Java: How to pass 2 dimensional ArrayList to a method/constructor

Question

I am trying to pass a 2d ArrayList to a constructor. The header of the constructor is as such:

public Table( ArrayList<ArrayList<?>> table )
{

After this I am trying to implement the following code in main:

ArrayList<ArrayList<Object>> 2dList = new ArrayList<ArrayList<Object>>(2); 

Table Data1 = new Table( 2dList );

However, when I attempt such code I receive the following error:

no suitable contructor found for Table(java.util.ArrayList<java.util.ArrayList<java.lang.Object>>)
    constructor Table.Table(java.util.ArrayList<java.util.ArrayList<?>>) is not applicable
    (argument mismatch; java.util.ArrayList<java.util.ArrayList<java.lang.Object>> cannot be converted to java.util.ArrayList<java.util.ArrayList<?>>)

What would be the correct implementation? Forgive me if I have misunderstood any basic idea or have made a silly mistake.

Thank You.

No, you're not making a silly mistake. Generics are totally baffling. I encountered this the other day (See http://stackoverflow.com/q/27465348/3973077). You can make it work by writing `Table(ArrayList extends ArrayList>> table)`, but mere mortals aren't meant to understand why. — Paul Boddington, Dec 16 '14 at 21:49

score 5 · Accepted Answer · edited May 23 '17 at 10:27

5

An ArrayList<ArrayList<Object>> is not an ArrayList<ArrayList<?>>, even though an ArrayList<Object> is an ArrayList<?>, for the same reason that a List<Dog> is not a List<Animal>, even if a Dog is an Animal.

To pass an ArrayList<ArrayList<Object>>, include another wildcard in the signature:

public Table( ArrayList<? extends ArrayList<?>> table )

edited May 23 '17 at 10:27

Community

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answered Dec 16 '14 at 21:46

rgettman

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I went back and looked over the wildcard implementation and the inheritance concept and now it makes complete sense to me. Thank You. – Rezwan Azfar Haleem Dec 16 '14 at 23:08

Logan Murphy · Answer 2 · 2014-12-16T21:58:50.717

0

Variable names cannot start with numbers! 2dList should be list2D or something like that

Also declare your constructor like so

public Table(ArrayList<? extends ArrayList<?>> list2D) {

}

Or like this

public <E> Table(ArrayList<ArrayList<E>> list2D) {

}

edited Dec 16 '14 at 21:58

answered Dec 16 '14 at 21:47

Logan Murphy

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Java: How to pass 2 dimensional ArrayList to a method/constructor

2 Answers2