using Java Generics with a specific type

Question

I'm working on some LeetCode practice with binary search trees. I'm wondering if someone might explain the following method signature:

 public List<List<Integer>> levelOrder(TreeNode root) {

I am confused by the use of Java Generics here -- what exactly List<List<Integer>> is saying in this case.

I understand Java Generics to be a way to "The <...> syntax allows you to write generic classes and methods that can handle multiple different types" according to https://programming.guide/java/less-than-greater-than-syntax.html

and I've seen it with with ArrayLists, but what does this specific definition of an integer list mean here?

Thanks

List<List<Integer>> can be parsed List<A> where A is List<Integer>. So what you have is a list containing lists of integers. — littlebenlittle
– littlebenlittle, Commented Feb 21, 2020 at 18:02

sdgfsdh · Accepted Answer · 2020-02-21 18:32:07Z

1

List<List<Integer>> should be read as "a list of list of integers".

If you were defining "a list of list of integers" in pseudo-code it might look like this:

[ [ 1, 2, 3 ], [], [ 4, 5 ], [] ]

An ArrayList is a kind of List. Every ArrayList is a List, but not every List is an ArrayList.

The reason Java does this is because you (usually) do not want your code to depend on a specific implementation of a list, but rather the contract of a list instead.

answered Feb 21, 2020 at 18:32

sdgfsdh

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