java arraylist ensureCapacity not working

Question

Either I\'m doing this wrong or i\'m not understanding how this method works.

ArrayList a = new ArrayList();
a.ensureCapacity(200         


        

Answer 1

ensureCapacity just makes sure that the underlying array's capacity is greater than or equal to the argument. It doesn't change the size of the ArrayList. It does't make any changes visible through the API, so you won't notice a difference except that it will probably be longer before the ArrayList resizes it's internal array.

Answer 2

  public static void fillArrayList(ArrayList<String> arrayList, long size) {
    for (int i = 0; i < size + 1; i++) {
      arrayList.add(i,"-1");
    }
  }

public static void main(String[] args) throws Exception {
  ArrayList<String> a = new ArrayList<String>(10);
  fillArrayList(a, 190);
  a.add(190,"test");
  System.out.println(a.get(190).toString());
}

Answer 3

So as others have mentioned ensureCapacity isn't for that. It looks like you want to start out with an ArrayList of 200 nulls? Then this would be the simplest way to do it:

ArrayList<String> a = new ArrayList<String>(Arrays.asList( new String[200] ));

Then if you want to replace element 190 with "test" do:

a.set(190, "test");

This is different from

a.add(190, "test");

which will add "test" in index 190 and shift the other 9 elements up, resulting in a list of size 201.

If you know you are always going to have 200 elements it might be better to just use an array.

Answer 4

Once again JavaDoc to clarify the situation:

Throws: IndexOutOfBoundsException 
    - if index is out of range (index < 0 || index > size()).

Note that size() returns the number of elements currently held by the List.

Answer 5

Adding 190 null entries to an ArrayList reeks of a misuse of the data structure.

1. Think about using a standard primitive array.

2. If you require a generics or want more efficient use of space then consider SparseArray or even a Map like a HashMap may be appropriate for your purposes.

Answer 6

ArrayList maintains its capacity (the size of the internal array) separately from its size (the number of elements added), and the 'set' method depends on the index already having been assigned to an element. There isn't a way to set the size. If you need this, you can add dummy elements with a loop:

for (int i = 200; --i >= 0;) a.add(null);