CODEWITHSUNDEEP
javainterfacearraylist
bdav
bdav

Reputation: 41

Compile error when providing interface as an arraylist type

I have an interface defined as 

interface ListItem {
    public String toString();
    public String getUUID();
}

And a class (BrowseItem) implementing that interface. When I try:

ArrayList<ListItem> = (method returning ArrayList of type BrowseItem)

I get an incompatible type error (found ArrayList<BrowseItem>, require ...<ListItem>)

Am I approaching this wrong?

Upvotes: 4

Views: 1457

Answers (3)

Matt Ball
Matt Ball

Reputation: 359826

Java generics are not covariant.

See (among many other questions on SO):

Solutions:

  • Change the return type of the problematic method. That is, change

    List<listItem> = (method returning List of type browseItem)
// to
List<listItem> = (method returning List of type listItem)

  • Use wildcard covariance (I think that's what this is called):

    List<? extends listItem> = (method returning List of type browseItem)

    Be aware that you cannot add items to the list if you take this route.

N.B. it is generally good practice to declare list types as List<T> and not ArrayList<T>. The pseudocode above reflects this.

Upvotes: 11

Dragon8
Dragon8

Reputation: 1805

your example doesnt work, but you can use

ArrayList<? extends ListItem> list = (method returning ArrayList of type browseItem)

this should work.

Upvotes: 3

Jigar Joshi
Jigar Joshi

Reputation: 240898

ArrayList<listItem> is not equal to ArrayList<browseItem>

They are strictly type safe

you can make use of ?

Upvotes: 2

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