How to declare multiple arraylists in Java

Question

I want to declare multiple arraylists but i failed to do so. Is there a way to it? here's my failed code:

List<Integer>[] table = new ArrayList<Integer>[]();

It seems no matter how and where do i put '[]', it just doesn't work. any help or suggestion would be appreciated

Always specify the error you're receiving at the same time as the code that doesn't work. I assume this is failing at compile time? What's the error? (Note that that syntax is broken to start with as you're not specifying the size of the array, and you've got () as if you were calling a constructor, which you're not.) — Jon Skeet
– Jon Skeet, Commented Nov 11, 2015 at 6:43
@karimmohsen: No, that creates a single list. The OP wants multiple lists. — Jon Skeet
– Jon Skeet, Commented Nov 11, 2015 at 6:45
Basically, the answer is that arrays and generics don't mix well in Java. You should consider using a List<List<Integer>> instead. — Jon Skeet
– Jon Skeet, Commented Nov 11, 2015 at 6:46
@karimmohsen: No need for it to be declared as an ArrayList<ArrayList<Integer>> - I'd just go with List<List<Integer>>, personally. Then you can add ArrayList<Integer> references to it - or other implementations. — Jon Skeet
– Jon Skeet, Commented Nov 11, 2015 at 6:52

Andrei Amarfii · Accepted Answer · 2015-11-11 07:20:12Z

2

You have to specify the size like this:

ArrayList<Integer>[] table = new ArrayList[10];

Creates an array of ArrayList with capacity of 10.

answered Nov 11, 2015 at 7:20

Andrei Amarfii

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2 Comments

CaffYang Over a year ago

it looks good, but i received 'uses unchecked or unsafe operations.' when compiling.

Andrei Amarfii Over a year ago

This is it, your array can't be sure that Lists that it holds will include just Integers. Yes this is unsafe, and actually using of arrays is unsafe and it's considered bad practice.

Anatoly Deyneka · Accepted Answer · 2015-11-11 07:17:27Z

1

@SuppressWarnings("unchecked")
List<Integer>[] table = (ArrayList<Integer>[]) Array.newInstance(ArrayList.class, size);

answered Nov 11, 2015 at 7:17

Anatoly Deyneka

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JGCW · Accepted Answer · 2015-11-11 07:28:58Z

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List<Integer> multiple = new ArrayList<String>(), myArray = new ArrayList<String>();

hope it helps.

answered Nov 11, 2015 at 7:28

JGCW

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blm Over a year ago

It seems clear that the poster is trying to allocate an array of ArrayLists (despite the misleading title). This doesn't help with that.

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