements the `java.util.List<T>` interface and thus passes the "is-a" test for this. This means that anything you can do with a List, including calling all of List public methods, you can do with an ArrayList.

It's data model however is backed by an array, and so it contains an array by composition. So this means that while it has some array-like behaviors obtained from the underlying array, you can't call array methods or obtain array fields on it directly. For instance, you cannot obtain items from the ArrayList using array indices. i.e., this won't work: `myArrayList[i]`, but you can call this indirectly via ArrayList's `get(int i)` method. And also you can't get a `length` field from it, but you can get it indirectly from its `size()` method. Also, note that obtaining data from an ArrayList will follow the same limiting big O behavior as that of an array.

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