Difference between ArrayList declarations

Question

I was reading online and I saw a declaration for an ArrayList as

ArrayList[] graph = new ArrayList[numCourses];

Traditionally, I thought that ArrayLists were always declared as

ArrayList<type> graph = new ArrayList<type>();

What's the difference between these two? The rest of the code seemed to utilize the same functions from the ArrayList class, but with specific indices.

The first one declares an array of raw `ArrayLists`: the second one declares a single `ArrayList`. — user207421, Jan 02 '20 at 05:23
@VishwaRatna He asked for the difference between the two declarations in the question. No thinking required. — user207421, Jan 02 '20 at 05:25

ernest_k · Accepted Answer · 2020-01-02T05:37:45.053

ArrayList[] declares an array of ArrayList objects. graph is then initialized with an array of length numCourses.

While ArrayList[] uses the raw type ArrayList, ArrayList<type> graph uses the generic type ArrayList<T> and passes type as the type argument.

These two are not interchangeable. An array cannot be used instead of its component type. ArrayList[] is probably being used to circumvent the limitation that prevents the instantiation of arrays of generic types.

score 1 · Answer 2 · edited Jun 20 '20 at 09:12

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From Oracle: Cannot Create Arrays of Parameterized Types

You cannot create arrays of parameterized types. For example, the following code does not compile:

List<Integer>[] arrayOfLists = new List<Integer>[2]; // compile-time error

So yeah you are right only correct way to declare List or collection in general is like below:

ArrayList<type> graph = new ArrayList<type>();

edited Jun 20 '20 at 09:12

Community

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answered Jan 02 '20 at 05:22

Sumit Singh

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score 1 · Answer 3 · answered Jan 02 '20 at 05:24

With brackets ([]) an array is created, whereas with "beaks" (<>) parameter type is passed.

Your second line creates an ArrayList of type type, whereas the first line creates an array of raw ArrayLists (with no type) - those are whole different objects.

If you want a collection of parametrized Lists, array is not the way to go. Better use nested lists: List<List<type>>.

Difference between ArrayList declarations

3 Answers3