How can I initialize an `ArrayList` in one line in Java?

General Discussions
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1

I need a concise way to create a list of options for testing. Initially, I wrote:

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

Then, I refactored it to:

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

Is there a better way to java arraylist initialize efficiently?

2

You can use List.of() if you don’t need a mutable list. This is the cleanest and most efficient option when you only need to read from the list.

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

:white_check_mark: Why?

  • Immutable: This ensures no accidental modifications.
  • More performant than Arrays.asList() (since List.of() doesn’t create a fixed-size array). :x: But: It’s immutable—no add() or remove() after initialization.
3

If you need the list to be mutable, you can combine List.of() with an ArrayList to create a modifiable version.

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

:white_check_mark: Why?

  • Concise and clean, yet still mutable.
  • No fixed size limitations like Arrays.asList(). :x: But: Slightly more overhead than a plain ArrayList initialization, but this is rarely an issue unless you’re doing heavy processing.
4

If your list needs to undergo some dynamic processing or transformation, using Streams can be a powerful approach for initializing your java arraylist initialize.

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

:white_check_mark: Why?

  • Ideal if you need to perform filtering, mapping, or other transformations right when initializing the list.
  • Powerful and flexible for more complex use cases. :x: But: Overkill for static, simple lists. If you’re just initializing with hardcoded values, this might be more complexity than needed.