java Arraylist 移除元素

Question

Suppose there are 10 elements in ArrayList and if i have deleted 2 elements from the middle, so the arraylist will contain 8 elements, but will the capacity be 10 or reduced to 8 at that time.

Answer 1

The API states:

Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost .

and you can always test this empirically in your debugger. After removing two elements, look at the array that backs the ArrayList , and see what it's size is. Most likely, it's 10.

Answer 2

The behaviour depends on the implementation of the ArrayList. You can't rely on this kind of implementation details. The only thing you must consider is that the capacity is always greater or equal to the number of elements in your list. If you need the capacity to be exactly the number of elements (because you may want to reduce memory usage), you will have to ask it explicitly with the function trimToSize() .

Answer 3

I think it may change because suppose we create new array list by default JVM allocate say 10 continuous locations in memory.if you put 8 or 2 or zero it will be same.

But is you put 15 element then it will increase it to say 20 memory locations.But if you below 10 again it should release that memory and reduce to 10 default size.

This is dynamic allocation.For 1000 element list if you are removing the element and reduce to 2 then it is logical that it must release that memory.It should depend on number of elements removed.