在 Java 中将 int 的原始数组转换为 ArrayList：“java.lang.ClassCastException: [I cannot be cast to java.lang.Integer”

Question

I'm wondering how I can convert a primitive array of integers to a list of Integer?

There's no compile error for:

    int[] nums = {0, 1};
    List<Integer> list = new ArrayList(Arrays.asList(nums));
    list.get(0);

But this one:

    int[] nums = {0, 1};
    List<Integer> list = new ArrayList(Arrays.asList(nums));
    int a = list.get(0);

fails with:

java.lang.ClassCastException: class [I cannot be cast to class java.lang.Integer ([I and java.lang.Integer are in module java.base of loader 'bootstrap') 

Answer 1

Solution 1:

In Java 8:

List<Integer> list = Arrays.stream(nums).boxed().collect(Collectors.toCollection(ArrayList::new));

Note: You might need to:

import java.util.stream.Collectors;

Solution 2:

Use for loop:

List<Integer> list = new ArrayList();
for(int n : nums) {
    list.add(n);
}

Solution 3:

Declare the original array as Ingeger[] instead of int[] :

Integer[] nums = {0, 1};

Answer 2

The code Arrays.asList(nums) won't translate to an ArrayList with 2 values, viz. 1 and 2. It just has one value, int[] { 1, 2 } . In order to create the right ArrayList , you should use

List<Integer> list = Arrays.stream(nums).boxed().collect(Collectors.toList());