Initialization of an ArrayList in one line

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"));

And import the correct package:

import java.util.Arrays;

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.

In Java 9 we can easily initialize an ArrayList in a single line:

List<String> places = List.of("Buenos Aires", "Córdoba", "La Plata");

or

List<String> places = new ArrayList<>(List.of("Buenos Aires", "Córdoba", "La Plata"));

This new approach of Java 9 has many advantages over the previous ones:

1. Space Efficiency
2. Immutability
3. Thread Safe

See this post for more details -> What is the difference between List.of and Arrays.asList?

Using Arrays.asList("Buenos Aires", "Córdoba", "La Plata"); is correct. but Any calls to Arrays.asList() with zero arguments or only one argument could be replaced with either a call to Collections.singletonList() or Collections.emptyList() which will save some memory.

Note: the list returned by Collections.singletonList() is immutable, while the list returned Arrays.asList() allows calling the set() method. This may break the code in rare cases.

interestingly no one-liner with the other overloaded Stream::collect method is listed

ArrayList<String> places = Stream.of( "Buenos Aires", "Córdoba", "La Plata" ).collect( ArrayList::new, ArrayList::add, ArrayList::addAll );

Simplest way : you can use this approach to add multiple elements to any type of collection like ArrayList and HashSet

ArrayList<String> allViews = new ArrayList<String>();
Collections.addAll(allViews,"hello","world","abc","def","ghi");