The class 'material' should begin with a capital letter, as in Material.
The ArrayList you've built is 1-dimensional rather than 3, and the single dimension is filled with Material objects.
Since each element in the arraylist is a Material object, Material methods could be used to obtain details about those objects/elements. For example, from the code you posted above, to get the metal from the first element, the code would look something like:
material_List.get( 0 ).getMetal();
Assuming there was a method Material.getMetal() that returns the metal type of the object.