Question

Which collection classes are implemented with an indexed structure (i.e., a simple array)?

Short answer

ArrayList and Vector are implemented with indexed structures.

Step-by-step solution

Understanding Indexed Structures

Indexed structures are typically collections that are based on arrays, where elements can be accessed directly via an index number. Arrays themselves are the simplest form of an indexed structure.

Identifying Java Collections with Indexed Structure

Collections such as ArrayList and Vector in Java are designed on top of arrays. These classes maintain elements in a contiguous memory area, allowing random access through an index, similar to a native Java array.

Differentiating from Non-Indexed Collections

While ArrayList and Vector are indexed, other collections like LinkedList, HashSet, and TreeSet are not indexed structures. LinkedList, for example, uses a doubly-linked list, and Set implementations generally do not maintain any order or index.

Conclusion

The classes that implement collections using indexed structures are ArrayList and Vector. They allow fast random access to elements via indices, unlike other collection classes.

Key concepts

Indexed Structures

Indexed structures are a key component in Java collections that allow efficient access to data elements. Think of them as organized storage units where each item is assigned a number or index. This system is highly advantageous because it allows for direct access to any element in the structure without going through each element sequentially.
One way to achieve indexed structure is using arrays, which are the foundational building blocks for other indexed collections like ArrayLists and Vectors. Using index numbers, you can read or update elements quickly, making array-based structures better suited for applications requiring numerous read and write operations without significant delays.
Collections designed with indexed structures efficiently handle tasks like searching, insertion, deletion, and updating, especially when you need to work with large sets of data in a predictable order.

ArrayList

An ArrayList in Java is a resizable array, which means you can easily add or remove elements dynamically without worrying about the underlying array size.
Unlike a fixed-size array, ArrayList grows as you add more elements, and it automatically manages the underlying array size. This adaptability makes ArrayList incredibly popular for scenarios where the number of elements is not known beforehand.
ArrayLists maintain elements in a contiguous block of memory, allowing quick access with an index, similar to an array. However, they also allow for more flexibility and easier manipulation of the contents. This balance of efficiency and flexibility is why ArrayLists are frequently used to store collections of data that require frequent updates or retrievals by position.

Vector

Vectors in Java were introduced earlier than ArrayLists and are part of the Java Collection Framework. Vectors also use a dynamically sized array, providing all the benefits of an indexed structure.
While Vectors and ArrayLists are similar, one notable difference is that Vector methods are synchronized, making them inherently thread-safe. This means Vectors are preferable in multi-threaded contexts where data consistency across threads is crucial.
Being synchronized, however, comes at the cost of performance. When thread safety is not a concern, ArrayList might be a more efficient choice due to less computational overhead. Vectors, like ArrayLists, allow random access to elements through index numbers, providing quick data retrieval.

Random Access

Random access refers to the ability to reach any element in a data structure directly by using its index. Indexed structures like arrays, ArrayLists, and Vectors support random access, enabling you to retrieve or modify an element without the need to traverse other elements first.
This efficient access pattern makes operations like reading or updating elements much faster compared to other structures like linked lists, where you must navigate through elements sequentially to reach your target.
Random access is one of the primary reasons for choosing indexed structures in performance-critical applications. Having the capability for quick access helps significantly in scenarios like implementing search functionalities, sorting algorithms, or when managing lists of data that require frequent updating.