CODEWITHSUNDEEP
javaarrayslist
Dhivya Dandapani
Dhivya Dandapani

Reputation: 431

How do Backed Collections work in Java?

In Java,I have the following array 'integerArray' and add 2 integers to it.

Integer[] integerArray = new Integer[3];
integerArray[0] = 1;
integerArray[1] = 2;

Now I create a list out of the Array.

List<Integer> integerList = Arrays.asList(integerArray);

At this point,the 'integerList' contains 1 and 2.

Now I add another element to the array.

integerArray[2] = 3;

At this point,if we examine the integerList,we see that it contains 1,2,3;

What mechanism is used so that any changes to the Array are reflected in the List as well? A simple implementation or example will really help.

Upvotes: 3

Views: 519

Answers (3)

aarti
aarti

Reputation: 2865

Arrays.asList is a static/utility method that invokes the constructor to create a new ArrayList. The ArrayList is backed by an Array. This is the case in a lot of programming languages, where advance data structures are simply backed by primitive arrays. You can look at the source code here Arrays.asList

Upvotes: 2

M A
M A

Reputation: 72844

Returns a fixed-size list backed by the specified array

This means that the returned list object, which is actually an ArrayList, has a reference to the array, as opposed to a copy. Since it has a reference to this array, any changes to it will be reflected in the list.

The method Arrays.asList just calls the constructor of ArrayList defined as follows:

ArrayList (E[] array) {
    if (array == null)
        throw new NullPointerException();
    a = array;
}

So the field a will store the reference to your initial array.

Upvotes: 4

StackFlowed
StackFlowed

Reputation: 6816

This is because it is the same variable but used as a list. If you read the Javadoc for Arrays.asList()

It states clearly

Returns a fixed-size list backed by the specified array. (Changes to the returned list "write through" to the array.) This method acts as bridge between array-based and collection-based APIs, in combination with Collection.toArray(). The returned list is serializable and implements RandomAccess.

Upvotes: 4

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