How to build an array of ArrayList in Java?

Question

I wrote a piece of code like this

ArrayList<Integer>[]list=new ArrayList<Integer>[128];

But Eclipse says

Cannot create a generic array of ArrayList

I also tried code like this

ArrayList<Integer>[]list=(ArrayList<Integer>[])new Object[128];

But Eclipse throws exception:

[Ljava.lang.Object; cannot be cast to [Ljava.util.ArrayList;

So how can I build an array of ArrayList< Integer > ?

Thanks a lot!!

score 1 · Answer 1 · answered Mar 05 '17 at 16:11

1

List<Integer> inp = new ArrayList<Integer>(10) to create list of integers whose size is 10.

answered Mar 05 '17 at 16:11

Pramod V

84
3

1

No, its size is 0. Its *capacity* is 10. – Andy Turner Mar 05 '17 at 16:25
yes, my bad. capacity is 10 and size is 0. thanks – Pramod V Mar 12 '18 at 10:22

Nulano · Answer 2 · 2017-03-05T16:47:51.223

0

Use this to create an ArrayList (remember, ArrayLists always have a theoretically indefinite capacity, since you can always add more elements to them - see a tutorial):

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

or this to make an array of ArrayLists:

ArrayList<Integer>[] lists = new ArrayList[128];

You will of course have to initialize your ArrayLists:

for (int i = 0; i < lists.length; i++)
    lists[i] = new ArrayList<>();

Alternatively, you can create an ArrayList of ArrayLists:

ArrayList<ArrayList<Integer>> lists2 = new ArrayList<>();
for (int i = 0; i < 128; i++)
     lists2.add(new ArrayList<>());

edited Mar 05 '17 at 16:47

answered Mar 05 '17 at 16:07

Nulano

1,148
13
27

No, its size is 0. Its *capacity* is 128. – Andy Turner Mar 05 '17 at 16:26
@AndyTurner Ah, yes. I got it mixed up with the behavior of vectors from C++. What do you think of my edit? – Nulano Mar 05 '17 at 16:29
I simply wouldn't have mentioned capacity, especially as you don't say why it is probably irrelevant. – Andy Turner Mar 05 '17 at 16:44
Thank you very much! Your method works!! @Nulano – Julian20151006 Mar 06 '17 at 02:28

score 0 · Answer 3 · answered Mar 05 '17 at 16:28

From what I see you are trying to create an ArrayList and an Array in the same step, which is impossible.

An Arraylist differs from arrays as it is a generic class, which means it has a lot more functionality.

For example:

In your code you are trying to specify a limit to the ArrayList, ArrayLists don't have a limit, they are expandable.

You can use the .add() function to add objects to ArrayLists, and get values using the .get(int index) function.

Example code:

ArrayList<Integer> myArray = new ArrayList<Integer>(); //initialized a new arrayList
myArray.add(7); //added element 7 at index 0
myArray.add(12); // added element 12 at index 1
print(myArray.get(1)) //output 12

You can check the documentations for the ArrayList class here.

Hope that helped.

score 0 · Answer 4 · answered Mar 05 '17 at 16:32

0

Your question isn't really clear, but to build an array list,this code should be sufficient

   ArrayList<Integer> list;
   list = new ArrayList<Integer>(128);

answered Mar 05 '17 at 16:32

Kennedy

547
4
23

How to build an array of ArrayList in Java?

4 Answers4