Why does this not compile : List> lss = new ArrayList>();

Question

The below code :

List<List<String>> lss = new ArrayList<ArrayList<String>>();

causes this compile time error :

Type mismatch: cannot convert from ArrayList<ArrayList<String>> to List<List<String>>

to fix I change the code to :

List<ArrayList<String>> lss = new ArrayList<ArrayList<String>>();

Why is this error being thrown ? Is this because the generic type List in List<List<String>> is instantiated and since List is an interface this is not possible ?

score 13 · Accepted Answer · answered Dec 20 '13 at 11:29

13

The problem is that type specifiers in generics do not (unless you tell it to) allow subclasses. You have to match exactly.

Try either:

List<List<String>> lss = new ArrayList<List<String>>();

or:

List<? extends List<String>> lss = new ArrayList<ArrayList<String>>();

answered Dec 20 '13 at 11:29

Jules

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Exactly, everything in <> braces should match. Only class before <> braces can be subtype. – Pawel Dec 20 '13 at 11:31

score 3 · Answer 2 · answered Dec 20 '13 at 11:32

From http://docs.oracle.com/javase/tutorial/java/generics/inheritance.html

Note: Given two concrete types A and B (for example, Number and Integer), MyClass<A> has no relationship to MyClass<B>, regardless of whether or not A and B are related. The common parent of MyClass<A> and MyClass<B> is Object.

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score 2 · Answer 3 · edited May 23 '17 at 12:19

2

Same reason

List<List> myList = new ArrayList<ArrayList>();

wont work.

You can see this question for more details.

edited May 23 '17 at 12:19

Community

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answered Dec 20 '13 at 11:28

Aniket Thakur

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Why does this not compile : List> lss = new ArrayList>();

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