Using types / casts

Question

Hi, I'm new to Java and I'm trying to understand a certain convention that i've noticed in reading source code posted by "official" or "knowledgeable" sources. I'll try my best to explain what I mean.
Often in source code, I'll see that the author has called a constructor but is casting the reference to that constructor differently than what you would expect from the constructor itself. Take, for instance the following code (taken from the 2002/09/10 issue of Core Java Technologies Tech Tips email I get):
public class ZeroDemo2 {
    public static void main(String args[]) {
    // set up ArrayList and add strings to it
    List stringlist = new ArrayList();
    stringlist.add("string 1");
    stringlist.add("string 2");
... etc ...My question pertains specifically to the line where the ArrayList() constructor is called but the reference "stringlist" is a List object, not an ArrayList object.
I have seen this in a lot of Java code -- the constructor is one thing, but the object that the constructor instantiates is not of the same type as the constructor! Why do we do this?
I can try to be more specific if I'm not making sense.

Answer 1

Imagine a school directory. Some people are going to
be students, some are faculty, others administrators,
etc etc. All of these groups are different, and have
to have their own methods (addClass() for students,
for example) but they all have things in common (like
a university ID)Right, I can see where it is helpful to have a more general handle on an object (especially for reasons of polymorphism, since that way you can just call a method and know that one of the various subclasses is going to have extended that method and it will work). But for such a short example as the one in the Tech Tips newsletter -- it seems like there was no point in referring to this object as a List and not as the more specific ArrayList. Here is the full code, reprinted below:
import java.util.*;
public class ZeroDemo2 {
    public static void main(String args[]) {
        // set up ArrayList and add strings to it
        List stringlist = new ArrayList();
        stringlist.add("string 1");
        stringlist.add("string 2");
        stringlist.add("string 3");
        // convert to String array
        String out[] = (String[])stringlist.toArray(
                                        new String[0]);
        for (int i = 0; i < out.length; i++) {
            System.out.println(out);
See what I mean? It's almost like it's convention to refer to an object by one of its ancestor types rather than just call it for what it is. (That is, to refer to the object as a List rather than an ArrayList I wonder why.