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 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
– Paul Boddington, Commented Dec 16, 2014 at 21:49

Community · Accepted Answer · 2017-05-23 10:27:30Z

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, 2017 at 10:27

CommunityBot

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answered Dec 16, 2014 at 21:46

rgettman

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1 Comment

Rezwan Azfar Haleem Over a year ago

I went back and looked over the wildcard implementation and the inheritance concept and now it makes complete sense to me. Thank You.

Logan Murphy · Accepted Answer · 2014-12-16 21:58:50Z

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, 2014 at 21:58

answered Dec 16, 2014 at 21:47

Logan Murphy

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