Initialization of an ArrayList in one line
I want to create a list of options for testing purposes. At first, I did this:
ArrayList<String> places = new ArrayList<String>();
places.add("Buenos Aires");
places.add("Córdoba");
places.add("La Plata");
Then I refactored the code as follows:
ArrayList<String> places = new ArrayList<String>(
Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));
Is there a better way to do this?
Actually, probably the "best" way to initialize the ArrayList is the method you wrote, as it does not need to create a new List in any way:
ArrayList<String> list = new ArrayList<String>();
list.add("A");
list.add("B");
list.add("C");
The catch is that there is quite a bit of typing required to refer to that list instance.
There are alternatives, such as making an anonymous inner class with an instance initializer (also known as an "double brace initialization"):
ArrayList<String> list = new ArrayList<String>() {{
add("A");
add("B");
add("C");
}};
However, I'm not too fond of that method because what you end up with is a subclass of ArrayList which has an instance initializer, and that class is created just to create one object -- that just seems like a little bit overkill to me.
What would have been nice was if the Collection Literals proposal for Project Coin was accepted (it was slated to be introduced in Java 7, but it's not likely to be part of Java 8 either.):
List<String> list = ["A", "B", "C"];
Unfortunately it won't help you here, as it will initialize an immutable List rather than an ArrayList , and furthermore, it's not available yet, if it ever will be.
It would be simpler if you were to just declare it as a List - does it have to be an ArrayList?
List<String> places = Arrays.asList("Buenos Aires", "Córdoba", "La Plata");
Or if you have only one element:
List<String> places = Collections.singletonList("Buenos Aires");
This would mean that places is immutable (trying to change it will cause an UnsupportedOperationException exception to be thrown).
To make a mutable list that is a concrete ArrayList you can create an ArrayList from the immutable list:
ArrayList<String> places = new ArrayList<>(Arrays.asList("Buenos Aires", "Córdoba", "La Plata"));
The simple answer
In Java 8 or earlier:
List<String> strings = Arrays.asList("foo", "bar", "baz");
This will give you a List backed by the array, so it cannot change length.
But you can call List.set , so it's still mutable.
In Java 9:
List<String> strings = List.of("foo", "bar", "baz");
This will give you an immutable List , so it cannot be changed.
Which is what you want in most cases where you're prepopulating it.
The shorter answer
You can make Arrays.asList even shorter with a static import:
List<String> strings = asList("foo", "bar", "baz");
The static import:
import static java.util.Arrays.asList;
Which any modern IDE will suggest and automatically do for you.
For example in IntelliJ IDEA you press Alt+Enter and select Static import method... .
However, i don't recommend shortening the Java 9 List.of method, because having just of becomes confusing.
List.of is already short enough and reads well.
Using Stream s
Why does it have to be a List ?
With Java 8 or later you can use a Stream which is more flexible:
Stream<String> strings = Stream.of("foo", "bar", "baz");
You can concatenate Stream s:
Stream<String> strings = Stream.concat(Stream.of("foo", "bar"),
Stream.of("baz", "qux"));
Or you can go from a Stream to a List :
List<String> strings = Stream.of("foo", "bar", "baz").collect(toList());
But preferably, just use the Stream without collecting it to a List .
If you really specifically need a java.util.ArrayList
(You probably don't.)
To quote JEP 269 (emphasis mine):
There is a small set of use cases for initializing a mutable collection instance with a predefined set of values. It's usually preferable to have those predefined values be in an immutable collection, and then to initialize the mutable collection via a copy constructor.
If you want to both prepopulate an ArrayList and add to it afterwards (why?), use
List<String> strings = new ArrayList<>(asList("foo", "bar", "baz"));
or in Java 9:
List<String> strings = new ArrayList<>(List.of("foo", "bar", "baz"));
or using Stream :
List<String> strings = Stream.of("foo", "bar", "baz")
.collect(toCollection(ArrayList::new));
But again, it's better to just use the Stream directly instead of collecting it to a List .
Program to interfaces, not to implementations
You said you've declared the list as an ArrayList in your code, but you should only do that if you're using some member of ArrayList that's not in List .
Which you are most likely not doing.
Usually you should just declare variables by the most general interface that you are going to use (eg Iterable , Collection , or List ), and initialize them with the specific implementation (eg ArrayList , LinkedList or Arrays.asList() ).
Otherwise you're limiting your code to that specific type, and it'll be harder to change when you want to.
For example:
// Iterable if you just need iteration, for (String s : strings):
Iterable<String> strings = new ArrayList<>();
// Collection if you also need .size() or .stream():
Collection<String> strings = new ArrayList<>();
// List if you also need .get(index):
List<String> strings = new ArrayList<>();
// Don't declare a specific list implementation
// unless you're sure you need it:
ArrayList<String> strings = new ArrayList<>(); // You don't need ArrayList
Another example would be always declaring variable an InputStream even though it is usually a FileInputStream or a BufferedInputStream , because one day soon you or somebody else will want to use some other kind of InputStream .
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