ArrayList.TrimToSize（）和Array之间的区别？

Question

Generally, They say that we have moved from Array to ArrayList for the following Reason

Arrays are fixed size where as Array Lists are not .

One of the disadvantages of ArrayList is:

When it reaches it's capacity , ArrayList becomes 3/2 of it's actual size. As a result , Memory can go wasted if we donot utilize the space properly.In this scenario, Arrays are preferred.

If we use ArrayList.TrimSize(), will that make Array List a unanimous choice? Eliminating the only advantage(fixed size) Array has over it?

Answer 1

One short answer would be: trimToSize doesn't solve everything, because shrinking an array after it has grown - is not the same as preventing growth in the first place; the former has the cost of copying + garbage collection.

The longer answer would be: int[] is low level, ArrayList is high level which means it's more convenient but gives you less control over the details. Thus in a business-oriented code (eg manipulating a short list of "Products") i'll prefer ArrayList so that I can forget about the technicalities and focus on the business. In a mathematically-oriented code i'll probably go for int[].

There are additional subtle differences, but i'm not sure how relevant they are to you. Eg concurrency: if you change the data of ArrayList from several threads simultaneously, it will intentionally fail, because that's the intuitive requirement for most business code. An int[] will allow you to do whatever you want, leaving it up to you to make sure it makes sense. Again, this can all be summarized as "low level"...

Answer 2

If you are developing an extremely memory critical application, need resizability as well and performance can be traded off, then trimming array list is your best bet. This is the only time, array list with trimming will be unanimous choice.

In other situations, what you are actually doing is:

1. You have created an array list. Default capacity of the list is 10.
2. Added an element and applied trim operation. So both size and capacity is now 1. How trim size works? It basically creates a new array with actual size of the list and copies old array data to new array. Old array is left for grabage collection.
3. You again added a new element. Since list is full, it will be reallocated with more 50% spaces. Again, procedure similar to 2 will be followed.
4. Again you call TrimSize and it follows same procedure as 2.
5. Things repeats...

So you see, we are incurring lots of performance overhead just to keep list capacity and size same. Fixed size is not offering you anything advantageous here except saving few more extra spaces which is hardly an issue in modern machines.

In a nutshell, if you want resizability without writing lots of boilerplate code, then array list is unanimous choice. But if size never changes and you don't need any dynamic function such as removal operation, then array is better choice. Few extra bytes are hardly an issue.