List 和 List 有什么区别<Integer[]>并列出<List<Integer> &gt; 在 Java 中

Question

What is the difference between List<Integer[]> and List<List> in java. Are both types of ArrayList? If they are different how to convert to each other. Thanks.

public static List<Integer[]> fourNumberSum(int[] candidates, int target)  {
    List<List<Integer>> res = new ArrayList<>()
    dfs(candidates, target, res, new ArrayList<Integer>(), 0);
    return res;
}

./Program.java:11: error: incompatible types: List<List<Integer>> cannot be converted to List<Integer[]>
    return res;
           ^

Answer 1

List<Integer[]>

This will create an array of type Integer inside your List .Which means you will have a List which contains it's elements of type Integer[]. But again the array size will be constant.So always you need to know the length of the data items which you are placing inside the Array.So all your List elements(Which are arrays)should have constant size.

List<Integer[]> sampleList = new ArrayList<>();

//Add array elements to list

Integer[] element1 = {15, 20, 40};//Here you need to know the size 
sampleList.add(element1);

//Retrieving the elements from the list 

Integer[] listElement1 = sampleList.get(0);

List<List<Integer>>

This will create a List inside a List .Not an array like before so you can place any amount of data to that particular list.So in this case you don't need to know about the size of data elements which you are going to store like before.

List<List<Integer>> sampleList = new ArrayList<>();

//Add list elements to list

List<Integer> element1 = new ArrayList<>();

element1.add(1);//You can set any number of elements

sampleList.add(element1);

//Retrieving the elements from the list 

List<Integer> listElement1 = sampleList.get(0);

Answer 2

它们不是Integer[]表示整数数组而List<Integer>表示列表，如果要将数组转换为列表，可以执行Arrays.asList(array)

Answer 3

I hope this example will help you understand

    List<Integer[]> x = new ArrayList<Integer[]>();
    Integer[] arr1 = new Integer[3];
    arr1[0] = 34;
    arr1[1] = 55;
    arr1[2] = 2;
    Integer[] arr2 = new Integer[3];
    arr2[0] = 34;
    arr2[1] = 55;
    arr2[2] = 2;         
    x.add(arr1);
    x.add(arr2);

    List<List<Integer>> a = new ArrayList<List<Integer>>();        
    List<Integer> y = new ArrayList<Integer>();
    y.add(1);
    y.add(2);
    List<Integer> z = new ArrayList<Integer>();
    z.add(1);
    z.add(2);
    a.add(y);
    a.add(z);