Declare an Array of ArrayLists of certain object

Question

i need to make an array of arraylists of an object.

i have tried two ways , but i can't access object's properties from the array elements.

i've tried :

     ArrayList<Word>[] SubList = (ArrayList<Word>[])new ArrayList(MainList.size());

AND :

    ArrayList<Word>[] SubList= new ArrayList[MainList.size()];

AND:

           ArrayList<Word>[] SubList= (ArrayList<Word> [])new ArrayList[MainList.size()];

What i would like to do is to call a method from the object inside the array

like this :

    SubList[0].get(0).method();

Thanks In Advance

Why not use an ArrayList of an ArrayList?

Johan Sjöberg
– Johan Sjöberg

2012-02-11 16:21:57 +00:00
Commented Feb 11, 2012 at 16:21 — Johan Sjöberg
– Johan Sjöberg, Commented Feb 11, 2012 at 16:21
i can use that , i just don't know how exactly.

user1203861
– user1203861

2012-02-11 16:24:44 +00:00
Commented Feb 11, 2012 at 16:24 — user1203861
– user1203861, Commented Feb 11, 2012 at 16:24

Johan Sjöberg · Accepted Answer · 2012-02-11 16:27:55Z

4

To avoid working with low level Arrays and to preserve type safety, you could make a list of a list

List<List<Word>> listOfList = 
         new ArrayList<List<Word>>(new ArrayList<List<Word>>());

Which you then could access like

Word word = listOfList.get(0).get(0);

answered Feb 11, 2012 at 16:27

Johan Sjöberg

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4 Comments

dantuch Over a year ago

why you use this syntax: ` new ArrayList<List<Word>>(new ArrayList<List<Word>>())? .... and not new ArrayList<List<Word>>()`?

user1203861 Over a year ago

this seems very good , i have defined many ArrayLists in my Code , do i need to re-write them as Lists? in order to store them in this list of lists

Johan Sjöberg Over a year ago

@user1203861, that sounds like a good idea since Lists are easier to work with than arrays.

Johan Sjöberg Over a year ago

@dantuch, the reason is that I inline the creation of the second list to keep it a one-liner.

rlegendi · Accepted Answer · 2012-02-11 16:26:19Z

1

Generally you cannot do that, because it is not type safe.

See Angelika Langer's Gererics FAQ about the issue (and some additional notes here).

What you can do however, is writing code without using generics and type-casting each time. You'll get a lot of warnings but note that you are sailing on dangerous waters indeed.

answered Feb 11, 2012 at 16:26

rlegendi

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Comments

Milosz · Accepted Answer · 2012-02-11 16:51:15Z

Java doesn't support Arrays of generics very well. However, if you insist on having the top-level structure being an Array type object, you could work around this limitation by creating a new class that extends the generic:

import java.util.ArrayList;

class Test {
    private static class StringArrayList extends ArrayList<String> {}

    public static void main(String[] args) {
        StringArrayList[] sublist = new StringArrayList[1];
        int stringLength;

        sublist[0] = new StringArrayList();
        sublist[0].add("Hello");

        stringLength = sublist[0].get(0).length();
        System.out.println(stringLength);
    }
}

It's not pretty, but it works. It's fine for a personal project but might be a bit too cluttered and C++-like for a large Java enterprise ;)

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