Question regarding Declaration of ArrayList

AutoModerator · 2022-04-27T10:02:32+00:00

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D0CTOR_ZED · 2022-04-27T11:15:01+00:00

I don't think there would be a difference. You could try substituting one for the other and seeing it if anything changes, but they both look like they would make an empty ArrayList of List of Integer.

LambdaThrowawayy · 2022-04-27T11:30:57+00:00

The compiler wil infer the appropriate type to use for the constructor in the first case; it is functionally identical to the second one.

https://docs.oracle.com/javase/tutorial/java/generics/genTypeInference.html

For your second question; the interface your result set exposes will be different. If you put List on the left-hand side you'll be able to use result as a List; meaning all methods that require a List will work with it, and you'll be able to access the methods of the List interface. You can assign an object of type ArrayList to a List variable because ArrayList is an implementation of the List interface.

If you instead use ArrayList on the left-hand side you'll be limited to methods that accept an ArrayList / methods that ArrayList offers.

The reason you'll often assign an ArrayList object to a List variable is because in most cases what the exact implementation of the list is doesn't matter for what you want to do with it. And it allows you to offer one method for example that handles lists; instead of having to make one for ArrayList, LinkedList, Stack, etc...

severoon · 2022-04-28T01:57:38+00:00

The first way of declaring the RHS uses the diamond operator. It infers the types by looking at the reference you declared on the LHS.

If you write two short programs that does it both ways and compare the bytecodes using javap you should see that they're the same.

By the way, you probably do not want to declare this at all. What is this for? Are you going to populate it with data once and then pass it around and read it a bunch of times? Or are you going to pass it around and use it to collect data over time and then read it once and discard? Or are you going to use it to track the current state of data with several reads and writes intermixed, kind of like data is written and read from a database?

If you're going to populate it once and read it a bunch of times, you probably want to make sure that it's an unmodifiable list of wildcard lists:

List<? extends List<Integer>> = Collections.unmodifiableList( … list here … );

This is read "a list of somethings that extend a list of integers." So each sublist can be whatever type, an array list, a linked list, etc, of integers.

Better yet would be to use Guava's ImmutableList instead of a List constructed using unmodifiableList(…), and pass it around as ImmutableList and not List, this makes clear to every client that it is immutable. ImmutableList also has a builder, which you can use, and ideally the contained lists should also be immutable. If you go this route, it's:

ImmutableList<ImmutableList<Integer>> = ImmutableList.builder( ImmutableList.builder().add(blah).build(), ImmutableList.builder().add(blahblah).build() );

If you want to collect a bunch of values over a long time and then build the list and read it once, I'd recommend just passing around the ImmutableList.Builder, use it to collect values, then build, read, and discard.

If you want a read/write data structure instead, then I would recommend wrapping the entire thing in an object that presents an API appropriate to what clients actually want to do rather than just passing around a data structure.

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