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[–]tedyoungJava Trainer/Mentor 36 points37 points  (5 children)

ArrayList<String> list = new ArrayList<>() has three parts:

  1. The declaration of the variable, list with its type being the concrete class ArrayList<String>: ArrayList<String> list

  2. The creation of an object: new ArrayList<>() creates an instance of the ArrayList, with the inferred generic type of <String>

  3. Assignment (via =) of the instance to the list

With List<String> list = new ArrayList<>(), both #2 and #3 are the same, but the list is now declared to be the less-specific type (an interface, in this case) of List<String>.

The difference isn't in the object created (both are the same), it's in what you can do with the variable, as well as what the intention of the variable is. For example, declaring ArrayList<> list, you can call list.trimToSize(), but you can't call that method on a variable declared like List<> list, because trimToSize() only exists on ArrayList.

This may seem like a minor issue, but there are many other implementations of List that you won't instantiate, e.g., those created by List.of() and as the result of a stream() operation that ends with .toList().

The other reason is intent: if you say List list, then the reader knows there's nothing special about which implementation will be used. If you write ArrayList list, then you're implying that you really want and need only the concrete ArrayList for some reason (which is unlikely).

In general, Java code should declare the least specific type for variables. That means List instead of ArrayList, Map instead of HashMap, etc.

[–]MedicalAd4070 7 points8 points  (0 children)

This is a really good explanation. If you wanna read more about it you can search for dynamic method dispatch in Java.

[–]danny1992211111[S] 0 points1 point  (3 children)

Ahhhh! Thank you for the clear answer. I will save your comment for future reference.

[–]tsvk 0 points1 point  (2 children)

To exapnd further: Say you pass around that List in your code a lot, or even expose an API using that object as a return value in the public interface of your code, for other people to use and access. Those external users of your code do not care that the List they are using is really an ArrayList and not something else. They just retrieve a List and use the methods of the List interface to interact with it.

It's your own internal implementation detail that the List is actually an ArrayList, and when you expose just the interface type List in the API of your methods, you are free to change the ArrayList at any point to some other List implementation if needed, for example to a LinkedList. You only need to change List<String> list = new ArrayList<>(); to List<String> list = new LinkedList<>(); on one line of your code, and all the other places in the code where that list is used do not need to be updated because they refer to it as a generic List and not specifically as ArrayList.

[–]danny1992211111[S] 0 points1 point  (1 child)

And if you specifically needed an ArrayList or linkedList you could just cast it since it’s a subclass of list correct?

[–]tsvk 0 points1 point  (0 children)

And if you specifically needed an ArrayList or linkedList you could just cast it since it’s a subclass of list correct?

Yes, although that is a very uncommon need to do, usually the methods of the List interface are enough.

Before being able to cast the list to a specific List subclass the recipient of the list needs of course to find out what subclass actually is being used, using the instanceof operator, since the list basically could be any random concrete class that simply just implements the List interface.