why arrays are initialized to default values but not arraylist in java?

Question

The implementation of ArrayList uses Array under the hood. However, Arrays are intialized to default values (0 or null) but ArrayList are just empty. why is this?

       int[] arr = new int[10];
       String[] arr1 = new String[11];
       System.out.println(Arrays.toString(arr));
       System.out.println(Arrays.toString(arr1));
      List<Integer> list = new ArrayList<Integer>(10);
      System.out.println(list);

      [0, 0, 0, 0, 0, 0, 0, 0, 0, 0]
      [null, null, null, null, null, null, null, null, null, null, null]
      []

This means every time I use, ArrayList, I need to fill stuff in; I was trying the below part in my code and it was throwing NoSuchElementException and then I realized that it is not defaulted, where as Arrays do

if (list.get(i)==null){
         list.add(i,x);
  else:
        list.add(i,list.get(i)+x)

EDIT:

even List<Integer> list = new ArrayList<Integer>(10);
prints [] although I initialized the size;

Solution 1

When you create an array, you specify the size. This is required because the size of arrays can't be changed after they are created. Something must go in each element of the array, then, and the most obvious thing to put is 0 or null.

On the other hand, ArrayLists are designed to be able to be resized. So you shouldn't have to specify the size when you create them. If the starting size is more then zero, it would have to initialize all those elements, and it's easier not to. So the starting size is zero.

Solution 2

When constructing an array, the number is the actual size of the array (and you can't change it later). The number in the ArrayList constructor is the initial capacity (space reserved for elements) but the size is zero. The size of an ArrayList can change after it is constructed.

Solution 3

You could do it like this:

List<Integer> items = Arrays.asList(new Integer[10]);

And you would get a list with 10 null elements.

Similarly,

Integer[] numbers = new Integer[10];
Arrays.fill(numbers,0);
List<Integer> list = Arrays.asList(numbers);

And you get a list with 10 elements initialised to 0;

Solution 4

The reason of null and 0 is difference between primitive and Object types. Their default values are null and 0(only false for boolean) correspondingly. See The Java Tutorial

ArrayList is not physically empty after creating. It has initial capacity, but it shows his size 0 even you initialized it exactly.Because Collection size() has logical meaning, not phisical.

List<Integer> list = new ArrayList<Integer>(10);

Also you can fill new Collection (List) with the following static method

Collections.fill(list, 0) // int 0 is auto-boxed to `Integers` 0 

Solution 5

As an ArrayList doesn't initially contain any elements, it can't default the value of those elements to null or any other value.

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