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The below line gives me error : 

Incompatible Types.

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

What is the reason?

EDIT

I understand if I change my second ArrayList to List, it does not give me error. I want to know the reason of error though. Thanks

Kraken
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6 Answers6

33

From Generics, Inheritance, and Subtypes

This is a common misunderstanding when it comes to programming with generics, but it is an important concept to learn.

enter image description here

Box<Integer> is not a subtype of Box even though Integer is a subtype of Number.

Community
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earthmover
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  • This doesn't answer the question. The question is "what is the reason?". All you've done is restate the question's premise, using different classes. – Dawood ibn Kareem Aug 09 '20 at 23:06
29

If you had a List<List<Integer>> then you'd be able to add a LinkedList<Integer> to it. But you can't do this for an ArrayList<ArrayList<Integer>>, so the latter can't possibly be a type of List<List<Integer>>.

Dawood ibn Kareem
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27

The correct writing should be: List<List<Integer>> ret = new ArrayList<List<Integer>>(); Since in this way, you can add not only ArrayList but also LinkedList to ret

spiralmoon
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9

The reason is that generics are not covariant.

Consider simpler case:

List<Integer> integers = new ArrayList<Integer>();
List<Number> numbers = integers; // cannot do this
numbers.add(new Float(1337.44));

Now List holds a Float, which is certainly bad.

Same for your case.

List<ArrayList<Integer>> al = new ArrayList<ArrayList<Integer>>();
List<List<Integer>> ll = al; // cannot do this
ll.add(new LinkedList<Integer>())

Now you have a list ll which holds LinkedList, but the al is declared as a List of ArrayLists.

lpiepiora
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6

It is clearly stated in Java Doc

In general, if Foo is a subtype (subclass or subinterface) of Bar, and G is some generic type declaration, it is not the case that G<Foo> is a subtype of G<Bar>. This is probably the hardest thing you need to learn about generics, because it goes against our deeply held intuitions.

Same thing happens here it's Bar = List<Integer> and Foo = ArrayList<Integer> as ArrayList<ArrayList<Integer>> is not sub type of List<List<Integer>>

ΔȺȾΔ
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3

Less text more fixes:

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

or

List<List<Integer>> lists = new ArrayList<List<Integer>>();
Pavel Shkleinik
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