java ArrayList<Integer> to int one-dimensional array, two-dimensional array toArray method

font color=#999AAA >Source:
Disclaimer: If I violate anyone's rights, please contact me and I will delete it
. Experts are welcome to spray me

ArrayList to one-dimensional array int[] —traversal/toArray()

 //1. 遍历
ArrayList<Interger> list = new ArrayList<>();
	list.add(1); list.add(2);
	int[]arr = new int[list.size()];
	int index= 0;
	for(int a : list){
		arr[index++] = a;
	}

 2. toArray()需要object做中转
         ArrayList<Integer> list = new ArrayList<>();

        for (int i = 0; i< 10; i++) {
			list.add(i);
		}

        Object[] arr = list.toArray();
        int[] rett = new int[arr.length];

        for(int i=0;i<arr.length;++i){
            rett[i] = (int)arr[i];
        }

        System.out.println(Arrays.toString(rett));

ArrayList<Integer> to one-dimensional array int[] —toArray()

List<int[]> merged = new ArrayList<int[]>();

merged.toArray(new int[merged.size()][]);

ArrayList<ArrayList<Integer>> to two-dimensional array int[] [] did not find a way (because in the process of writing the program, the length of the two-dimensional array and the length of the one-dimensional array are uncertain, I feel ArrayList<ArrayList<Integer >> It will be convenient, but it is not easy to change back), so when this conversion is required in the program, it is recommended to use List<int[]> merged instead of ArrayList<ArrayList<Integer>>, which is very convenient

    @Test
    public void test5(){
        int[] array = {1, 2, 5, 5, 5, 5, 6, 6, 7, 2, 9, 2};
 
        /*int[]转list*/
        //方法一：需要导入apache commons-lang3  jar 
        List<Integer> list = Arrays.asList(ArrayUtils.toObject(array));
        //方法二：java8及以上版本
        List<Integer> list1 = Arrays.stream(array).boxed().collect(Collectors.toList());
 
        /*list转int[]*/
        //方法一：
        Integer[] intArr =  list.toArray(new Integer[list.size()]);
        //方法二：java8及以上版本
        int[] intArr1 =  list.stream().mapToInt(Integer::valueOf).toArray();
        
    }