ArrayList 容量限制限制

Question

I have searched for ArrayList capacity questions but could not find a complete answer. so asking it again here.

I understand that size the number of elements we add in ArrayList and capacity is how much data we can put in that list with the default value to be 10.

So the question here is while declaring if give the capacity to be like this

List<String> list = new ArrayList<>(1);

Then also I can keep on adding the elements up to 10 or 20. So is this capacity declaration useful only to internal reallocation which happens when it reaches the capacity?

Or by giving the capacity limit can we restrict only to that point of add elements?

Answer 1

The initial capacity doesn't determine how many elements you can add to the ArrayList . The capacity is automatically increased when necessary as elements are added.

The motivation to specify an initial capacity is performance. If you know that your ArrayList will contain a million elements, it's more efficient to create the ArrayList with an initial capacity of 1000000, since that will save the need to resize the ArrayList capacity multiple times as elements are added.

Answer 2

ArrayList are backed by an array which max size is Integer.MAX_VALUE - 8.

This is referred in these answers:

• why-the-maximum-array-size-of-arraylist-is-integer-max-value-8
• Max array size

Answer 3

ArrayLists and other collections will grow as their capacity is used up. However how they do this depends on what data-structures are used 'behind the scenes'

An array list is backed by a Java Array. When that array is close to being filled a new Array is created, normally with twice the capacity, then all the elements are copied from the old to the new array, and the ArrayList then points at the new array.

Since the arrays double each time, adding an element to an array is still a constant time operation or "O(1)" in bigO notation. It is done in "amortized time"

However if you know your ArrayList is likely to contain n elements, you can declare it with an initial size of n . This makes the initial Array big enough to hold the elements you expect it to without having to do any "resizing". (Creating bigger arrays and copying elements over).

However it's not an issue if you underestimate n as the ArrayList will still grow to handle the extra elements.