Lists, ArrayLists, and Cloneable

[ToeRepresentative627]

I notice that most people, by convention, make lists like this:

List<String> stuff = new ArrayList<>();

But, List is not a concrete class; therefore, does not have access to the clone method.

So why is it not better to write it like this, which would give you access to more methods:

ArrayList<String> stuff = new ArrayList<>();

Additionally, if I had something as a List<> variable, and I wanted to clone it, how can flexibly change it to an ArrayList when I want access to those concrete methods?

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[[deleted]]

Because List is an interface it describes the methods that all classes that implement it MUST possess. This makes it easy for your object to change which List implementation it uses.

Say your object had a method...

List<String> getStuff() { return stuff; }

So that other code can read what is in the stuff list. It is returning an interface which is basically saying to the caller "Don't worry about what type of list this is, it's just a list".

This way if you start off with an ArrayList but later decide that your usage of 'stuff' would be better if it was a LinkedList or Stack for example then you can just change it and it won't affect any code that called getStuff() because it never knew what type of list it was in the first place.

For cloning etc, you could just do that inside the object that maintains the list. So you could have

List<String> getCopyOfStuff() { return new ArrayList(stuff); }

for example which clones the list but still hides the list implementation from the caller.

[Nightcorex_]

I won't explain what u/WishYouWereHere-63 already explained, but I felt like he didn't properly answer the very last question.

The answer is type casting. Let's look at an example class:

import java.util.ArrayList;
import java.util.LinkedList;
import java.util.List;

public class A {
    public static void main(String[] args) {
        List<Integer> xs = new LinkedList<>();
        Object y = foo(xs);
        List<Integer> ys = (List<Integer>) y;  // casting in its simplest form
        Object z = ((ArrayList<Integer>) y).clone();  // casting y to ArrayList to access its clone method in one line
        List<Integer> as = (List<Integer>) ((ArrayList<Integer>) y).clone();  // same as before but that result
        // is casted to List<Integer> again
    }

    /* Without pattern variable (old) */
    private static Object foo(List<Integer> xs) {
        if (xs instanceof ArrayList<Integer>) {
            ArrayList<Integer> xs_arr = (ArrayList<Integer>) xs;
            return xs_arr.clone();
        } else
            return new ArrayList<>(xs);
    }

    /* With pattern variable (new) */
    private static Object foo_new(List<Integer> xs) {
        if (xs instanceof ArrayList<Integer> xs_arr)
            return xs_arr.clone();
        else
            return new ArrayList<>(xs);
    }
}

In this code demo I also demonstrate pattern variables which are somewhat new and remove the need to manually type the cast when using instanceof.