ArrayList> cannot be converted to List>

Question

I'm doing a problem on leetcode: given a list of integers, return a list of lists that contains all the possible permutation of these numbers. The class and method are the following:

class Solution {
    public List<List<Integer>> permute(int[] nums) {
        List<List<Integer>> permu = new ArrayList<ArrayList<Integer>>();
        more code here ...
        }
    }

The compiler threw an error "ArrayList<ArrayList<Integer>> cannot be converted to List<List<Integer>>". I know that the return type must be List<List<Integer>> instead of ArrayList<ArrayList<Integer>>, but I can't declare List<List<Integer>> permu = new List<List<Integer>>(); because list is an interface. How should I declare permu correctly, if I'm not allowed to change the return type in the permute method?

In this example `List` would be new `ArrayList>` and `List` would be `new ArrayList>`. — Tom, May 15 '21 at 17:10

score 1 · Accepted Answer · answered May 15 '21 at 17:06

1

You can create it like this:

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

Or even

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

answered May 15 '21 at 17:06

Lukas Eder

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score 0 · Answer 2 · answered May 15 '21 at 17:07

0

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

This should work since it can implicitly understand what types should be there.

answered May 15 '21 at 17:07

Asis

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ArrayList> cannot be converted to List>

2 Answers2