将多个项目添加到 Java 中已初始化的数组列表

Question

My arraylist might be populated differently based on a user setting, so I've initialized it with

ArrayList<Integer> arList = new ArrayList<Integer>();

How can I add hundreds of integers without doing it one by one with arList.add(55); ?

Answer 1

If you have another list that contains all the items you would like to add you can do arList.addAll(otherList) . Alternatively, if you will always add the same elements to the list you could create a new list that is initialized to contain all your values and use the addAll() method, with something like

Integer[] otherList = new Integer[] {1, 2, 3, 4, 5};
arList.addAll(Arrays.asList(otherList));

or, if you don't want to create that unnecessary array:

arList.addAll(Arrays.asList(1, 2, 3, 4, 5));

Otherwise you will have to have some sort of loop that adds the values to the list individually.

Answer 2

What is the "source" of those integers? If it is something that you need to hard code in your source code, you may do

arList.addAll(Arrays.asList(1,1,2,3,5,8,13,21));

Answer 3

Collections.addAll is a varargs method which allows us to add any number of items to a collection in a single statement:

List<Integer> list = new ArrayList<>();
Collections.addAll(list, 1, 2, 3, 4, 5);

It can also be used to add array elements to a collection:

Integer[] arr = ...;
Collections.addAll(list, arr);

Answer 4

If you are looking to avoid multiple code lines to save space, maybe this syntax could be useful:

        java.util.ArrayList lisFieldNames = new ArrayList() {
            {
                add("value1"); 
                add("value2");
            }
        };

Removing new lines, you can show it compressed as:

        java.util.ArrayList lisFieldNames = new ArrayList() {
            {
                add("value1"); add("value2"); (...);
            }
        };

Answer 5

Java 9 + now allows this:

List<Integer> arList = List.of(1,2,3,4,5);

The list will be immutable though.

Answer 6

If you needed to add a lot of integers it'd probably be easiest to use a for loop. For example, adding 28 days to a daysInFebruary array.

ArrayList<Integer> daysInFebruary = new ArrayList<>();

for(int i = 1; i <= 28; i++) {
    daysInFebruary.add(i);
}

Answer 7

In a Kotlin way;

val arList = ArrayList<String>()
arList.addAll(listOf(1,2,3,4,5))

Answer 8

I believe scaevity's answer is incorrect. The proper way to initialize with multiple values would be this...

int[] otherList = {1,2,3,4,5};

So the full answer with the proper initialization would look like this

int[] otherList = {1,2,3,4,5};
arList.addAll(Arrays.asList(otherList));