What’s an ArrayList?

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An ArrayList in computer programming is a data structure that behaves like a computer array but also implements the ability to dynamically grow the size of the array as needed. Unlike an intrinsic array data type, which cannot be resized during program execution, the ArrayList structure can grow and shrink the size of the array in response to adding or deleting elements. It has a very favorable performance profile, enabling fast random access to data collection. There are two cases however where it is slower than some other data structures, namely adding and removing elements from the middle of the array. Most object-oriented programming languages ​​have some kind of implementation of such lists, although they are sometimes called dynamic arrays.

Using an ArrayList gives a program the ability to instantly access data objects with an index number instead of having to traverse an entire sequence of data to find an address, which is necessary with linked lists. With the ability to grow the array size as needed, it’s a very balanced approach that considers both flexibility and speed. Also, when items are removed from that list, the size of the array is reduced, freeing up memory space.

One advantage of using an ArrayList over other data structures is that you don’t need a wrapper object to hold the stored data. In the case of a linked list or hashtable, a separate object is usually needed to hold the technique used to contain and manipulate the collection. With an ArrayList, the only information needed about the data objects is the object’s address in memory. This means that there will be less memory usage when working with this type of list.

A potential problem with using an ArrayList can come from the implementation and memory management system. Most arrays are allocated as consecutive memory locations. So, to use an ArrayList of a certain size, at least that amount of memory must be available in an uninterrupted sequence of blocks. The dynamic array might resize itself many times, so memory fragmentation can occur and lead to memory allocation failure, halting program execution.

The performance of an ArrayList is similar to using a standard array, although access times are slightly slower because the array is encapsulated in an object. One case where a dynamic array can slow down significantly, depending on your implementation, is when you need to change the size of the array. This can result in the current array being copied to a new array that has been allocated to the new desired size, causing a temporary performance degradation. The same problem can occur when adding or removing an item from the middle of the list, causing all subsequent items to need to be moved to a new location.

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